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SC/BES/MCS/2006/13
December 2006
Original: English
Organic Chemistry
Microscience Experiments
Teaching and Learning Materials
MANUAL FOR TEACHERS
Compiled by Beverly Bell & Christopher Gunter
Edited by Prof. JD Bradley and Jane Spriggs
© 2006 RADMASTE Centre
µscience
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre
University of the Witwatersrand, Johannesburg
Prepared under UNESCO Contract No. 4500021563
This Booklet of Organic Chemistry Microscience
Experiments has been Prepared in Cooperation
with UNESCO, IOCD and IUPAC
UNITED NATIONS EDUCATIONAL,
SCIENTIFIC AND CULTURAL
ORGANIZATION
INTERNATIONAL ORGANIZATION
FOR CHEMICAL SCIENCES
IN DEVELOPMENT
INTERNATIONAL UNION OF PURE
AND APPLIED CHEMISTRY – IUPAC
IN COLLABORATION WITH
µscience
THE UNESCO-ASSOCIATED CENTRE FOR
MICROSCIENCE EXPERIMENTS
THE RADMASTE CENTRE
UNIVERSITY OF THE WITWATERSRAND
JOHANNESBURG, SOUTH AFRICA
THE INTERNATIONAL FOUNDATION FOR
SCIENCE EDUCATION
JOHANNESBURG, SOUTH AFRICA
SESI
SCIENCE EDUCATION SOLUTIONS
INTERNATIONAL
P.O. BOX 681, WITS 2050,
JOHANNESBURG, SOUTH AFRICA
Worksheets Prepared By :
Ms B. Bell & Mr C. Gunter (UNESCO-Associated Centre for Microscience
Experiments, The RADMASTE Centre, University of the Witwatersrand)
µscience
Worksheets Edited By :
Prof. J. Bradley (International Foundation for Science Education);
Ms J. Spriggs (The RADMASTE Centre)
Introductory Note to Teachers
The Organic Chemistry Microscience experiments in this book have been designed for use
with an upgraded RADMASTE Advanced Microchemistry Kit. Users of the Advanced
Microchemistry Kit can acquire an Organic Microchemistry Accessories Kit to upgrade their
kits. It contains the required additional items of equipment and is available from the
RADMASTE Centre at low cost.
Organic Chemistry
Microscience Experiments
FOREWORD
7
INTRODUCTION
8
Components of the RADMASTE Organic Microchemistry Kit 9
CHAPTER 1
10
Qualitative Testing of Organic Compounds
1.
Preparing Esters
2.
Tests for Aldehydes and Ketones
3.
The Iodoform Reaction
4.
Tests for Identifying and Classifying Alcohols
30
5.
Tests for Unsaturated Hydrocarbon Compounds
40
6.
Investigating Intermolecular Forces in Organic Compounds
by Measuring the Boiling Point
48
Melting Point Determinations
51
7.
12
17
25
CHAPTER 2
53
General Procedures
1.
2.
Assembling the Wire Microvial-holder
Vacuum Filtration and Washing of the Collected Solid
55
56
3.
4.
5.
Making Capillary tubes for Melting and Boiling Point
Determinations
General Procedure for Boiling Point Determinations
General Procedure for a Melting Point Determination
58
58
60
6.
7.
Making TLC Spotting Capillary Tubes
General Procedure for a Thin Layer Chromatogram (TLC)
62
63
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
CHAPTER 3
65
Preparation and Reaction of Organic Compounds
1.
2.
3.
4.
5.
6.
Preparation and Testing of Ethyne (Acetylene)
Making Esters – the Synthesis of Aspirin
Recrystallisation and Purification of Aspirin
The Oxyacetylene Flame
Acid-base Extraction of Benzoic Acid from Naphthalene
Using Esters: Saponification
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
67
72
76
78
81
85
FOREWORD
The International Organisation for Chemical Sciences in Development (IOCD) was
established more than 22 years ago. Since its inception, it has been assisting its partners
in many different parts of the world - with special emphasis on the developing countries to create and develop projects on chemical sciences. Professor Glen Seaborg and I,
together with all our colleagues, have always tried to find the best solutions during the
development stages of such projects. We have worked with the different
Intergovernmental (IGO) and Non-governmental (NGO) organisations, and especially with
UNESCO - the unique organisation of the United Nations system with the international
mandate to develop programme activities within the basic science disciplines and
mathematics. Amongst these the chemical sciences have been described as the
"essential science" by Professor Glen Seaborg.
Today we are very pleased to present to the world community, the new teaching and
learning packages on Organic Chemistry Microscience Experiments. These materials can
be considered as an extension of the previous Advanced Teaching and Learning
Packages which use microchemistry activities to address general and inorganic chemistry.
The Microchemistry Packages have been published by UNESCO in different languages,
and have recently become available on the Internet for free access and use by UNESCO
member states.
The organic chemistry materials open up new possibilities of using microscience kits at
different educational levels to do practical laboratory work. In some countries, the
educational curriculum allows for organic chemistry at secondary school level to be part of
a general chemistry syllabus; in some other countries, organic chemistry is an
independent part of chemical sciences at secondary level. Whatever the case may be,
these educational materials will provide simple and cost-effective methods to do new
practical laboratory work with the microscience kits.
We hope, too, that these Organic Chemistry Microscience Experiments will soon be
installed on the Internet for free use.
Some years ago, Professor John Bradley, President of the International Foundation for
Science Education (IFSE), Director of the RADMASTE Centre, and formerly Chairman of
IUPAC-CTC, received the Pierre Crabbe IOCD Award for outstanding contributions to the
advancement of science education in the developing countries. In his introductory
statements which follow, you can find some concrete details on the new educational
materials contained in this publication.
Finally, please let me express my hope that these new microscience materials will be
successfully used in the different countries of the Globe, and that this will lead to different
language versions being prepared so that all may have access to practical organic
chemistry.
Professor Jan Marie Lehn
President, IOCD
Nobel Prize winner
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
7
INTRODUCTION
All over the world, science educators declare that practical experiences are an essential
part of learning science. However, in many countries these experiences are not provided
in the majority of their primary and secondary schools. There are several reasons for this:
cost, safety, waste disposal and teacher preparation. In the last few years a global
programme under the aegis of UNESCO, and in association with a number of
organisations and donors, has attempted to address and overcome that situation. The
programme has promoted the idea that practical experiences can be provided at much
lower cost and with much lower environmental impact by working on a small scale and
using less expensive materials.
The programme has had much success. When the idea has been introduced (so far in
more than 70 countries) it has always had a positive reception. In several countries, local
initiatives have taken the idea further through pilot projects and on to wider national
implementation. This programme of introducing the concept is continuing.
This satisfying record is however not enough. Not surprisingly, the original repertoire of
experiences and low-cost equipment designs, has some limitations. In chemistry, for
example, major areas of the subject have been ignored. One of these is organic chemistry.
In several countries, the welcome accorded to the basic ideas, has been coupled with
questions about these important missing areas. This workbook responds to these
questions, by offering a limited repertoire of basic organic chemistry experiences that can
be provided with low-cost small scale equipment. They have been developed at the
RADMASTE Centre, University of the Witwatersrand in South Africa, the Centre which
created the original materials referred to above. The worksheets and teacher guide layout
will therefore be familiar to those who have used the previous chemistry books, which
were published by UNESCO. Much of the equipment used will also be familiar, but there
are some new and modified items.
In modern laboratories around the world, chemistry is increasingly done on the small
scale. Organic chemistry has been one of the leaders in this trend and it is therefore fitting
that this new publication offers experiences in this field to those who are starting their
studies of it. We hope that both teachers and learners will enjoy the experiments, and will
improve and modify them as a result of their own experience. Although many of the
experiments can be carried out at the secondary school level, it is suggested that some of
the activities be attempted by tertiary level students only.
Professor JD Bradley
Director, RADMASTE Centre &
UNESCO Associated Centre for
Microscience Experiments
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
8
R
AD
CE
MAST
E
N T RE
COMPONENTS OF THE RADMASTE ORGANIC MICROCHEMISTRY KIT
silicone tube
microspatula
gas collecting tube
lid 1
plastic
forceps
lid 2
propette
1
2
wooden toothpick
4
3
5
6
7
8
9
10
11
12
A
string
B
C
microfunnel
D
wire
vial-holder
E
fusion
tube
F
C
5 0
4 0
100
glass rod
8 0
1
2
3
4
6
5
Organic comboplate7
2,5
2 0
6 0
4 0
2 0
0 0
combustion tube
pH
1-4
acid
5
6
7
neutral
8
9
10
11-14
basic
pH indicator chart
alcohol
thermometer
glass
vial with
lid
organic
vial
syringe
microburner
microstand
9
CHAPTER 1
Qualitative Testing of
Organic Compounds
10
Qualitative Testing of Organic Compounds
1.
Preparing Esters
12
2.
Tests for Aldehydes and Ketones
17
3.
The Iodoform Reaction
25
4.
Tests for Identifying and Classifying Alcohols
30
5.
Tests for Unsaturated Hydrocarbon Compounds 40
6.
Investigating Intermolecular Forces in Organic
Compounds by Measuring the Boiling Point
48
Melting Point Determinations
51
7.
11
PREPARING ESTERS
TEACHER'S GUIDE AND MODEL ANSWERS
INTRODUCTION
Esters are a group of organic compounds which can be made by reacting an alcohol with a
carboxylic acid in the presence of a sulphuric acid catalyst. The formation of an ester - or
esterification - is an example of a general class of reactions called condensations. The
condensation reaction results in the formation of water as the ester is formed. The general reaction
equation is:
O
R1OH
+
R2COH
O
R2C
O
R1
+
H2O
R1 and R2 are hydrocarbon groups that may be the same or different.
Esters have characteristic odours that differ substantially from the sometimes foul smelling
alcohols and carboxylic acids from which they are derived. Many of the pleasant odours of fruits
and flowers are due to the presence of low molecular weight esters. Both the pharmaceutical and
food industries use esters to enhance the flavours and fragrances of various products. For
example, benzyl ethanoate is the oil of jasmine used in perfumes while pentyl ethanoate
contributes to the pear fragrance of pear drop sweets.
1. Chemicals
All the chemicals required for the esterification reactions are found in the requirements table
preceding the experiment. If the organic acids and alcohols required are not available, others can
be used. Some examples are isoamyl alcohol reacted with ethanoic acid, ethanol reacted with
benzoic acid, pentanol reacted with butanoic acid and methanol reacted with butanoic acid.
Consult relevant textbooks for further combinations of alcohols and acids.
2. Apparatus
The propettes, microspatula, glass rod, microburner and matches are all components of the
RADMASTE Microchemistry kits. The organic comboplate7 is manufactured from a material which
is resistant to organic solvents and must be used in this experiment, because the conventional
comboplatewill be dissolved by the organic acids and most of the alcohols. A scissors is an
optional requirement for cutting a piece of paper towel into smaller squares. Students may tear the
towelling into smaller pieces instead. Gloves should be worn throughout the experiment to protect
the skin from the corrosive nature of the chemicals used. The teacher must carry out a risk
assessment to decide whether protective glasses should be worn by the students. If the teacher is
not comfortable using the open flame of a microburner in the vicinity of organic substances, a
water bath may be set up by pouring boiling water into a plastic container. This option is reliant on
the availability of apparatus for boiling water, such as an electric kettle, and plastic containers must
be large enough for a comboplate7 to fit into. The teacher must also ensure that a waste container
is provided into which all organic waste can be disposed of.
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
12
3. Hints
Since all the chemicals used in the ester preparations are colourless, it is a good idea to advise
students to label their propettes prior to the experiment to prevent these from becoming muddled.
Two or three students can share one propette of alcohol/acid per ester made because only
three or four drops of each liquid is required. The same applies to the use of the concentrated
sulphuric acid.
When heating the glass rod, it must not be held for a long time in the flame of the microburner
as this will cause the glass to become too hot. The liquid mixture in the wells will be boiled. Since
many of the alcohols have low boiling points, they may evaporate quickly and the experiment will
have to repeated. The end of the glass rod must be waved five or six times through the flame for
the best results.
Stirring the glass rod through contents of the well will allow the heat to be distributed throughout
the mixture. The gentle immersion heating should be repeated three times to ensure that the ester
is formed.
If a boiling water bath is used, the boiling water should be poured into a plastic container just
prior to heating the comboplate7 , which must then be floated in the water bath for about three
minutes for complete esterification. Care must be taken not to push the comboplate7 into the water
because the wells will be flooded. Results with the water bath are normally better than with
immersion heating as the odours of the different esters are more evident. However, use of a water
bath requires replenishment of the boiling water from time to time and takes slightly longer than the
immersion process.
If the teacher is confident that the students are skilled enough to carry out the experiment
without all the step-by-step instructions, then a table such as the one shown below could be
provided after step 11 in the procedure.
Quantity of
Alcohol
Quantity of
Organic Acid
Methanol and
salicylic acid
(Well A3)
5 drops
1 small spatula
using narrow end
of microspatula
Pentanol and
ethanoic acid
(Well A5)
5 drops
5 drops
Isobutanol and
formic acid
(Well A7)
5 drops
5 drops
Mixture/Well
Quantity of
H2SO4(aq)
Heating
3 x immersion
heatings with glass
rod or 3 minutes in
water bath
1 drop
As above
As above
Students will still need instructions for the smelling of the starting materials and resulting esters
to be able to complete the results table.
Each well should be dried out with paper towel after the odour of the ester has been identified.
This is especially necessary when using a water bath for heating, as the odours of the different
esters may mix and the student may not be able to detect the characteristic odour of a specific
ester.
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
13
During the formation of pentyl ethanoate and isobutyl formate, the colour of the liquids may
become yellow. This is not part of the esterification reactions. Similarly, the formation of white
crystals during synthesis of methyl salicylate is also not part of the esterification.
The predict, explore, explain exercise for showing the catalytic function of the sulphuric acid can
be incorporated into the main experiment if liked.
Try to get students to each bring an empty container or label of one food item and one cosmetic
product. Let them examine the list of ingredients and identify which of these are esters.
4. Cautions
Ensure that all students are aware of the following safety advice:
Never point or hold the tip of a propette containing an organic liquid facing upwards. Unlike
aqueous solutions, organic liquids escape readily from the propette stem and can suddenly squirt
from the propette if it is held with the tip pointing upwards.
Do not inhale the acids and alcohols deeply! The vapours are easily detectable. Serious
respiratory damage and dulled receptiveness may occur if this rule is not adhered to. In particular,
the glacial acetic acid, formic acid and isobutanol have sharp, burning odours that also irritate the
eyes.
Move all organic reagents away from the flame when using the microburner. They are
flammable!
Concentrated sulphuric acid is extremely corrosive. If any acid is spilt on the skin, the affected
area must immediately be rinsed with copious amounts of water. Severe burns must receive
medical attention.
Never point a propette or a syringe containing acid upwards. A momentary lapse of
concentration can result in a nasty accident. If any acid is squirted into the eye, immediately rinse
the eye out under running water. Always have a dilute solution of sodium hydrogencarbonate
(household baking soda), or milk close by to apply to the injury. These substances will help
neutralise the acid in the eye. The patient should be referred to a doctor.
Dispose of all organic waste in the waste container provided, and not down the drain!
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
14
MODEL ANSWERS TO QUESTIONS
Q1. Complete Table 1 by naming each of the esters formed during the reactions.
A1. TABLE 1: DESCRIPTION OF THE ODOURS OF EACH ALCOHOL, CARBOXYLIC ACID
AND THE RESULTING ESTER WHEN THESE ARE REACTED
ALCOHOL AND
DESCRIPTION OF
ODOUR
CARBOXYLIC ACID
AND DESCRIPTION
OF ODOUR
Ethanol
Glacial acetic
acid/Ethanoic acid
Smells like alcohol in
alcoholic beverages e.g
cane
Strong smell of vinegar
Methanol
Salicylic acid
Similar to methylated
spirits
Odourless
Pentanol/Amyl alcohol
Glacial acetic
acid/Ethanoic acid
Sweet smelling
Strong smell of vinegar
Isobutanol/2 methyl
propanol
Formic acid
Sharp, burning odour
Sharp burning odour
DESCRIPTION OF
ODOUR OF
PRODUCT
NAME OF ESTER
Smells like nail polish
remover
Ethyl ethanoate or ethyl
acetate
Smells like oil of
wintergreen
Methyl salicylate
Smells like pear (e.g in
pear drop sweets)
Pentyl ethanoate or
pentyl acetate or amyl
acetate
Smells fruity - similar to
raspberry
Isobutyl formate or
2-methylpropyl formate
Q2. What is the name given to the type of reaction by which esters form from a carboxylic acid
and an alcohol?
A2. An esterification reaction.
Q3.
What evidence is there that esters formed in the reactions studied?
A3. Students should identify that the ester smells more pleasant than the original
reactants.
Q4. Complete the following predict, explore, explain exercise to determine the role of the sulphuric acid in each of the reactions:
PREDICT
Will an esterification reaction take place in the absence of concentrated sulphuric acid? The students must make their own predictions and test these by completing the procedure below.
A4.
The correct prediction is that no ester can be formed without the concentrated
H2SO4(aq).
EXPLORE
Has the expected ester been formed? How do you know this?
No. The odour is that of sour vinegar due to the presence of glacial acetic acid. There is no
smell of the expected ethyl ethanoate (nail polish remover).
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
15
EXPLAIN
Was your prediction correct? This answer will depend on the original prediction made.
What is the role of concentrated sulphuric acid in the esterification reaction? Concentrated
sulphuric acid catalyses the esterification reaction. Without it, an ester forms very slowly.
Q5. The introduction to this experiment shows the general equation for an esterification reaction.
Use the equation as a guide to write down balanced equations representing each of the
esterification reactions you have performed. The formulae of each alcohol and carboxylic
acid are given under the chemical requirements. If possible, use a different colour pen to
show the alcohol.
A5.
a) Ethanol and acetic acid:
C2H5OH + CH3COOH → C2H5OOCCH3 + H2O
b) Methanol and salicylic acid: CH3OH + C6H4OHCOOH → CH3OOCOHC6H4 + H2O
c) Pentanol and acetic acid:
C5H11OH + CH3COOH → C5H11OOCCH3 + H2O
d) Isobutanol and formic acid: (CH3)2CHCH2OH + HCOOH → (CH3)2CHCH2OOCH + H2O
Q6. Using the description of the odours of each ester you have prepared in this experiment,
choose two industries in which you think esters are used commercially. Give reasons for your
answers.
A6. a)
The food industry, because some esters like pentyl ethanoate and isobutyl
formate have a fruity aroma that can be used to enhance the flavour of certain
foods.
b)
The cosmetic industry, because some esters like methyl salicylate and ethyl
ethanoate may be used as fragrances for creams, gels and other cosmetic
products.
Q7. If possible, try and obtain the containers of some food and cosmetic products. Read the lists
of ingredients and identify any esters which appear on the labels.
A7. There are a number of examples. One common ester in food products is monosodium
glutamate (MSG), which is used as a flavour enhancer. Benzyl acetate (benzyl
ethanoate) is the oil of jasmine in perfumes.
Q8. Do all ethyl esters have the same smell? Do all formates have the same smell? Design, carry
out and report on a piece of research to answer these questions.
A8. Students should react ethanol with salicylic acid and formic acid to show that ethyl
salicylate, ethyl formate and ethyl ethanoate smell different. The second investigation
would require the reaction of pentanol and methanol with formic acid. The pentyl
formate and methyl formate have different odours, as does the ethyl formate already
synthesised. This is because the smell of an ester is related to its structure and since
all the esters have different structures, they all smell differently.
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
16
FUNCTIONAL GROUP ANALYSIS
Tests for Aldehydes and Ketones
TEACHER'S GUIDE AND MODEL ANSWERS
Part 1:
The 2,4-Dinitrophenylhydrazine Test for Aldehydes and Ketones
The molecules of aldehydes and ketones contain the carbonyl, -C=O, group. Other organic
compounds such as carboxylic acids and their derivatives, also contain the carbonyl group as part
of their molecular structure. The molecules of compounds like alcohols contain the -C-O- group. It
is therefore important to be able to distinguish ketones and aldehydes from these other carbonyl
and oxygen containing compounds. In this experiment you will mix an aldehyde, a ketone, an
alcohol, an ester and a carboxylic acid with a solution called 2,4-dinitrophenylhydrazine (2,4-DNP)
reagent. The reactions of each of these compounds with the 2,4-DNP will be used to differentiate
the aldehyde and ketone.
Part 2.1: Reduction of Ammoniacal Silver Nitrate Solution - Tollen’s Test
The 2,4-DNP test can help the chemist establish which substances are aldehydes and/or ketones
from a variety of carbonyl containing compounds. However, it cannot be used to determine which
of these substances are the aldehydes and which are the ketones because both classes of
compounds form precipitates with the 2,4-DNP reagent. In this experiment, the differing reactions
of aldehydes and ketones with the Ag(NH3)2+(aq) ions in ammoniacal silver nitrate solution
(Tollen’s reagent) will distinguish an aldehyde from a ketone.
Part 2.2: Schiff’s Test to Distinguish between Aldehydes and Ketones
Schiff’s reagent contains a dye compound which has been decolourised by sulphur dioxide. The
colourless compound is unstable and will yield a violet-purple colour when reacted with certain
substances. The task in this experiment is to determine how the colour reaction with Schiff’s
reagent can be used to distinguish between aldehydes and ketones.
1. Chemicals
Part 1:
The 2,4-Dinitrophenylhydrazine Test for Aldehydes and Ketones
All the chemicals required for Part 1 are found in the requirements table preceding the
experiment. The 2,4-dinitrophenylhydrazine reagent (Brady’s reagent) can be prepared by
suspending 2.0 g of solid 2,4-dinitrophenylhydrazine in 100 ml of methanol. To dissolve the solid,
4.0 ml of concentrated sulphuric acid must be added cautiously and slowly. If the solution is not
clear, filtering may be necessary. This volume of reagent should be sufficient for an entire class.
If the methanol, formic acid and ethyl acetate are not available, then another alcohol, carboxylic
acid and ester may be used to show that 2,4-DNP does not react with these compounds.
It is not essential that formaldehyde and propanone are used as the aldehyde and ketone to be
tested. Other aliphatic aldehydes and ketones are also suitable. (For the purpose of providing
model answers, the results of the 2,4-DNP test with butyraldehyde have also been supplied.)
Glacial acetic acid may be needed for removing solid residue from the wells of the comboplate7
after the experiment.
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
17
Part 2.1: Reduction of Ammoniacal Silver Nitrate Solution - Tollen’s Test
All the chemicals required for Part 2.1 are found in the requirements table preceding the
experiment.
The 0.3 M silver nitrate solution can be prepared by dissolving 5.096 g of solid silver nitrate in a
little distilled water in a volumetric flask, and then making up the volume to 100 ml.
Other aliphatic aldehydes and ketones can be mixed with the ammoniacal silver nitrate solution,
but aromatic ketones are not recommended for testing because these - together with
α-hydroxyketones and α-diketones - reduce Tollen’s reagent and give a positive result.
Caution should be exercised when testing unknown compounds with Tollen’s reagent as some
other reducing substances like hydroxylamines, and salts and esters of formic acid can also
produce a silver mirror.
Part 2.2: Schiff’s Test to Distinguish between Aldehydes and Ketones
All the chemicals required for Part 2.2 are found in the requirements table preceding the
experiment. Schiff’s reagent is readily available from commercial suppliers, but can be prepared by
dissolving 2 g of sodium bisulphite in a solution of 0.2 g of p-rosaniline hydrochloride (fuchsin) and
2 ml of hydrochloric acid in 200 ml of distilled water. The reagent is susceptible to decomposition
by light, heat, alkalis and alkali salts. It should therefore be kept in a well stoppered bottle and
protected from exposure to heat and light. If the bottle is not adequately stoppered, the solution will
gradually lose sulphur dioxide and the purple colour will return.
As in the previous parts of the experiment, other aliphatic aldehydes and ketones can be tested
with Schiff’s reagent. The result of this test with butyraldehyde has been provided in the model
answers, together with that for formaldehyde.
The use of aromatic aldehydes and ketones should be avoided as some aromatic aldehydes in
particular, give a negative result with Schiff’s reagent.
2. Apparatus
The propettes and microspatulas are components of the RADMASTE Microchemistry kits. The
organic comboplate® is manufactured from a material which is resistant to organic solvents and
must be used in this experiment, because the conventional comboplate® will be dissolved by the
aldehyde, ketone, carboxylic acid and ester. Gloves should be worn throughout the experiment to
protect the skin from the corrosive nature of the chemicals used. The teacher must carry out a risk
assessment to decide whether protective glasses should be worn by the students. The teacher
must also ensure that a waste container is provided into which all organic waste can be disposed
of.
3. Hints
Since most of the chemicals used in all parts of the experiment are colourless, it is a good idea to
advise students to label their propettes prior to the experiment to prevent these from becoming
muddled.
Two or three students can share propettes of the 2,4-DNP reagent, the Schiff’s reagent,
formaldehyde, propanone etc. because only a few drops of each is required.
The UNESCO-Associated Centre for Microscience Experiments
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Part 1:
The 2,4-Dinitrophenylhydrazine Test for Aldehydes and Ketones
The aim in Part 1 is to show that the 2,4-DNP test can be used to distinguish aldehydes and
ketones from other compounds whose molecules also contain an oxygen atom bonded to a carbon
atom, as well as from other carbonyl-containing compounds. For this reason, the procedure
requires that an alcohol, a carboxylic acid and an ester (a carboxylic acid derivative) are mixed with
the 2,4-DNP reagent. Only the ketone and aldehyde will react to form 2,4-dinitrophenylhydrazones.
Formaldehyde reacts immediately with the 2,4-DNP in the absence of agitation. Propanone, like
other ketones, requires stirring to ensure mixing of the ketone and the 2,4-DNP. A crystalline
derivative is observed after a few minutes. Other aldehydes and ketones, such as butyraldehyde,
will react with the 2,4-DNP but require further agitation to induce crystallisation. A standing period
is therefore allowed for in the experiment, after which stirring may still be required before the
formation of crystals is observed.
The red, yellow or orange precipitates should be washed immediately from the wells of the
comboplate® as soon as Part 1 has been completed. Staining may occur if the precipitates are left
for a prolonged period of time in the wells. Boiling water can be used to flush the precipitates from
the wells. If the stains have still not been removed, a few drops of glacial acetic acid can be added
to each affected well. The stains can then be rubbed off using a cotton bud, or the pointed end of a
wooden skewer around which a small strand of cotton wool has been wrapped.
Part 2.1:
Reduction of Ammoniacal Silver Nitrate Solution - Tollen’s Test
When preparing the “Tollen’s reagent” in the large wells of the comboplate®, brown silver oxide
(Ag2O(s)) is formed by adding sodium hydroxide solution to the aqueous silver nitrate. This
precipitate must be dissolved with the ammonia so that [Ag(NH3)2]+ ions can be formed in solution.
An excess of ammonia must be avoided, therefore the ammonia solution must be added dropwise
with stirring. Sometimes it may appear that the silver oxide has not completely dissolved, but if
stirring is continued for a short while all the small, brown particles of silver oxide will eventually be
dissolved. No more than 25 drops of 1 M NH3(aq) should be added.
The number of drops of aldehyde added to the reagent varies between 1 and 3, depending on
the aldehyde used. Only one drop of formaldehyde is required to form a conspicuous silver mirror.
The silver mirror formed with butyraldehyde is however not as evident when one drop of the
aldehyde is used. Three drops of this aldehyde is preferable if a better silver mirror is desired.
A waiting period of five to ten minutes is required to allow the silver precipitate to accumulate on
the bottom and sides of the well. Formaldehyde reacts immediately and the silver mirror is normally
detected before five minutes has lapsed. Other aldehydes, like butyraldehyde, react more slowly
and the silver is only evident after some time. If another aldehyde is used by the teacher and a
silver mirror is not observed after 10 minutes, the comboplate® should be floated in a container of
warm - but not boiling - water (approximately 350C). A silver mirror should form within a few
minutes.
The appearance of the silver mirror varies with the aldehyde used. For example, the mirror
formed with formaldehyde is very shiny whereas that formed with butyraldehyde is somewhat
darker. It is usually necessary to tilt the comboplate® towards the light to show the silver deposit on
the sides of the wells.
The silver solutions should never be heated excessively, nor left to stand unnecessarily since
explosive silver fulminate may be formed. Once the experiment has been completed, the residue
from each well must be thoroughly rinsed way. Dilute nitric acid must then be added to each well to
remove any silver mirror and prevent the formation of fulminates. Sometimes there is still some
black residue that remains in each well after washing with nitric acid. This can be removed with a
cotton bud or wooden skewer as previously described.
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Part 2.2: Schiff’s Test to Distinguish between Aldehydes and Ketones
Schiff’s reagent is very easily decomposed by heat, light, alkali, alkali salts of weak acids and
when exposed in small quantities to the air for some time. Mineral acids also reduce the sensitivity
of the test. As a result the Schiff’s reagent becomes pale purple in colour, which darkens with
further deterioration of the solution. If the teacher finds that the Schiff’s reagent is no longer
colourless but already pale purple in colour prior to performing the experiment, the following
procedure should be used: Place 10 drops of the reagent in wells A1, A2 and A3. Place 1 drop of
the aldehyde in well A2 and 1 drop of the ketone in well A3. The reagent in well A1 will be used as
a colour comparison. The aldehyde will react with the reagent to produce a darker purple colour
within two minutes. The ketone will not react and the colour in well A3 will be the same as the
control in well A1. The student will still be able to deduce that the aldehyde gives a positive result
with Schiff’s reagent, whereas that with the ketone is negative. If the Schiff’s reagent has
deteriorated to such an extent that the solution is dark purple in colour, it cannot be used and
should be discarded. Students must be advised to keep the bottle containing the Schiff’s reagent
closed at all times.
Since some compounds may contain traces of other substances that react with Schiff’s reagent,
a time limit of two minutes has been set in this experiment for observing the change in colour of
the reagent. Any colour change taking place after two minutes should be disregarded.
At the end of the test, rinse the comboplate® thoroughly with water as the purple colour may
stain the wells. Stubborn stains can be removed by dissolving with a few drops of 5.5 M
hydrochloric acid, and rubbing with a cotton bud or wooden skewer as previously described.
The Iodoform Test for Methyl Ketones
If the 2,4-DNP test indicates that an aldehyde or ketone is present, and if the Tollen’s and Schiff’s
tests suggest that the compound is not an aldehyde, then the iodoform test can be used to
distinguish methyl ketones from other ketones. Iodoform (CHI3) is produced when a methyl ketone
reacts with iodine solution in the presence of aqueous sodium hydroxide. Other ketones do not
form iodoform. The iodoform reaction must be performed after:
i.
the 2,4-DNP test to ensure that alcohols are excluded, since secondary alcohols will also
give a positive result, and
ii.
the Tollen’s or Schiff’s tests since acetaldehyde will give a positive result.
4. Cautions
Ensure that all students are aware of the following safety advice:
Never point or hold the tip of a propette containing an organic liquid facing upwards. Unlike
aqueous solutions, organic liquids escape readily from the propette stem and can suddenly squirt
from the propette if it is held with the tip pointing upwards.
Avoid dropping any of the organic reagents onto the skin. If any of these chemicals is spilled
onto the skin, wash the affected area with copious amounts of water.
Do not allow any flames to be brought near the organic reagents used in this experiment. They
are all flammable.
Do not inhale the vapours of the alcohol, ester, aldehyde, ketone and carboxylic acid as they are
poisonous and may cause respiratory damage.
Silver nitrate solution is poisonous and can cause heavy metal poisoning. Take care when using
this solution. Silver is also an expensive metal, therefore the solution must not be wasted!
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The mixture of the silver nitrate, sodium hydroxide and ammonia solutions (“Tollen’s reagent”)
should never be heated. The solution formed from the reaction of an aldehyde with Tollen’s
reagent should also never be heated, as it may become explosive.
The acetic, nitric and hydrochloric acids recommended for cleaning are corrosive. If any acid is
spilt on the skin, the affected area must immediately be rinsed with copious amounts of water.
Severe burns must receive medical attention.
Never point a propette or a syringe containing acid upwards. A momentary lapse of
concentration can result in a nasty accident. If any acid is squirted into the eye, immediately rinse
the eye out under running water. Always have a dilute solution of sodium hydrogencarbonate
(household baking soda), or milk close by to apply to the injury. These substances will help
neutralise the acid in the eye. The patient should be referred to a doctor.
Dispose of all organic waste in the waste container provided, and not down the drain!
MODEL ANSWERS TO QUESTIONS
Part 1:
The 2,4-Dinitrophenylhydrazine Test for Aldehydes and Ketones
Q1. What do you observe immediately and after 5 minutes? Prepare a table like the one below.
A1. TABLE 2: TESTING OF VARIOUS COMPOUNDS WITH 2,4-DNP REAGENT
IMMEDIATE REACTION
WITH 2,4 DNP REAGENT
REACTION AFTER
STANDING (~ 5 MINUTES)
none
none
Formaldehyde (aqueous)
Precipitate of bright,
orange-yellow crystals
forms.
A greater volume of
precipitate has deposited
in the w ell.
Propanone
Colour of solution
changes from yellow to
orange.
A precipitate of fine,
needle-like orange crystals
has formed.
Formic acid
None, or the solution may
become pale orange in
colour. The colour fades
w ith standing.
N one
Ethyl acetate
N one
N one
*Butyraldehyde
Colour changes slightly to
pale orange but may not
be very evident.
A yellow precipitate forms
w ith stirring
COMPOUND
Methanol
* The results for butyraldehyde as an alternative aldehyde are also given.
Q2. Name the class to which each of the compounds used in this experiment belongs (eg.
alcohol, alkane, ester, etc.)
A2. methanol
alcohol
formic acid
carboxylic acid
aldehyde
ethyl acetate
ester
formaldehyde
ketone
*butyraldehyde aldehyde
propanone
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Q3. Use the results in Table 2 and your answer to question 1 to explain how the 2,4-DNP test
helps to distinguish aldehydes and ketones from other -C-O- and -C=O containing
compounds.
A3. Aldehydes and ketones form yellow, orange or red precipitates when they react with
2,4-dinitrophenylhydrazine reagent. Other carbonyl containing compounds like esters
and carboxylic acids do not form precipitates with 2,4-DNP. Compounds containing
the -C-O- group, such as alcohols, also do not react with the 2,4-DNP.
Part 2:
Tests to Distinguish Between Aldehydes and Ketones
2.1: Reduction of Ammoniacal Silver Nitrate Solution - Tollen’s Test
Q1. What do you observe after 5 to 10 minutes? Prepare a table like the one below.
A1. TABLE 3: TESTING OF ALDEHYDES AND KETONES WITH AMMONIACAL
SILVER NITRATE SOLUTION
NAME OF ALDEHYDE/KETONE
REACTION WITH AMMONIACAL SILVER
NITRATE SOLUTION
Formaldehyde (aqueous)
The solution blackens as soon as
formaldehyde is added. A very shiny
precipitate of silver forms and deposits on
the bottom of the w ell. After 5 minutes, the
entire bottom and sides of the w ell are
covered w ith a silver mirror.
Propanone
No reaction. There is no silver mirror.
*Butyraldehyde
The solution blackens after a few seconds
and a silver precipitate is detected after a
short w hile. The silver also deposits on
the w alls and bottom of the w ell, but is
slightly darker and not as shiny as the
silver mirror formed w ith formaldehyde.
Q2. How can Tollen’s test be used to differentiate between an aldehyde and a ketone?
A2. Aldehydes react with the ammoniacal silver nitrate solution to form a silver mirror.
Ketones do not react and therefore a silver mirror cannot be detected.
Q3. The reaction which occurs when an aldehyde is added to Tollen’s reagent is:
RCHO(l) + 2[Ag(NH3)2]OH(aq) → RCOONH4(aq) + 2Ag(s) + 3NH3(aq) + H2O(l)
i.
ii.
What type of reaction is this?
Explain your answer to i above.
A3.
i.
ii.
It is a redox reaction.
The silver ion in the [Ag(NH3)2]OH molecule has an oxidation number of +1. The
silver atom in the silver precipitate has an oxidation number of 0. The silver ion
has gained an electron and has therefore been reduced. The carbon atom in the
aldehyde molecule (RCHO) has an oxidation number of 0. The carbon atom in
the RCOONH4 molecule has an oxidation number of +2. The carbon atom has lost
electrons and has therefore been oxidised.
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Part 2:
Tests to Distinguish between Aldehydes and Ketones
2.2: Schiff’s Test to Distinguish between Aldehydes and Ketones
Q1. What do you observe in each well? Prepare a table like the one below.
A1. TABLE 4: TESTING OF ALDEHYDES AND KETONES WITH SCHIFF’S REAGENT
ALDEHYDE/KETONE TESTED
REACTION WITH SCHIFF'S REAGENT
Formaldehyde (aqueous)
The solution changes immediately to a dark
violet colour. Stirring is not usually necessary.
Propanone
The solution remains colourless. No reaction
takes place.
*Butyraldehyde
Within a few seconds of stirring, the solution
changes to violet in colour. The colour
becomes more intense w ith time.
Q2. Based upon the results in Table 4 above, explain how the Schiff’s test allows one to
distinguish between aldehydes and ketones.
A2. Aldehydes react with Schiff’s reagent and form a violet or purple coloured product in
solution. Ketones do not react, therefore a colourless solution remains.
Q3. Examine the structures of formaldehyde and propanone below.
i.
ii.
When formaldehyde reacts with Tollen’s reagent, what is it converted to? Write equations to
illustrate your answer.
Explain why propanone cannot react in the same way.
A3.
i.
Formaldehyde is converted to formic acid, which in the presence of ammonia
gives ammonium formate.
CH2O(aq) + 2[Ag(NH3)2]OH(aq) → HCOONH4(aq) + 2Ag(s) + 3NH3(aq) + H2O(l)
ii.
The structure of propanone shows that it does not have a hydrogen atom bonded
to the carbonyl carbon atom. It is therefore not oxidised by Tollen’s reagent.
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Q4. The figure below shows a scheme for testing substances to check whether they are
aldehydes or ketones. Complete the chart by filling in the missing words related to the
experiments completed in this worksheet.
A4.
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FUNCTIONAL GROUP ANALYSIS
The Iodoform Reaction
TEACHER'S GUIDE AND MODEL ANSWERS
Introduction
Some ketones have a methyl group bonded to the carbonyl carbon atom. These are called methyl
ketones. Other ketones, called non-methyl ketones, do not have a methyl group adjacent to the
carbonyl group. It is possible to distinguish between these two kinds of ketones, by observing
whether iodoform (CHI3(s)) is formed when a ketone is mixed with iodine and sodium hydroxide
solutions. In this experiment learners will react various ketones, an aldehyde and some alcohols
with iodine and sodium hydroxide solutions. The results of these reactions will help them determine
which of the ketones is a methyl ketone and which is not. It will also show that other compounds
can be oxidised to methyl ketones, and hence form iodoform.
1. Chemicals
All the chemicals required are found in the requirements table preceding the experiment. If the
ketones and alcohols listed are not available, then other ketones and alcohols can be used.
Remember to provide a methyl ketone and a non-methyl ketone so that it can be shown that the
iodoform test distinguishes between the two. It is a good idea to also test a number of primary,
secondary and tertiary alcohols.
The potassium iodide-iodine reagent (sometimes referred to as iodine solution) should be kept
in a cool dark place because it is decomposed by heat and light.
If the teacher would like the students to test an unknown substance, then the compound
provided should not be a quinone or hydroquinone as these also give a positive result in the
iodoform test. If the compound to be tested is insoluble in water, it may be necessary to dissolve it
in a little dioxan. However, the dioxan may be contaminated with impurities that give a positive
iodoform reaction, so it is necessary to subject the dioxan alone to the iodoform test.
2. Apparatus
The propettes and microspatulas are components of the RADMASTE Microchemistry kits. The
organic comboplate is manufactured from a material which is resistant to organic solvents and
must be used in this experiment, because the conventional comboplate will be dissolved by the
ketones, aldehyde and some of the alcohols used. Gloves should be worn throughout the
experiment to protect the skin from the corrosive nature of the chemicals used. The teacher must
carry out a risk assessment to decide whether protective glasses should be worn by the students.
The teacher must also ensure that a waste container is provided into which all organic waste can
be disposed.
3. Hints
Since most of the chemicals used in the experiment are colourless, it is a good idea to advise
students to label their propettes prior to the experiment to prevent these from becoming muddled.
Two or three students can share propettes of the ketones, aldehyde and alcohols because only
a few drops of each is required.
Since there are more than six compounds to be tested, students should be encouraged to work
in pairs. One student can test some of the compounds, while the other tests the remainder. The
results can be shared.
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Propanone has the tendency to flow from the propette tip, especially when the temperature of
the surroundings is relatively high. The propette containing the propanone should be handled
carefully and very little pressure applied to the bulb as drops of the liquid may fall freely into the
well. Do not add an excess of the propanone or other compounds to be tested because the
iodoform reaction is only successful when the iodine and sodium hydroxide solutions are added in
excess.
The ketones have been chosen in such a way that one ketone is a methyl ketone (propanone)
and the other is not (cyclohexanone). Only the former gives a positive iodoform reaction. The
formaldehyde is representative of the majority of aldehydes (except acetaldehyde) which do not
react with hypoiodite solution. The alcohols have been selected in such a way to show that only
certain alcohols give a positive iodoform reaction, as explained earlier.
Propanone reacts immediately with the hypoiodite solution to produce iodoform as a yellow
suspension. This gives the solution a milky yellow/cloudy appearance. The ethanol and secondary
butyl alcohol do not react as quickly as the propanone. For this reason, a waiting period of 2 to 3
minutes is allowed for in the experiment, after which time these solutions also appear milky yellow.
The cyclohexanone is slightly soluble in cold water, therefore when the sodium hydroxide and
iodine solutions are added, two layers may be seen. After addition of the iodine solution the upper
organic layer may retain the brown colour of iodine but this fades quickly, and it is obvious that no
iodoform has been obtained with cyclohexanone.
Iodoform crystals will not have settled at the bottom of the relevant wells after the 2 minute
standing period. The procedure therefore advises that the comboplate be left to stand for another 5
to 10 minutes, during which time the iodoform precipitates at the bottom of each well in which a
positive result is obtained. The longer the waiting period, the greater the volume of iodoform that
will precipitate and the easier the solid will be detected at the bottom of a particular well.
Individual crystals of iodoform cannot be seen but rather a fine, pale yellow precipitate settles at
the bottom of a well. Sometimes a solid is difficult to detect because the entire solution is still
cloudy. There are two ways in which it can be proved that there is a precipitate in the well: the
bottom of the well can be scratched with the narrow end of a microspatula and/or the liquid
contents in the comboplate can be discarded as waste. With the former method, scratch marks
can be seen in the precipitate at the bottom of the well proving that a solid is present. When the
latter method is used, the precipitates adhere to the bottom of the relevant wells and can easily be
seen. Those wells in which a negative result is obtained will of course be empty when the solution
is discarded.
The precipitates can be removed from the comboplate by first rinsing with water and then using
a cotton bud - or the pointed end of a wooden skewer around which a small strand of cotton wool
has been wrapped - to remove any solid residue.
4. Cautions
Ensure that all students are aware of the following safety advice:
Never point or hold the tip of a propette containing an organic liquid facing upwards. Unlike
aqueous solutions, organic liquids escape readily from the propette stem and can suddenly squirt
from the propette if it is held with the tip pointing upwards. Propanone, in particular, has a low
surface tension and drops of the liquid may fall from the tip of the propette without pressure being
placed on the bulb.
Avoid dropping any of the organic reagents onto the skin. If any of these chemicals is spilled
onto the skin, wash the affected area with copious amounts of water.
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Do not allow any flames to be brought near the organic reagents used in this experiment. They
are all flammable.
Do not inhale the vapours of the ketones, aldehyde and alcohols as they are poisonous and
may cause respiratory damage. Keep the bottles of all organic liquids closed so as to minimise
vaporisation.
The halogen solutions are corrosive. Do not drop the iodine solution onto skin or fabrics. If
contact is made with the skin, wash the affected area with copious amounts of water. Keep the
bottle containing the iodine solution in a cool, dark place as it decomposes in the presence of light
and heat.
Never point a propette or a syringe containing acid or base upwards. A momentary lapse of
concentration can result in a nasty accident. If any base is squirted into the eye, immediately rinse
the eye out under running water. Always have a dilute solution of boric acid close by to apply to the
injury. This will help neutralise the base in the eye. The patient should be referred to a doctor.
Dispose of all organic waste in the waste container provided, and not down the drain!
MODEL ANSWERS TO QUESTIONS
Q1. What do you observe after 2-3 and after 5-10 minutes? Prepare a table like the one below.
A1. TABLE 1: TESTING OF VARIOUS COMPOUNDS WITH HYPOIODITE (I2/KI - NaOH)
SOLUTION
COMPOUND
APPEARANCE OF WELL CONTENTS AFTER
2 TO 3 MINUTES
PRECIPITATE (CHI3(s))
FORMATION?
Propanone
A milky yellow turbidity is present.
Yes
Cyclohexanone
Tw o layers may be seen. The low er layer is
colourless w hile the upper layer has a faint
iodine (brow n) colour. This later fades.
No
Formaldehyde (aqueous)
The solution is colourless and clear.
No
Methanol
A clear, colourless solution remains.
No
Ethanol
Appears cloudy (milky yellow ).
Yes
Secondary butyl alcohol
A milky yellow turbidity is present.
Yes
Tertiary butyl alcohol
A colourless, clear solution remains.
No
*Propan-1-ol
A colourless, clear solution remains.
No
Q2. What do you notice in the wells where turbidity was observed earlier?
A2. A pale, yellow solid has settled at the bottom of those wells in which the contents
were cloudy (i.e those wells which contained propanone, ethanol and secondary butyl
alcohol).
Q3a. What do you see in step 7 of the procedure?
A3a. Scratch marks are evident in the pale yellow solid at the bottom of the wells in which
propanone, ethanol and secondary butyl alcohol were tested. This proves that a
precipitate has formed with these compounds.
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Q3b. What do you notice in step 8 of the procedure?
A3b. There is a pale yellow precipitate at the bottom of the wells where propanone, ethanol
and secondary butyl alcohol were tested.
Q4. Describe the odour.
A4. The solid has a characteristic odour similar to that of some antiseptic substances.
Q5. Examine the structures of propanone and cyclohexanone alongside. What is the difference in
structure between these two ketones, besides the fact that one ketone is cyclic and the other
aliphatic?
A5.
Propanone is a methyl ketone i.e its molecules contain a methyl group bonded to the
carbonyl carbon atom. Cyclohexanone is not a methyl ketone.
Q6. Based upon your answer to question 1 and the observations made in this experiment, how
does the iodoform test help to distinguish methyl ketones from non-methyl ketones?
A6. Methyl ketones react with hypoiodite solution to form iodoform, seen as turbidity or a
precipitate of pale yellow crystals. Non-methyl ketones do not form iodoform.
Q7. The reaction of a methyl ketone with iodine solution in the presence of aqueous sodium
hydroxide is shown in the equation below:
RCOCH3(l) + 3I2(aq) + 4NaOH(aq) → RCOONa(aq) + CHI3(s) + 3NaI(aq) + 3H2O(l)
where R = H, alkyl or aryl group
i.
ii.
A7.
Which part of the ketone molecule has been involved in the formation of an iodoform
molecule?
Use your answer to i to explain why cyclohexanone gives a negative reaction in the iodoform
test.
i.
The methyl group attached to the carbonyl carbon atom in the ketone molecule is
involved in the reaction which produces iodoform molecules.
ii.
Cyclohexanone does not contain a methyl group attached to the carbonyl carbon
atom, and therefore iodoform cannot be produced. (Reaction with iodine does
occur but the product is not iodoform.)
Q8. Formaldehyde gives a negative result in the iodoform test. Do you think that the iodoform
test could be used to distinguish methyl ketones from all aldehydes? Explain your answer
with reference to the equation given in question 3.
A8. No. The equation shows that a positive iodoform test will be obtained with a
compound having the structure RCOCH3 where R is an H atom, alkyl or aryl group. The
compound HCOCH3 is an aldehyde (acetaldehyde). Acetaldehyde will also produce
iodoform with hypoiodite solution because, like methyl ketones, its molecules
possess a methyl group bonded to the carbonyl carbon atom:
HCOCH3(l) + 3I2(aq) + 4NaOH(aq) → HCOONa(aq) + CHI3(s) + 3NaI(aq) + 3H2O(l)
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Q9. The equation for the initial reaction between an alcohol (primary or secondary) and iodine
solution in the presence of aqueous sodium hydroxide is:
RCH(OH)R’(l) + I2(aq) + 2NaOH(aq) →
RCOR’(aq) + 2NaI(aq) + 2H2O(l)
where R and R’ = H, alkyl or aryl group
i.
ii.
iii.
A9.
According to the equation, which alcohols should react with the hypoiodite solution?
Use the equation to explain why certain alcohols are able to give a positive iodoform
reaction.
Why does ethanol give a positive iodoform test but methanol does not? (Hint: look at the
reaction equation).
i.
Alcohols whose molecules have the CH3-C-OH structure. (Let the students write
down the structures of various kinds of alcohols that have the formula
RCH(OH)R’. This will make it easier to identify those alcohols which do not have
a methyl group adjacent to the OH-bonded carbon atom, and which will therefore
not produce a methyl ketone.)
ii.
The equation shows that under the conditions of the reaction, certain alcohols
are oxidised to methyl ketones. We know that such ketones react further with the
hypoiodite solution to produce iodoform.
iii. Alcohols having the structure RCH(OH)CH3 will give a positive result, where R is
an H atom, alkyl or aryl group. The compound HCH(OH)CH3 is ethanol. The
ethanol is oxidised to acetaldehyde which, as discussed previously, reacts with
hypoiodite solution to produce iodoform. Methanol has the structure CH3OH.
There is no other methyl group attached to the OH-bearing carbon atom,
therefore a negative result is obtained with methanol. (A methanol molecule has
a methyl group, but it is bonded directly to the OH.)
Q10. Refer back to Table 1. Write down the reactions taking place for those ketones and alcohols
which gave a positive iodoform test.
A10. Propanone
Ethanol
sec-Butyl alcohol
CH3COCH3 + 3I2 + 4NaOH → CH3COONa + CHI3 + 3NaI + 3H2O
a) CH3CH2OH + I2 + 2NaOH → HCOCH3 + 2NaI + 2H2O
b) HCOCH3 + 3I2 + 4NaOH → HCOONa + CHI3 + 3NaI + 3H2O
a) CH3CH2CH(OH)CH3 + I2 + 2NaOH → CH3CH2COCH3 + 2NaI + 2H2O
b) CH3CH2COCH3 + 3I2 + 4NaOH → CH3CH2COONa + CHI3 + 3NaI + 3H2O
Q11. You are working in a company laboratory. You have been asked to use 1-propanol in a
particular experiment. You find a cupboard in which there is a shelf for alcohols, but the
bottles are not labelled. You know that ethanol and 1-propanol are used often. Explain how
you will use the iodoform test to locate a bottle of 1-propanol in the presence of bottles of
ethanol. Predict the outcome of your investigation and refer to the structures of the alcohol
compounds when necessary.
A11. The student should recognise that ethanol gives a positive iodoform test.
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FUNCTIONAL GROUP ANALYSIS
Tests for Identifying and Classifying Alcohols
TEACHER'S GUIDE AND MODEL ANSWERS
Introduction
Both alcohols (R-O-H) and ethers (R-O-R’) are neutral, oxygen containing compounds. The
purpose of the sodium test (Part 1) is to enable one to distinguish between alcohols and ethers by
observing how each compound reacts when sodium metal is added to it.
The two tests in Part 2 allow one to distinguish primary, secondary and tertiary alcohols from one
another, by observing the reactivity each class of alcohol displays.
Secondary and tertiary alcohols both form turbid mixtures with Lucas’ Reagent. When an unknown
alcohol is tested and forms a turbid mixture, the chemist may not be able to determine whether the
alcohol is secondary or tertiary in structure. To determine conclusively which of the alcohols is the
secondary and which is the tertiary structure, the chemist needs to mix each of the alcohols with
concentrated hydrochloric acid.
Chromic Anhydride Reagent, also called Jones' Reagent, is a solution of chromic anhydride
(chromium trioxide - CrO3) in dilute sulphuric acid. It rapidly oxidises primary and secondary
alcohols. Tertiary alcohols do not react visibly in less than two minutes under the test conditions.
Primary alcohols are oxidised to carboxylic acids, and secondary alcohols form ketones:
3RCH2OH(l) + 4CrO3(aq) + 6H2SO4(aq) → 3RCOOH(l) + 2Cr2(SO4)3(s) + 9H2O(l)
3RR'CHOH(l) + 2CrO3(aq) + 3H2SO4(aq) → 3RR'CO(l) + Cr2(SO4)3(s) + 6H2O(l)
The chromium sulphate (Cr2(SO4)3(s)) is a greenish-blue precipitate. As it is formed, it gives the
test solution a blue-green, opaque appearance. The precipitate later settles and the test solution
becomes clear.
1. Chemicals
All the chemicals required are listed in the worksheet.
Ice is required as well as boiling water - at least enough to fill every student's lunch box or to fill
a communal container eg. a bucket of some sort (see step 3).
Instead of ice, students can use cold water to cool the reaction mixture. This can be tap water
(depending on the ambient temperature) or cold water from a fridge.
2. Apparatus
The vials, propettes, microspatula, microburner, microretort stand, are all components of the
RADMASTE Advanced Microchemistry kit.
The organic comboplate is manufactured from plastic which is resistant to organic solvents and
must be used in this experiment, because the comboplate used with aqueous solutions can be
affected by the organic reagents.
A kettle (or a stove with a pot) may be required to boil the water in Part 2.1.
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3. Hints
Since the organic reagents used in all parts of the experiment are colourless, it is a good idea to
advise students to label their propettes prior to the experiment to prevent these from becoming
muddled.
Two or three students can share propettes of the alcohols and Lucas Reagent in Part 2.1, as
well as the acetone and Chromic Anhydride Reagent in Part 2.2 because only a few drops of each
is required.
Part 1:
The Sodium Test to Distinguish Between Alcohols and Ethers
The aim in Part 1 is to show that the reaction between an alcohol and metallic sodium can be
used to distinguish alcohols from ethers, which do not react with sodium. Since the presence of
water in samples of the ethanol and ether is unavoidable, Part 1 focuses mainly on the differences
in the rate of hydrogen production when sodium is added to ethanol and ether.
The reaction of sodium with ethanol is vigorous (shown by the fizzing sound and the movement or
‘darting’ of the sodium across the surface of the ethanol) and rapid (hydrogen bubbles are given off
very quickly). The sodium is used up in the reaction after a short while. In contrast, when sodium is
added to the ether the reaction is slow without any fizzing or rapid movement of the piece of
sodium. The sodium reacts with water which was not removed during the drying of ether with
magnesium sulphate. Since the water is present only as a trace impurity in the ether, the bubbles
of hydrogen are given off more slowly with time as the concentration of water molecules
decreases. After about 10 minutes, the reaction has ceased and the size of the piece of sodium is
not much smaller than when first added to the ether. Coarse granules of a white solid are evident
at the bottom of the well. This is sodium hydroxide which has precipitated in the ether during the
reaction of sodium with water:
2H2O(l) + 2Na(s) → 2NaOH(s) + H2(g)
Students must be made aware that ether escapes very readily from the propette, which must
therefore not be pointed upwards.
Most of the water may usually be removed with anhydrous magnesium or calcium sulphate, or
alternatives. At the drying step in Part 1 students should be advised to keep the container of
magnesium sulphate closed at all times, otherwise the solid will absorb moisture from the
atmosphere (therefore rendering it useless as a drying agent).
The unreacted sodium in the well containing the ether must be destroyed by adding a little cold
ethanol or methanol. Once all the sodium has reacted, the remaining solution can be washed down
the sink.
Part 2.1: Lucas’ Test to Distinguish Between 10, 20 and 30 Alcohols
If the room temperature is below 250C, students should be advised to keep the propettes
containing tertiary butyl alcohol in a little warm water. Tertiary butyl alcohol freezes in the
temperature range 24 - 25.50C, and if not kept at a temperature above freezing it will form crystals
that block the stem and bulb of the propette.
The temperature of the water must be maintained at 27 to 280C by mixing different volumes of
hot and cold water as required. It is suggested that the water temperature be raised to 280C before
the comboplate is placed in the bath. During the 5 minute reaction period, the temperature should
decrease by one degree only to 270C. This will prevent the need to add hot water to the bath while
the comboplate is being warmed. The volume of water used in the water bath should be kept low,
and the comboplate must not be pushed into the water as it may become immersed in the bath.
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When Lucas’ Reagent is added to the alcohols listed n-butyl alcohol does not react, secondary
butyl alcohol does not react immediately but will form the alkyl chloride after warming for about 5
minutes at 27 to 280C, and tertiary butyl alcohol reacts almost immediately to produce a turbid
solution which becomes more turbid while the comboplate is on the water bath. In order for
students to be aware of the different reaction rates, the test solutions are observed immediately
after adding Lucas’ Reagent to the alcohols, as well as after being kept at 27 to 280C. The
conclusion can then be made that primary alcohols do not react with Lucas’ Reagent, secondary
alcohols react slowly with heating and tertiary alcohols react quickly even in the cold.
The colour of the turbid solutions produced after incubation is almost the same as the colour of
the opaque plastic of the Organic comboplate. This may make it difficult for students to observe
the turbidity in the well to which the secondary butyl alcohol was added, since the volume of alkyl
chloride produced with the secondary alcohol is small (much less than that for tertiary butyl
alcohol).
In this case, students should be advised to hold the comboplate up to the light and observe the
wells from both above and the underside of the comboplate.
Lucas’ Test has an extension which allows one to distinguish between secondary and tertiary
alcohols after the initial reaction with the hydrochloric acid-zinc chloride reagent has been used to
exclude primary alcohols. In Part 2.1, the procedure requires that both secondary and tertiary butyl
alcohol are mixed with 11 M hydrochloric acid. The secondary alcohol does not react and the
solution remains clear.
The tertiary butyl alcohol reacts within 15 seconds to produce a suspension of tertiary butyl
chloride, which gives the solution a milky, white appearance:
Since the volumes of alcohols and Lucas’ Reagent used are small, the waste solutions may be
decanted onto a thick pile of paper towel. The organic residue evaporates quicky in the open air
and the towel can then be thrown away. If this is not a possible option, then the chlorinated organic
waste much be discarded into a specially designated waste container to ensure that it is not mixed
with non-chlorinated waste.
Part 2.2:
The Chromic Anhydride Test to Distinguish 10 and 20 Alcohols from 30 Alcohols
The oxidation of n-butyl alcohol and secondary butyl alcohol with Chromic Anhydride Reagent is
instantaneous. The reactions are accompanied by vigorous bubbling, after which an opaque bluegreen coloured solution is produced. The solution formed with the secondary butyl alcohol also has
a yellow tinge. The opaque solutions are visible within 3 to 5 seconds of adding the Chromic
Anhydride Reagent to the alcohols, after which the solution becomes clear and a dark blue
precipitate settles at the bottom of each well. It is this precipitate which actually produces the initial
opaque blue-green solution, as the solid does not settle until later.
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Tertiary butyl alcohol does not react and there is no change in the deep orange colour of the
Chromic Anhydride Reagent.
Students need to be careful when adding the reagent to the primary and secondary alcohols, as
the bubbling is so vigorous that the solutions may spill out of the wells. The comboplate should not
be lifted for observation until after the reaction has stopped.
The Chromic Anhydride Reagent is a strong oxidant and may oxidise aldehydes, enols, phenols
and other substances that occur as impurities in the alcohol samples. These reactions are slower
than the oxidation of the alcohols, and a time limit of 5 seconds has been set in which observations
can be made.
Any other oxidations taking place after 5 seconds are to be disregarded. The precipitation of the
blue-green chromium sulphate solid, however, takes place after 5 seconds. Students should be
made aware that the settling of the precipitate is not due to a chemical reaction, but is a change in
the appearance of the solutions following the initial oxidation reactions. The changes should still be
noted shortly after the oxidations of the primary and secondary alcohols have occurred, usually
within 30 seconds.
As in Part 2.1, the volumes of organic waste produced are small and can be decanted onto a
thick pile of paper towel to allow evaporation to take place in the open air. Alternatively, the waste
can be discarded into a container before rinsing the comboplate with water.
The Iodoform Test for Methyl Ketones
If it is suspected that an alcohol may be secondary or tertiary in structure, then the iodoform test
may also be carried out on the alcohol provided that other tests have been done to exclude the
presence of an aldehyde and ketone. The molecules of secondary alcohols may contain a methyl
group adjacent to the -OH bearing carbon atom. Under the conditions of the iodoform test the
secondary alcohol is oxidised to a methyl ketone, which in turn produces iodoform - CHI3. (See
Chapter 1: Experiment 3 for details.) Tertiary alcohols do not react with the sodium hypoiodite
solution and iodoform is not produced.
4. Cautions
Ensure that all students are aware of the following safety advice:
Never point or hold the tip of a propette containing an organic liquid facing upwards. Unlike
aqueous solutions, organic liquids escape readily from the propette stem and can suddenly squirt
from the propette if it is held with the tip pointing upwards. Diethyl ether is a special example.
Avoid dropping any of the organic reagents onto the skin. If any of these chemicals is spilled
onto the skin, wash the affected area with copious amounts of water.
Avoid dropping the Lucas’ and Chromic Anhydride Reagents onto the skin as they are highly
corrosive. (Lucas’ Reagent is prepared with concentrated hydrochloric acid, while the Chromium
Anhydride Reagent contains sulphuric acid.) If spillage occurs, rinse the affected area with copious
amounts of water.
Do not allow any flames to be brought near the organic reagents used in this experiment. They
are all flammable. Do not attempt to test for hydrogen gas (Part 1) using a flame as the ethanol
and comboplate will catch alight.
Do not inhale the vapours of the alcohols, acetone and ether as they are poisonous and may
cause respiratory damage.
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11 M hydrochloric acid is extremely corrosive. Take care when handling the acid. Never point a
propette or a syringe containing acid upwards. A momentary lapse of concentration can result in a
nasty accident. If any acid is squirted into the eye, immediately rinse the eye out under running
water. Always have a dilute solution of sodium hydrogencarbonate (household baking soda), or
milk close by to apply to the injury. These substances will help neutralise the acid in the eye. The
patient should be referred to a doctor.
Do not dispose of any unreacted sodium in the sink. Ensure that all the sodium has reacted with
cold ethanol before rinsing the comboplate.
Dispose of all other organic waste in the waste containers provided, and not down the drain!
MODEL ANSWERS
Part 1: The Sodium Test: Using Metallic Sodium to Distinguish Alcohols from Ethers
Q1. Prepare a table like the one below and record your observations.
A1. TABLE 1: TESTING OF ALCOHOL AND ETHER WITH METALLIC SODIUM
COMPOUND
OBSERVATIONS
Ethanol
The sodium reacts rapidly w ith the alcohol, as show n by small
bubbles quickly being given off. The piece of sodium "darts"
around the w ell on the surface of the ethanol and a fiz z ing sound is
heard. After about 3 to 5 minutes, all of the sodium has reacted.
Diethyl Ether
The piece of sodium sinks to the bottom of the w ell and small
bubbles can be seen rising to the surface of the ether. There is no
fiz z ing sound as the rate of reaction is slow er than for ethanol. The
reaction slow s dow n considerably w ith time until only a few
bubbles are given off at intervals. After about 8 to 10 minutes, the
reaction has stopped. The piece of sodium has not decreased much
in siz e and there are granules of a w hite solid at the bottom of the
w ell. (Note that if the teacher has obtained a fresh, pure sample of
ether w hich has not been contaminated w ith w ater, there should not
be any reaction w ith sodium .)
Q2. How can one distinguish between an alcohol and an ether using the sodium test?
A2. Sodium reacts rapidly and vigorously with an alcohol. Hydrogen gas is formed and the
sodium is used up in the reaction. Sodium reacts sluggishly with (the water in) an
ether. Very little hydrogen gas is generated. The sodium is not all used up in the
reaction and a precipitate is produced.
Q3. Write down the equation for the reaction of ethanol with sodium.
A3. 2CH3CH2OH(l) + 2Na(s) → 2CH3CH2ONa(l) + H2(g)
Q4. Write down the reaction of water with sodium metal.
Q4. 2H2O(l) + 2Na(s) → 2NaOH(aq) + H2(g)
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Q5. The reaction of anhydrous magnesium sulphate with water is as follows:
MgSO4(s) + 7H2O(l) → MgSO4.7H2O(s)
i.
ii.
What is the function of the anhydrous magnesium sulphate in the sodium test?
Why is it important to add anhydrous magnesium sulphate to the alcohol and ether prior to
reaction with sodium metal? (Hint: look at your answer to question 1 above.)
iii.
Ether does not react with water. Explain why you observed a reaction when sodium was
added to the diethyl ether. (Hint: look at your answer to question 1 above.)
iv.
Use your answer to iii. to explain why there was a difference in the rates of reaction when
sodium was added to ethanol and diethyl ether.
v.
What is the name of the precipitate that settled at the bottom of the well containing the
ether?
A5. i.
It is a dehydrating agent. (It removes water from the alcohol and ether by forming
a hydrate.)
ii.
The alcohol and ether may contain water as an impurity. Since water reacts with
sodium metal to produce hydrogen, it must be removed from the alcohol and
ether so that the results are interpreted correctly.
iii. Water was present as an impurity in the diethyl ether and was not all removed
with the magnesium sulphate. The small concentration of water molecules in the
ether (and not the ether) reacted with the sodium metal.
iv.
A large concentration of ethanol molecules reacted with all of the sodium metal
atoms, causing the rapid evolution of hydrogen gas. In contrast, there was only a
small concentration of water molecules in the ether. The reaction of water with
the sodium was therefore slower than that with ethanol. All the water molecules
were used up before all the sodium atoms reacted, resulting in cessation of
hydrogen production.
v.
Sodium hydroxide (not soluble in ether).
Part 2.1
Lucas’ Test for Differentiating Between Primary, Secondary and Tertiary Alcohols
Q1. Prepare a table like the one below and record your results.
A1. TABLE 2: REACTION OF PRIMARY, SECONDARY AND TERTIARY ALCOHOLS WITH
LUCAS’ REAGENT
ALC OH OL
IMMED IATE R EAC TION WITH
LU C AS' R EAGEN T
OB SER VATION S AFTER 5
MIN U TES AT 27 TO 280C
n-butyl alcohol
N o reaction. The solution remains
clear.
N o reaction. The solution is still
clear.
secondary butyl
alcohol
N o reaction. The solution is clear.
A (slow ) reaction has occurred, as
show n by a slight turbidity in the
w ell.
terti ary butyl
alcohol
A reaction occurs w ithin a few
seconds of adding Lucas' R eagent
and the w ell contents become
turbid.
The reaction w hich started at room
temperature has continued
because the turbidity in the w ell
has increased.
*ethanol
N o reaction. Solution is clear.
N o reaction. Solution is still clear.
* The results for ethanol as an alternative primary alcohol have also been provided.
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Q2. Prepare a table like the one below and record your observations.
A2. TABLE 3: REACTION OF SECONDARY AND TERTIARY ALCOHOLS WITH
CONCENTRATED HYDROCHLORIC ACID
ALC OH OL
OB SER VATION S
secondary butyl alcohol
N o reaction. The solution remains clear.
terti ary butyl alcohol
A reaction occurs w ithin 10 to 15 seconds to
produce a milky, w hite mixture in the w ell.
Q3. Classify the alcohols used in this experiment as primary, secondary or tertiary.
A3. n-butyl alcohol
primary
secondary butyl alcohol
secondary
tertiary butyl alcohol
tertiary
*ethanol
primary
(Note: if students find it difficult to classify certain alcohols as primary, secondary or
tertiary, let them draw the structures of each alcohol so that they can see the number
of carbon substituents bonded to the OH-bearing carbon atom.)
Q4. Use your answer to question 1 and the results in Table 2 to explain how mixing an alcohol of
unknown structure with Lucas’ Reagent can help one to determine whether it is primary,
secondary or tertiary.
A4. If an alcohol of unknown structure is mixed with Lucas’ Reagent and the solution
remains clear after 5 minutes at 27 - 280 C, the alcohol is primary in structure. If the
solution becomes turbid after 5 minutes at 27 to 280 C, the alcohol is secondary in
structure. If the solution becomes turbid immediately (i.e in the cold), the alcohol is
tertiary in structure.
Q5. Write down an equation for the reaction of tertiary butyl alcohol with concentrated
hydrochloric acid.
A5.
Q6. Explain why the solution became milky white in appearance after adding 11 M HCl(aq) to the
tertiary butyl alcohol.
A6. Tertiary butyl chloride formed as a result of the reaction between the alcohol and
concentrated hydrochloric acid. The liquid droplets remained in suspension, making
the solution appear milky white.
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Q7. A student is given two alcohols with which to perform Lucas’ Test. These are cyclohexanol
and 2-methyl-2-propanol. The bottles containing each alcohol are labelled, but the propettes
used are not. The student accidentally muddles the propettes. When drops of each alcohol
are mixed with Lucas’ Reagent and kept at 270 C, both solutions become turbid. How can the
student find out which alcohol has been placed in which propette?
A7. Students should give answers similar to the following:
The student should mix each alcohol with 11 M hydrochloric acid in the comboplate
and observe the reaction mixtures. If the alcohol from a particular propette reacts with
the HCll(aq) to form a cloudy solution almost immediately, then that propette contains
the tertiary alcohol. If the alcohol from a particular propette shows no reaction with
the acid (the solution remains clear in the comboplate), then that propette contains a
secondary alcohol. Cyclohexanol is a secondary alcohol and did not react with
HCll(aq). 2-methyl-2-propanol is tertiary butyl alcohol and formed a cloudy mixture
with HCll(aq).
Part 2.2:
The Chromic Anhydride Test: Distinguishing Tertiary Alcohols from Primary and
Secondary Alcohols Using Chromic Anhydride Reagent (Jones= Reagent)
Q1. Prepare a table like the one below and record your observations.
A1. TABLE 4: REACTION OF PRIMARY, SECONDARY AND TERTIARY ALCOHOLS WITH
CHROMIC ANHYDRIDE REAGENT
ALCOHOL
REACTION WITHIN 3 TO 5 SECONDS OF
ADDING CHROMIC ANHYDRIDE REAGENT
CHANGE IN APPEARANCE OF WELL
CONTENTS AFTER INITIAL REACTION
n-butyl alcohol
Reacts vigorously w ith bubbling w ithin
about 4 seconds. The test solution becomes
an opaque, green/blue colour.
The solution loses turbidity (becomes
clear w ith a blue tint). A dark blue/green
precipitate settles at the bottom of the
w ell.
secondary butyl
alcohol
Reacts vigorously w ithin about 4 seconds.
There is bubbling and an opaque
yellow /green solution is formed.
The solution becomes clear w ith a slight
green tint. A dark blue/green precipitate
settles at the bottom of the w ell.
tertiary butyl
alcohol
No reaction. The solution remains deep
orange in colour.
No change. The solution is still deep
orange in colour.
*ethanol
Reacts vigorously w ith bubbling w ithin the 3
to 5 second time span. An opaque
blue/green solution results.
The turbidity clears and a dark
blue/green precipitate settles at the
bottom of the w ell.
* The results for ethanol as an alternative primary alcohol have been provided.
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Q2. How does the Chromic Anhydride Test distinguish primary and secondary alcohols from
tertiary alcohols?
A2. Primary and secondary alcohols react with the Chromic Anhydride Reagent within 5
seconds to form opaque, blue-green mixtures. Tertiary alcohols do not react and the
deep orange colour of the Chromic Anhydride Reagent remains.
Q3i. Write down an equation for the reaction of n-butyl alcohol with Chromic Anhydride Reagent.
(The reagent = CrO3 + H2SO4)
A3i.
Q3ii. What type of compound is formed in the reaction between a primary alcohol and Chromic
Anhydride Reagent? (Hint: use the answer to 3i. above.)
A3ii. A carboxylic acid.
Q4i. Write down an equation for the reaction of secondary butyl alcohol with Chromic Anhydride
Reagent.
A4i.
Q4ii. What type of compound is formed in the reaction between a secondary alcohol and the
Chromic Anhydride Reagent? (Hint: use the answer to A4i. above.)
A4ii. A ketone.
Q5. During the mixing of a primary or secondary alcohol with Chromic Anhydride Reagent, the
following change occurs (equation unbalanced):
CrO3(aq) + H2SO4(aq) → Cr2(SO4)3(s)
Q5i. What kind of reaction has taken place? Explain by examining the partial reaction equation
given.
A5i. A redox reaction. The oxidation number of chromium has changed from +6 in the red
CrO3 to +3 in the blue-green Cr2(SO4)3. Chromium has therefore gained electrons and
has been reduced. (In each reaction, the primary or secondary alcohol is oxidised.)
Q5ii. What is the blue-green precipitate that was observed at the bottom of each well in which a
reaction occurred?
A5ii. Chromium sulphate, Cr2(SO4)3(s)
Q5iii. Why did the test solutions containing primary and secondary alcohols, initially become bluegreen in colour?
A5iii. When the redox reaction occurred in each well, chromium sulphate was formed. It
remained in suspension for a few seconds before precipitating. The solid particles
made the test solutions appear blue-green in colour.
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Q6. Draw the chart below and complete it by filling in the missing words related to the
experiments carried out in this worksheet.
A6.
k
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FUNCTIONAL GROUP ANALYSIS
Tests for Unsaturated Hydrocarbon Compounds
TEACHER'S GUIDE AND MODEL ANSWERS
Introduction
Alkanes are hydrocarbon compounds whose molecules contain carbon-carbon single bonds. All
have the general formula CnH2n+2 where n is any integer. They are often referred to as saturated
hydrocarbons because their molecules contain the maximum possible number of hydrogen atoms
per carbon atom, and because they consist only of carbon and hydrogen.
Alkenes are hydrocarbon compounds whose molecules contain one or more carbon-carbon
double bonds. They are often referred to as being unsaturated because the presence of a double
bond means that they lack hydrogen atoms as compared with related alkanes. The general
formula for alkenes is CnH2n.
The molecules of alkyne compounds contain one or more carbon-carbon triple bonds. Like
alkenes, they are often termed as unsaturated hydrocarbons.
Tests to Distinguish Alkanes from Alkenes and Alkynes
Alkanes are chemically inert to most reagents mainly because all the electrons in an alkane
molecule are involved in carbon-carbon and carbon-hydrogen bonding. (Alkanes do however react
with oxygen under appropriate conditions.)
In contrast, alkenes and alkynes react with a number of other compounds. This is because the
carbon-carbon double and triple bonds in alkene and alkyne molecules have greater electron
density than a carbon-carbon single bond. These electron rich sites can donate a pair of electrons
to an electrophile (an electron-poor or electron-seeking reagent). This difference in reactivity
between saturated and unsaturated hydrocarbon compounds is used as the chemical basis for
differentiating between the two.
One test involves the Addition of Bromine to Alkenes and Alkynes. When a dilute solution of
bromine in water (or carbon tetrachloride) is mixed with an alkane, no reaction occurs due to the
inertness of saturated hydrocarbon compounds to bromine. The brown colour of the bromine
remains. Mixing the same bromine solution with an alkene or alkyne results in the rapid
disappearance of the brown colour of the bromine, until a colourless mixture is obtained.
Another test to distinguish between alkenes and alkynes is based on their reaction with cold, dilute
potassium permanganate solution - called Baeyer's Test. Potassium permanganate is an oxidant.
When mixed with an alkene, it oxidises the double bond in alkene molecules to yield 1,2- diols i.e.
an -OH group is added to each of the two alkene carbon atoms. Such compounds are also called
glycols.
Alkynes are also oxidised by potassium permanganate to yield carboxylic acids. The purple
permanganate (MnO4-) ion is reduced to brown manganese(IV) oxide, as seen by the change in
colour of the solution from purple to brown when mixed with an alkene or alkyne.
1. Chemicals
All the chemicals required are found in the requirements table preceding Parts 1 and 2 of the
experiment.
The aqueous bromine solution (commonly called bromine water) has a high vapour pressure
and should be stored in a cool place to minimize loss of bromine vapour, which will cause a
decrease in the concentration of bromine molecules in solution. The solution is also decomposed
in the presence of light and should be kept in an amber bottle in a dark place.
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A solution of bromine in carbon tetrachloride may be used as a substitute for the aqueous
bromine solution. However, due to its toxicity, the organic solution will no longer be readily available
from chemical suppliers and will be very expensive to obtain commercially. Bromine is more
soluble in carbon tetrachloride than in water and therefore the carbon tetrachloride solution does
not lose bromine as readily as the aqueous solution. It also deteriorates with heat and light and
should be stored as explained for the aqueous solution.
The 0.1% potassium permanganate solution can be prepared by dissolving 0.100 g of solid
potassium permanganate in a little water in a 100 ml volumetric flask, and then making the solution
up to 100 ml. It may be better to prepare a stock solution of 1% potassium permanganate and
then dilute this to 0.1% as required. Potassium permanganate solutions are decomposed by heat
and light. The decomposition reaction produces manganese dioxide which catalyses further
decomposition of the permanganate. For this reason the solutions must be protected adequately
from heat and light, and should be stored in clean containers because dust particles can also
promote degradation of the solution.
A 0.1% solution of potassium permanganate in acetone can be used in place of the aqueous
solution, but the acetone based solutions deteriorate rapidly. This option may be considered if the
entire solution is used on the day of experiment.
If the teacher would like the students to test unknown substances, then other saturated
(alkanes) and unsaturated (alkenes and alkynes) hydrocarbon compounds may be used. It should
be remembered that other classes of organic compounds may also rapidly decolourise both the
bromine and permanganate solutions.
2. Apparatus
The propettes and microspatulas are components of the RADMASTE Microchemistry kits. An
ordinary comboplate may be used in this experiment if cyclohexane and cyclohexene are tested.
However, the Organic comboplate is manufactured from a material which is resistant to organic
solvents and is recommended for this experiment, because alkanes and alkenes other than
cyclohexane and cyclohexene may dissolve the plastic of the conventional comboplate. Gloves
should be worn throughout the experiment to protect the skin from the corrosive nature of the
chemicals used. The teacher must carry out a risk assessment to decide whether protective
glasses should be worn by the students. The teacher must also ensure that a waste container is
provided into which all organic waste can be disposed.
3. Hints
Students should be advised to label their propettes containing cyclohexane and cyclohexene as
these are both colourless and can easily be confused.
Two or three students can share propettes of the alkane, alkene, bromine solution and
potassium permanganate solution because only a few drops of each is required.
Cyclohexene (and solutions of bromine in carbon tetrachloride), tend to flow easily from the tip
of a propette. Propettes containing these liquids should never be pointed upwards.
Bromine solutions are decomposed by heat and light. Light also initiates the substitution
reaction between bromine and an alkane, where a hydrogen atom on the alkane molecule is
substituted with a bromine atom. Hydrogen bromide (a gas) is produced.
The bromoalkane product is colourless, leading the student to the conclusion that the alkane
has reacted with the bromine and is therefore an unsaturated compound. To avoid such situations,
the bromine solutions should be protected from light at all times. Students should be advised to
share propettes of the bromine solution so that the bottle containing the solution can be stored
away in a cool, dark place for the remainder of the experiment.
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Note that the bromine solution in the propette may also fade during the activity and it may be
necessary to use a room with subdued lighting while carrying out the experiment. This can be
achieved by switching off overhead lights or blocking direct sunlight from external sources, such as
closing curtains or dropping blinds at the windows.
The student worksheet suggests that the cyclohexane-bromine and cyclohexene-bromine
mixtures are left in the comboplate until after Baeyer’s Test. The objective of leaving the mixtures
for an extended period of time is to show the effect of light on the cyclohexane-bromine mixture.
The latter is colourless at the end of the lesson because of the substitution of cyclohexane by
bromine. There are some questions after Part 2 which intend to help make the substitution reaction
known to the student, and to show that this photo-reaction is not an indication of the presence of
an unsaturated hydrocarbon compound.
When an aqueous solution of bromine is added to both the cyclohexane and cyclohexene, two
layers are observed in each well because non-polar organic compounds are immiscible with water.
Stirring the contents vigorously disperses the two layers into a suspension.
Bromine does not react with cyclohexane and as a result, the contents of the well containing
cyclohexane appear brown in colour. After a while, two layers separate out again and the lower
layer remains brown in colour. If the comboplate is left to stand for longer than about 3 minutes,
the brown colour of the bromine begins to fade as a result of the substitution reaction explained
previously. It is therefore important for students to make observations and record results soon after
stirring of the well contents.
During the stirring of the bromine and cyclohexene layers, the bromine reacts with the alkene
and the brown colour fades as the colourless addition product (1,2-dibromocyclohexane) is formed.
With standing, two layers may still be evident but the brown colour of the lower layer continues to
fade as the bromine is used up. After 1 to 3 minutes, the bromine is either pale brown or
colourless. The latter is the usual observation.
The time taken for the bromine to become colourless is also dependent on the concentration of
aqueous bromine test solution used. If the bromine solution is pale brown (low concentration of Br2
molecules) at the start of the experiment, less time will be needed for the brown colour to
disappear than if the bromine solution is dark brown (high concentration of Br2 molecules).
If a solution of bromine in carbon tetrachloride is used instead of an aqueous bromine solution,
the student should notice that when the Br2/CCl4(l) is stirred with the cyclohexane, the entire
solution appears brown in colour. If water is present in the alkane to be tested, two layers will be
evident. The lower layer is brown in colour. As with the aqueous bromine solution, the brown colour
also fades with time because a substitution reaction slowly occurs between the bromine and
alkane.
Adding a solution of bromine in carbon tetrachloride to cyclohexene results in immediate
decolourisation of the bromine. (As a drop of bromine solution enters the cyclohexene, the brown
colour disappears.) There is therefore no agitation required because they are miscible. Note that
the model answers contain results obtained with Br2(aq) only.
In Baeyer’s Test for unsaturation, the test is only positive if an excess of the potassium
permanganate solution turns brown. For this reason the volume of potassium permanganate used
is double that of the alkane or alkene to be tested.
When cold 0.1% potassium permanganate solution is added to cyclohexane, two layers are
seen with the purple colour in the lower aqueous layer. Stirring the contents of the well disperses
the two layers, but these separate again. The lower layer remains purple as there is no reaction
between the permanganate and alkane.
The permanganate changes colour from purple to brown within a few seconds of adding the
solution to cyclohexene (seen as a dark brown, lower layer in the well). Stirring is not essential but
often helps to show the colour change better.
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Other alkanes and alkenes, as well as alkynes, can be tested with the bromine and potassium
permanganate solutions. The Organic comboplate should be used if the effect of such compounds
on the plastic of an ordinary comboplate is not known. It is important to apply both tests to each
compound because some alkenes react slowly under the conditions of the bromine test.
The mixtures in both parts of the experiment are easily discarded into a waste container and the
comboplate can be rinsed clean with water (assuming the use of aqueous bromine and
permanganate solutions).
4. Cautions
Ensure that all students are aware of the following safety advice:
Never point or hold the tip of a propette containing an organic liquid facing upwards. Unlike
aqueous solutions, organic liquids escape readily from the propette stem and can suddenly squirt
from the propette if it is held with the tip pointing upwards. Cyclohexene and the solution of
bromine in carbon tetrachloride tend to flow easily from the propette tip.
Avoid dropping any of the organic reagents onto the skin. If any of these chemicals is spilled
onto the skin, wash the affected area with copious amounts of water.
Do not allow any flames to be brought near the organic reagents used in this experiment. They
are all flammable (with the exception of carbon tetrachloride if it is used).
Do not inhale the vapours of the cyclohexane, cyclohexene and bromine solutions as they are
poisonous and may cause respiratory damage. Keep the bottles of all organic liquids closed so as
to minimise vaporisation. Note that carbon tetrachloride is extremely dangerous.
The bromine solutions are corrosive. Avoid dropping the bromine solution onto skin or fabrics. If
contact is made with the skin, wash the affected area with copious amounts of water. Keep the
bottle containing the bromine solution in a cool, dark place as it decomposes in the presence of
light and heat. (Bromine has a high vapour pressure: the vapour above the solution will expand
with an increase in temperature.)
Potassium permanganate solutions are poisonous. Rinse your hands thoroughly if contact is
made with the skin.
Dispose of all organic waste in the waste container provided, and not down the drain!
MODEL ANSWERS
Part 1:
The Bromine Test for Unsaturated Hydrocarbon Compounds
Q1. What happens when the bromine solution is first added to each compound?
A1. Two layers can be seen in each well when the Br2(aq) is added to the cyclohexane and
cyclohexene. The lower, aqueous layer in each well is brown in colour.
Q2. What do you notice when the bromine solution is stirred with each compound?
A2. The bromine remains brown in the well containing the cyclohexane, even with
vigorous stirring. The aqueous solution of bromine begins to lose colour during
stirring with cyclohexene and appears pale brown in colour.
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Q3. Prepare a table like Table 1 below. Record your observations for wells A1 and A2.
A3. TABLE 1: TESTING OF SATURATED AND UNSATURATED COMPOUNDS WITH
BROMINE SOLUTION
COMPOUND
Cyclohexane
Cyclohexene
APPEARANCE OF WELL
CONTENTS AFTER 1 MINUTE
REACTION WITH
BROMINE (Br2 (aq))?
There are tw o layers. The
bromine in the low er layer has
retained its brow n colour.
No
There are tw o layers. The
low er layer is pale brow n or
colourless.
Yes
Q4. Write down the structure of cyclohexane. Is the compound saturated or unsaturated? Explain
your answer.
cyclohexane, C6H12
A4.
The cyclohexane is saturated. There are only carbon-carbon and carbon-hydrogen
single bonds in the molecule. One molecule of the compound has the maximum
number of hydrogen atoms per carbon atoms.
Q5. Write down the structure of cyclohexene. Explain whether the compound is saturated or
unsaturated.
cyclohexene, C6H10
A5.
The cyclohexene is unsaturated. A molecule of the compound contains one carboncarbon double bond. The double bond replaces two possible carbon-hydrogen bonds
i.e cyclohexene molecules have two hydrogen atoms less than cyclohexane
molecules.
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Q6. Based on your answers to questions 4 and 5 above and on the results of the bromine test in
your table, describe how mixing a bromine solution with a hydrocarbon compound of
unknown structure can help one to tell whether it is saturated or unsaturated.
A6. Saturated hydrocarbon compounds like cyclohexane do not react with bromine
solution so that the bromine retains its brown colour. Unsaturated hydrocarbon
compounds like cyclohexene react with bromine solution so that the bromine is
decolourised. If a hydrocarbon compound of unknown structure is mixed with a
bromine solution and the brown colour disappears, the compound is unsaturated. If
the brown colour of bromine remains, the compound is saturated.
Q7. Write down a balanced equation for the reaction of bromine with cyclohexene. Is the product
of the reaction saturated or unsaturated? Explain your answer by describing what type of
reaction is involved.
A7.
H Br
H
H
H
H
H
H
H
H
H
+
Br2
H
Br
H
H
H
H
H
H
H
H
cyclohexene, C6H10
H
1,2-dibromocyclohexane, C6H10Br2
The product, 1,2-dibromocyclohexane, is saturated. An addition reaction has occurred
i.e the bromine molecule has added across the double bond of the cyclohexene
molecule.
Q8. Explain why the brown colour of bromine disappears when it is mixed with cyclohexene.
(Hint: examine the equation written for question 7 above.)
A8. The bromine molecules are used up in the addition reaction. The product of the
addition reaction (1,2-dibromocyclohexane) is colourless. The decrease in
concentration of brown bromine molecules and the increase in the concentration of
the colourless product molecules causes the mixture to fade, and eventually become
colourless as all the bromine molecules are consumed.
Q9. The bromine addition test does not distinguish between a double or triple bond. The bromine
always adds across the multiple bond to form a trans addition product/s. Write down the
equation/s for the reaction/s involved when bromine is mixed with ethyne (acetylene).
A9.
Ethyne
1,2-dibromoethene
tetrabromoethane
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Part 2:
Baeyer's Test for Unsaturation: Reaction of Cold, Dilute Potassium
Permanganate solution with Unsaturated Hydrocarbon Compounds
Q1. What do you notice in each well immediately after adding the potassium permanganate
solution?
A1. There are two layers in each well. The lower aqueous layer is purple in the well
containing the cyclohexane, but the lower layer in the well with the cyclohexene is
already changing colour to brown.
Q2. Describe the appearance of the cyclohexane-permanganate mixture in well A4 after stirring.
A2. There are still two layers. No colour change has occurred in the lower layer i.e it has
remained purple.
Q3. What has happened to the cyclohexene-permanganate mixture after stirring?
A3. The lower layer has changed in colour from purple to dark brown. (The permanganate
actually decolourises immediately but the colour change is often seen better with
stirring.)
Q4. How can a dilute potassium permanganate solution help the chemist to distinguish between
saturated and unsaturated hydrocarbon compounds?
A4. Unsaturated hydrocarbon compounds react with potassium permanganate, causing
the solution to change colour from purple to brown. Saturated hydrocarbon
compounds do not react with the permanganate and it is therefore not decolourised.
Q5. If you have been able to leave the bromine-cyclohexane and bromine-cyclohexene mixtures
from Part 1 in the comboplate® until the end of the lesson, answer the questions which follow:
i.
What do you notice about the appearance of the bromine-cyclohexane mixture?
ii.
If cyclohexane is a saturated hydrocarbon compound, why has the brown colour faded? Use
the following reaction equation to explain your answer.
iii.
A5.
Name the kind of reaction shown in the given equation.
i.
It is colourless (or very pale brown in colour).
ii.
One hydrogen atom on the cyclohexane molecule has been substituted with one
bromine atom from the bromine molecule. The bromocyclohexane product is
colourless and hydrogen bromide is a gas. The increase in concentration of the
colourless product and the simultaneous decrease in the concentration of brown
bromine molecules causes the mixture to decolourise. The reaction is initiated
by light.
iii. A substitution reaction. (One atom is substituted with another kind of atom.)
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Q6. The figure below shows a scheme for testing hydrocarbon compounds to determine whether
they are saturated or unsaturated. Draw the scheme and complete it by filling in the missing
words related to the experiments completed in the worksheets of Parts 1 and 2.
A6.
Note: that even if the results of both the bromine and Baeyer's tests are positive, there remains an
element of doubt.
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INVESTIGATING INTERMOLECULAR FORCES IN ORGANIC COMPOUNDS BY
MEASURING THE BOILING POINT
TEACHER'S GUIDE AND MODEL ANSWERS
Introduction
The boiling of organic compounds involves the weakening or the breaking of intermolecular forces
(or Van der Waal's forces) between molecules. The temperature of boiling of a substance is
determined by the kinds and magnitude of intermolecular forces between molecules. There are two
types of intermolecular forces:
London forces - these are weak instantaneous forces due to distortion of electron clouds.
Dipole-dipole forces - these are a result of permanent polarity in molecules leading to stronger
attractive forces.
Hydrogen bonding is a special type of dipole-dipole force. It occurs when a hydrogen atom is
bonded to a highly electronegative F, O or N atom. Because it is stronger than most other
intermolecular forces, its presence can significantly increase the boiling point of a substance.
1. Chemicals
All the chemicals required are found in the requirements section preceding the experiment. In
this investigation, ethanol and 1-butanol will be used as examples.
2. Apparatus
The vials, propettes, microspatula, microburner, microretort stand, are all components of the
RADMASTE Advanced Microchemistry Kit.
The organic comboplate is manufactured from plastic which is resistant to organic solvents and
must be used in this experiment, because the conventional comboplate can be affected by the
organic reagents.
3. Hints
You may want to underpin this experiment with a lesson on intermolecular forces (hydrogen
bonding, dipole-dipole forces, and induced-dipole forces).
Two or three students can share one propette to dispense the 1-butanol because only five drops
are used per student.
When heating the fusion tube to evaporate 1-butanol, be very cautious that you don't melt the
retort stand. If you have a suitable metal replacement, use that instead. You can even use a
wooden peg to hold it over the flame.
Don't leave the glass vial over the hot flame continuously for more than a minute or two without
moving it around to change the position of the flame. If the flame is concentrated to one spot on
the vial for too long this can cause the glass to crack and the vial to break. Always extinguish the
flame of the microburner after a boiling point determination and let the glass cool. Do not leave the
burner under the vial.
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4. Cautions
(Information from Biggs, P. The Physical and Theoretical Chemistry Laboratory: Chemical and
Other Safety Information. Oxford University http://physchem.ox.ac.uk/MSDS/#MSDS).
Methylated spirits is a mixture of ethanol (95%) and methanol (5%). Methanol is toxic by
inhalation, ingestion or skin absorption. Use gloves in a well-ventilated room. Ethanol can cause
nausea, vomiting and inebriation upon ingestion, as well as skin and eye irritation. Work in a wellventilated room with gloves.
1-butanol: Is a harmful substance when inhaled, ingested or when absorped into skin. It is an
irritant and a narcotic which is a CNS depressant. Work in a well-ventilated place, with safety
glasses and gloves. Keep it away from any sources of ignition.
MODEL ANSWERS TO QUESTIONS
Q1. Report the observed boiling points for ethanol and 1-butanol.
Q2: Why is it important that the thermometer and fusion tube do not touch the bottom of the
glass vial.
A2: The glass vial is in direct contact with the flame, and thus its temperature is likely to
be higher than the oil that it contains. Convection in the oil ensures a fairly uniform
temperature for objects immersed in it.
Q3: Why do bubbles emerge from the capillary tube ? Explain why the boiling point is recorded
when they stop.
A3: The vapour pressure of a liquid increases with increasing temperature. When the
vapour pressure is equal to the atmospheric pressure, boiling occurs (and bubbles
emerge). Boiling stops (escape of bubbles stops) when the temperature of the oil falls
below the boiling point. Some bubbles may escape from the tube at temperatures well
below boiling point. Initially this is due to expansion of air in the capillary tube as the
temperature rises.
Q4: Why is it necessary to record the atmospheric pressure ?
A4: Since boiling occurs when the vapour pressure is equal to the atmospheric pressure,
the boiling point depends on the atmospheric pressure. Hence, atmospheric pressure
should always be recorded when reporting a boiling point. Tables of reference books
usually give boiling points at 1 atm/760 mm pressure.
Q5: Draw out the line drawings for 1-butanol and ethanol.
A5:
Q6: What kinds of intermolecular forces occur between the 1-butanol molecules and between the
ethanol molecules.
A6: Hydrogen bonds, dipole induced dipole bonds and Van der Waal's dispersion forces.
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Q7. Explain the difference in boiling points between 1-butanol and ethanol in terms of the
intermolecular forces.
A7: The magnitude of the dispersion forces between molecules depends on the nature
and number of the atoms in the molecules. Comparing the molecules of ethanol and 1butanol, the atoms are the same kind in both (C, H, O). However the number of atoms
is greater in 1-butanol molecules and hence larger intermolecular forces occur.
EXTENSION QUESTION
Q8. Predict the boiling point of 1-propanol, giving an explanation of your prediction.
A8. The boiling point of 1-propanol should be approximately the average of those of
ethanol and 1-butanol. As explained previously, the atoms in 1-propanol molecules are
the same as those in ethanol and 1-butanol molecules. The number of these atoms
(12), is intermediate between the number in ethanol (9) and 1-butanol (15) molecules.
Hence approximately the intermolecular forces between 1-propanol molecules are
intermediate in strength and the bp is midway between that of ethanol and 1-butanol,
ie. 1/2(118+78) = 98oC. The literature value is 97oC. Note that ethanol, 1-propanol and 1butanol are members of a homologous series (the normal alcohols) and that it is
typical of such a series that there is a gradual, regular trend in all physical properties
along such a series.
Q9. Predict the boiling point of 2-methyl-2-propanol, giving an explanation of your prediction.
A9. 2-methyl-2-propanol is a structural isomer of 1-butanol. Its molecules therefore have
the same atomic composition as those of 1-butanol but they are arranged in different
sequences within the molecule. This is shown in the line drawings:
Because they are structural isomers, the intermolecular forces are of the same kind in
the two. Also because the atomic composition is the same the total number of
electrons is the same. However, the structural differences imply that the molecules
cannot interact with their neighbours in the same way. The molecules of 2-methyl-2propanol are more compact and ball-shaped: hence they have a lesser surface area of
contact with neighbouring molecules. This means a lesser overall intermolecular force
and hence a lower boiling point for 2-methyl-2-propanol.
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MELTING POINT DETERMINATIONS
TEACHER'S GUIDE AND MODEL ANSWERS
Introduction
The melting point of any particular organic substance is a very useful property since it can help the
organic chemist to determine the purity and identity of the compound in question. Pure organic
substances normally have sharp melting points, although some solid compounds do not. It is a
general rule that the addition of an impurity to a pure compound lowers its melting point.
1. Chemicals
All the chemicals required are found in the requirements table preceding the experiment.
2. Apparatus
The vials, microspatula, microburner, microretort stand, are all components of the RADMASTE
Advanced Microchemistry Kit.
The organic comboplate is manufactured from plastic which is resistant to organic solvents and
must be used in this experiment, because the conventional comboplate can be affected by the
organic reagents.
The thermometer is used in the combostill kit.
3. Hints
You may want to underpin this experiment with a lesson on intermolecular forces (hydrogen
bonding, dipole-dipole forces, induced dipole forces).
Don't leave the glass vial over the hot flame continuously for long periods of time (30-60 mins)
as this will cause the glass to get too hot and crack.
4. Cautions
Benzoic acid is harmful if swallowed, can act as an eye or respiratory irritant and may cause
allergic respiratory or skin reaction. Wear safety glasses and gloves when using it.
Methylated spirits is a mixture of ethyl alcohol (95%) and methyl alcohol (5%). Methanol is toxic
by inhalation, ingestion or skin absorption. Use gloves in a well ventilated room. Ethanol can cause
nausea, vomiting and inebriation upon ingestion, as well as skin and eye irritation. Work in a well
ventilated room with gloves.
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MODEL ANSWERS
Q1: Look up the melting point of benzoic acid in the literature. Compare it to your melting point
and explain any differences.
A1: 122 oC. Differences may be due to impurities. It also may be due to inaccurate
thermometers (since we didn't calibrate them).
Q2: Explain why the amount of sample in the capillary tube should be kept as small as possible
for a melting point determination.
A2: The more sample in the capillary tube, the more heat is required to melt all the sample.
If too much sample is placed in the capillary tube, it won't all melt uniformly, it will
melt over a period of time. The less sample in the tube, the more likely the melting
point will be reached instantaneously and the more likely the reading will be made
accurately.
Q3:
A3:
Draw out the line drawing for benzoic acid.
Q4: What kinds of intermolecular bonds occur between the benzoic acid molecules?
A4: There are hydrogen bonds between the -OH groups and dipole-dipole forces between
C=O groups. There are also instantaneous dipole-dipole forces between the
hydrocarbon parts of the molecules.
Q5: Do you think that the melting point of benzene will be higher or lower than that of benzoic
acid ? Explain your reasonings.
A5: Higher, because there are no hydrogen bonds, only instantaneous dipole-dipole
forces.
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CHAPTER 2
General Procedures
53
General Procedures
1.
Assembling the Wire Microvial-holder
55
2.
Vacuum Filtration and Washing of the Collected
solid
56
Making Capillary tubes for Melting and Boiling
Point Determinations
58
4.
Boiling Point Determinations
58
5.
Melting Point Determinations
60
6.
Making TLC Spotting Capillary Tubes
62
7.
Thin Layer Chromatogram (TLC)
63
3.
54
GENERAL PROCEDURES
The general procedures are experimental procedures or techniques that can be used in many,
many experiments (especially organic chemistry ones) using the RADMASTE microchemistry kits.
1.
Assembling the Wire Microvial-holder
Apparatus:
Organic comboplate, lid 1, glass vial, elastic band, microburner,
iron wire (15 cm x 1.6 cm diameter).
1.
Curl the 15 mm long piece of iron wire around the center of the glass vial, so that it forms a
loop that fits exactly around the outside of the vial.
2.
Place lid 1 into well E1.
3.
Place the other end of the wire in the small opening in lid 1 so that it is touching the bottom of
the comboplate as shown in the figure, and bend it at the end of the loop so that it can hold
the glass vial vertically.
4.
Wrap a piece of elastic around the centre of the glass vial and place it in the loop of the wire
vial-holder.
5.
Place the microburner under the vial.
6.
Make sure the wire is bent in such a way so that the vial is about 2-3 mm above the flame of
the microburner.
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2.
Vacuum Filtration and Washing of the Collected Solid
microstand in well
D5 of comboplate
Apparatus:
Organic comboplate, lid 1, glass vial with solid-liquid mixture to be filtered,
silicone tubing, plastic microburner vial, cotton wool (or string), microfunnel,
paper clip, syringe, glass rod, plastic microretort stand.
1.
Unfold a paper clip.
2.
Place a small piece of cotton wool into the opening of the microfunnel.
3.
Force the cotton wool into the stem of the microfunnel using an unfolded paper clip. (In some
experiments, string is used instead of cotton wool because cotton wool is attacked by the
chemicals).
Ensure that there is enough cotton wool in the funnel and that it is tightly wedged into the
stem of the funnel so that it fills the opening ensuring that all the liquid has to pass
through the cotton wool upon filtration.
If you are using string, tie a double knot in it with both knots on top of one another. Then
use a pair of scissors to cut the two loose ends of the string off.
4.
Place the plastic microburner vial (without the lid) into well F1 in the comboplate and place lid
1 into it.
5.
Place the syringe into the larger opening in lid 1.
6.
Place a piece of silicone tubing over the smaller opening of lid 1.
7.
Place the stem of the microfunnel into the other end of the silicone tubing.
8.
Place the plastic microretort stand into well D5 of the comboplate and clamp the silicone tube
into the arm just below the stem of the microfunnel so that it is not touching the syringe. This
is to hold the funnel upright and to make sure that it is steady during filtration.
9.
Make sure that the microfunnel is held as nearly perpendicular as possible so as to minimise
spilling of liquid during the filtration.
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10.
Make sure all the connections are sealed tightly otherwise no vacuum will be produced.
11.
Slowly pour the solid/liquid mixture in the glass vial into the microfunnel until it is ¾ full.
12.
Slowly pull the plunger out of the syringe (this creates a vacuum in the plastic microvial),
drawing the liquid out of the microfunnel into the plastic vial leaving the solid residue in the
microfunnel.
13.
Repeat this process until all the solid has been transferred from the vial to the funnel.
14.
Use a glass rod to scrape any solid remaining in the vial into the funnel.
15.
You can also use a little water (or suitable solvent) (about 0.1 ml) to get any solid that sticks
to the sides of the vial into the microfunnel.
16.
Having filtered all of the mixture, add a further 0.5 ml water (or other suitable solvent) to the
original container to wash out into the funnel any crystals that are still adhering to its sides.
Filter these washings through the funnel using the syringe. The aim is to get as much of the
solid into the funnel as possible.
17.
Use a microspatula (or a glass rod) to scrape any remaining crystals out of the original
container into the microfunnel.
18.
Continue this process until all the solid has been collected in the microfunnel.
19.
Some of the original liquid may adhere to the surface of the crystals in the funnel. This can
be washed away using a little water (or other suitable solvent), and then continuing the
vacuum filtration.
20.
Draw up a small amount of water (or other suitable solvent) - about 1 ml - into the syringe or
a propette.
21.
Slowly add it to the funnel containing the solid.
22.
Using the suction action of the syringe, draw out this liquid from the funnel.
23.
The solid has now been collected and washed.
1
If the plunger is completely pulled out before all of the liquid has been drawn from the
microfunnel, remove the syringe from lid 1, and push the plunger completely in.
Replace the syringe in lid 1 and draw the plunger out slowly again to continue filtration.
2
The microburner vial may become full, at some point. This means that no more liquid
will be suctioned from the funnel. If this is the case, slowly remove the lid from the vial
and empty its contents into the organic waste container - make sure you don't lose any
crystals in the process - and then replace the lid.
3
Note the difference between ordinary 'gravity' filtration and vacuum filtration. Gravity
filtration relies on the force of gravity to draw the liquid through the filter paper into the
collection vial. This however, is not always effective, because solid can clog up the filter
causing the liquid phase to be unable to pass through the filter paper (ie. the force of
gravity is not strong enough to draw the liquid through the filter paper because of the
collected solid that clogs it) and the filtration process can take a very, very long time.
Vacuum filtration speeds up the process generating an additional force to gravity that
draws the liquid through the filter.
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3.
Making Capillary Tubes for Melting and Boiling Point Determinations
1.
Light the microburner flame and hold the one end of a capillary tube horizontally, directly into
the flame (see figure 1). The glass should begin to melt and the end of the capillary tube
should close.
2.
As the glass is melting, twirl the capillary tube between your fingers to make sure it remains
straight as it melts.
3.
Once the end of the capillary tube is totally sealed, pull it out of the flame and let it cool to
room temperature.
4.
Boiling Point Determinations
REQUIREMENTS
Apparatus: 1 x Organic comboplate®; lid 1, 1 x glass vial, wire vial-holder, 1 x microburner,
matches,1 x paper clip, 2 x elastic bands, 2 x capillary tubes,
1 x 150oC thermometer, 1 x microretort stand, 1 x fusion tube, 1 x propette,
3 capillary tubes, tissue paper.
Chemicals: Boiling chips, paraffin oil, methylated spirits, sample.
PROCEDURE
1.
Take a capillary tube sealed at one end. (see General Procedures, section 3).
2.
Place lid 1 in well E1 of the comboplate.
3.
Assemble the wire vial-holder (see General Procedures, section 1).
4.
Place the wire vial holder into the small opening in lid 1 so that it is touching the bottom of the
well (see figure 2).
5.
Wrap the elastic band around the center of the glass vial, so that it fits tightly.
6.
Fill the glass vial with paraffin oil to a depth of approximately 1 cm and place 2-3 boiling chips
into the oil.
7.
Rest the glass vial in the wire vial-holder.
8.
Clip the thermometer into the microretort stand and attach the fusion tube with an elastic
rubber band.
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9.
Place the microretort stand in well D12 and submerge the thermometer and fusion tube
halfway into the oil (see figure 3).
Do not let the thermometer or the capillary tube touch the bottom of the vial during
heating. Also keep the elastic band out of the oil.
10. Using a propette place the liquid sample into the fusion tube until it is ¾ full.
11. Place the open end of the capillary tube into the fusion tube (see figure 3).
12. Light the microburner and place it directly under the vial (see figure 2). Watch the rise in
temperature.
13. Bubbles will emerge from the open end of the capillary tube, slowly at first, but eventually
almost continuously. When the stream of bubbles is continuous, stop heating by removing the
flame of the microburner from beneath the vial. Extinguish the microburner flame.
14. Observe and record the temperature when the evolution of bubbles just ceases as the liquid
cools. This is the boiling point of the liquid. Also note the atmospheric pressure.
Do this experiment two times. This is because the boiling temperature is unexpected the
first time you conduct the experiment and your first measurement may not be read
accurately. Thus, conduct this experiment the first time to obtain a rough estimate, then
conduct it again to get a more accurate reading.
15. Take the fusion tube out of the oil and allow it to cool to room temperature.
You may have to top up the fusion tube with new sample so that it is ¾ full because some
of it may have evaporated in the previous boiling point determination.
16. Keep the rate of temperature rise low for the 2nd determination. This is done by removing the
microburner from under the oil every few seconds and replacing it. The closer you get to the
boiling point, the slower the rise in temperature should be.
When you have completed your 2 observations, discard the capillary tubes. Wash the
fusion tube with water, clean out the fusion tube with a piece of tissue paper and leave it in
the sun to dry.
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5.
Melting Point Determinations
REQUIREMENTS
Apparatus: Organic comboplate, lid 1, lid 2, glass vial, wire vial-holder, 1 x microburner,
2 x elastic bands, 2 x capillary tubes, 3 x microspatulas, 1 x microretort stand,
1 x 150oC thermometer.
Chemicals: Boiling chips, paraffin oil, benzoic acid (C7H6O2(s)), methylated spirits, unknown solid,
matches.
PROCEDURE
1.
Prepare 2 capillary tubes for a melting point determination (see General Procedures, section
3)
2.
Place 4 microspatulas of finely powdered solid into well A1 of the comboplate.
3.
Push the open end of the capillary tube through this portion of the finely powdered solid,
making sure that some of the solid is pushed into it.
4.
Tap the closed end of the tube on the desk so that the solid collects at the sealed end.
5.
A height of solid about 3 mm - 5 mm is suitable.
6.
Place lid 1 in well E1 of the comboplate.
7.
Wrap the elastic band around the centre of the glass vial, so that it fits tightly.
8.
Place the wire vial-holder (see General Procedures, section 1) into the small opening in lid 1
so that it is touching the bottom of the comboplate.
9.
Rest the glass vial inside the loop.
10. Fill the glass vial with oil to a depth of approximately 1 cm and place 4-5 boiling chips into the
oil to prevent the oil from spluttering out of the vial.
11. Attach the capillary tube to the thermometer using a rubber band (See figure 1). Clip the
thermometer into the microretort stand and place it in well D2 of the comboplate.
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12. Submerge the thermometer and capillary tube halfway into the oil.
Do not let the thermometer or the capillary tube touch the bottom of the vial. Do not
let the elastic band touch the oil.
13. Light the microburner and place it directly under the vial.
14. Continue to hold the vial over the flame until the solid melts.
15. Note the temperature at which the solid suddenly melts in the tube.
16. Discard the capillary tube.
Repeat this experiment to obtain a second, more accurate result. This is because the
melting temperature is reached fairly quickly and your first measurement may not be read
accurately. Thus, conduct this experiment the first time to obtain a rough estimate of the
melting point. Then conduct it again to get a more accurate result.
17. Keep the rate of temperature rise low for the 2nd determination. This is done by removing the
microburner from under the oil every few seconds and replacing it. The closer you get to the
melting point, the slower the rise in temperature should be.
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6.
Making TLC spotting capillary tubes
1.
Light the microburner.
2.
Hold the middle of a capillary tube about 3 mm over the flame and rotate it with your fingers
so that the heat is evenly distributed.
3.
The center part of the capillary tube will become supple from the heat.
4.
At this point quickly pull it out of the flame and pull the two ends away from each other
making the middle part long and thin.
Break here
Don't hold the tube over the flame too long or it will split in half because of the heat before
you have drawn it out.
5.
Break the extended tube in half.
6.
Each half is a TLC spotting capillary tube.
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7.
A Thin Layer Chromatogram (TLC)
Do not touch the surface of the TLC plate with your fingers. Hold it on it's sides
only - like a photographer would hold a photographic plate so as not to get any
finger marks on it.
1.
Draw a pencil line across the TLC plate about 10 mm from the bottom.
Do not use an ink pen, the ink will run and make the spots difficult to see.
2.
Draw as many pencil dots at evenly spaced points across the plate as substances there are
to be analysed. These dots are where you spot the TLC plate.
3.
Label each dot according to the substance you plan to spot there in pencil.
4.
Draw out as many TLC spotting capillaries as spots to be made on the plate (see General
Procedures, section 6).
5.
Place 1 microspatula of each solid to be spotted into one of the wells of the comboplate.
6.
Using a propette, place enough solvent into each well so as to dissolve the solid.
7.
Dip a spotting capillary into well E1; it should draw up some of the liquid.
8.
Gently dab the end of the capillary onto the TLC plate in the place where you wish the spot to
be.
A sp
.B
A
9.
N
B
Do this for each of the substances you wish to analyse.
Do not make the spot too big by holding the capillary tube against the TLC plate for
too long. A diameter of no more than 2 mm is sufficient.
10.
Fill the glass vial to a height of about 5 mm with the designated developing solvent (which is
normally a mixture of substances).
11.
Place the TLC plate inside the vial so that the top is
resting against the side of the vial.
12.
Place the lid upside down on top of the vial.
Asp.
BA
N B
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13.
Allow the developing solvent to run up the TLC plate until it is about ¾ of the way up and
then remove the plate from the vial.
14.
Immediately trace the developing solvent front with a pencil to indicate where it is. This must
be done quickly since the solvent will evaporate and the solvent front won't be visible.
15.
Allow the plate to dry for some time.
16.
Empty the remaining developing solvent into the waste bottle provided. Not down the sink.
17.
Wash the vial with water and dry it with a piece of tissue paper.
18.
Put one or two pieces of iodine crystals into the vial and put the lid on properly this time.
19.
Pour boiling water into the plastic lunch box and hold the vial over in the steaming water for
1-2 minutes.
20.
After a while, the presence of iodine vapour will be observable in the vial.
21.
Place your TLC plate into the vial in the same way as before.
22.
Brown spots begin to form on the plate corresponding to where the iodine has interacted with
the substances spotted.
23.
After a minute or two remove the plate from the vial and immediately mark the postion of all
spots, by outlining them on the plate with a pencil. This is because the iodine fades after
some time.
24.
Measure the distance from the line drawn at the bottom of the plate to the midpoint of each
spot and record it.
25.
Measure the distance of the solvent front from the line drawn at the bottom of the plate to the
solvent front directly above each spot.
26.
Calculate the Rf value of each spot by dividing distance travelled by the spot by the distance
the solvent front travelled.
27.
Use a table like the one below to report your results.
Substance
28.
Distance travelled
by spot /mm (1)
Distance of solvent front
directly above spot /mm (2)
Rf value = (1)/(2)
Draw a rough sketch (similar to the one below) of your TLC plate in your report indicating all
distances travelled.
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CHAPTER 3
Preparation and Reaction
of Organic Compounds
65
Preparation and Reaction of Organic
Compounds
1.
Preparation and Testing of Ethyne (Acetylene)
67
2.
Making Esters – the Synthesis of Aspirin
72
3.
Recrystallisation and Purification of Aspirin
76
4.
The Oxyacetylene Flame
78
5.
Acid-base Extraction of Benzoic Acid from
Naphthalene
81
Using Esters: Saponification
85
6.
66
PREPARATION AND TESTING OF ETHYNE (ACETYLENE)
TEACHERS GUIDE AND MODEL ANSWERS
Introduction
Calcium carbide is produced industrially by heating a mixture of calcium oxide and coke to 20000 C
in an electric furnace. It is then treated with water to produce ethyne (acetylene) gas:
CaC2(s) + 2H2O(l) → C2H2(g) + Ca(OH)2(s)
Ethyne belongs to the alkyne group of hydrocarbons i.e the molecule contains a carbon-carbon
triple bond and is represented as HC CH. Electrons which form part of this bond react readily
with certain reagents. Bromine and chlorine add to ethyne to give addition products:
Solutions of bromine in water and in carbon tetrachloride are brown in colour. As the bromoaddition products are formed, the concentration of bromine molecules in these solutions decreases
and the solutions eventually become colourless. This is one method of detecting the presence of
ethyne gas.
When ethyne is reacted with powerful oxidising agents such as potassium permanganate, the triple
bond undergoes oxidative cleavage to form methanoic acid (formic acid) and carbon dioxide:
The purple KMnO4 (Mn(VII) is reduced to the orange/brown MnO2 (Mn(IV)). This reaction can also
be used to show the presence of ethyne gas because a solution of potassium permanganate in
propanone will change from purple to brown when ethyne gas is bubbled through it.
Ethyne was previously used as the starting material for the preparation of acetaldehyde, acetic
acid, vinyl chloride and some other chemicals. Today it is exploited mainly for its flammability and is
burned together with propyne and oxygen in oxyacetylene torches for welding and steel cutting. It
is also still used in the preparation of acrylic polymers.
1. Chemicals
All the chemicals required are listed in the requirements table preceding the experiment. Calcium
carbide samples should be as fresh as possible. Older samples may not have the desired reactivity
because water in the atmosphere may have already reacted with the powder. The solution of
potassium permanganate in propanone must be freshly prepared by adding a very small quantity
(use the narrow end of a microspatula) of solid potassium permanganate to approximately 50 ml
of propanone (acetone) with stirring. The solution should preferably be stored in an amber bottle
and protected from heat and light.
Such a solution is dark purple in colour and an appreciable colour change will not be seen during
the experiment unless the solution is diluted to a pale violet colour. The extra propanone is
therefore required to dilute the potassium permanganate solution.
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Alternatively, a dilute solution of bromine in water or carbon tetrachloride can be used to detect the
presence of the ethyne triple bond. If bromine solutions are too dark in colour, they will also need
to be diluted until they are pale brown. Water is used to dilute the aqueous bromine solutions and
extra carbon tetrachloride is required to dilute solutions of bromine in carbon tetrachloride.
2. Apparatus
The propettes, syringe, lids, silicone tubing, microspatulas, glass tube and arms of the microstand
are all components of the RADMASTE Microchemistry kits.
The organic comboplate® is manufactured from a material which is resistant to organic solvents
and must be used in this experiment, because the reaction between calcium carbide and water is
so vigorous that the plastic of the conventional comboplate® will crack around the large wells being
used. Propanone will also destroy the plastic of a conventional comboplate®. The teacher must
carry out a risk assessment to decide whether gloves and protective glasses should be worn by the
students. The teacher must provide a waste container into which organic waste can be disposed
of.
3. Hints
The teacher should ideally perform the experiment prior to the practical session to assess the
reactivity of the calcium carbide sample. Samples which are old or have been exposed to the
atmosphere will not be as reactive as those that are fresh. Often, it is not the entire sample which
is unreactive but rather the first one or two centimetres of powder close to the surface of the
sample. The teacher should then instruct the students to remove this quantity of powder from the
chemical bottle/s and dispose of it safely. The experimental procedure requires 3 to 5
microspatulas of calcium carbide powder for a reaction that produces a sufficient volume of ethyne
gas. If the sample is fresh, only 3 microspatulas will be needed. However, if the reaction with water
is poor then a quantity in excess of 5 spatulas of powder may be added. Caution must be
exercised here because if too great a quantity of a relatively fresh sample is used, the reaction in
F1 will be too vigorous and will cause some of the reaction mixture to flow through the silicone tube
into F2.
Make sure that the comboplate® and the microspatula used to add the calcium carbide powder
are both dry, as the powder will react immediately with any water it comes into contact with.
The potassium permanganate solution must be diluted with propanone as described in the
worksheet to give a pale violet/pink colour. Such a solution will be sufficiently reduced so that a
pale brown-coloured mixture remains after ethyne gas has been bubbled through it. If the
potassium permanganate solution is not diluted, the volume of ethyne gas produced from the
reaction in F1 will be too little to cause any appreciable colour change when bubbled through the
solution.
Similarly, bromine solutions need to be diluted until they are pale brown in colour otherwise they
will not be colourless at the end of the experiment.
Two or three students can share one propette of potassium permanganate or bromine solution
because only five to ten drops of each liquid is required.
Well F2 should not be filled to more than b of its capacity, otherwise solution will spill out of the
vent in lid 2 as ethyne gas bubbles rapidly through it.
Ensure that the wells are securely sealed. If the lids are not tight, ethyne gas will escape from
the wells and the desired reactions may be incomplete.
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When adding the water from the syringe, it may sometimes happen that the water does not
drop directly onto the powder in the well but collects on the inside of lid 1. For this reason, only
about 0,1 ml of water must be added initially. If the water does not immediately drop onto the
calcium carbide powder, the top of the syringe must be tapped gently to force the water into the
well from the inside of the lid. The remainder of the water must then be added slowly from the
syringe to prevent the contents of F1 from being pushed through the silicone tubing into F2.
Students may observe the escape of a white, foul smelling vapour from the vent in lid 2 during
the experiment. Remind them that ethyne is not foul smelling and that these observations are due
to an impurity found in the calcium carbide.
During the experiment it is normal for the lids to swell slightly, and it may be difficult to remove
them afterwards. The swelling subsides after some time. It is however important to prevent solution
being sucked back from well F2 into well F1 as soon as the bubbling in well F2 ceases. If the lid on
well F2 fits too tightly to be removed, the silicone tubing connecting lids 1 and 2 can be removed
instead to avoid suck-back.
4. Cautions
Ensure that all students are aware of the following safety advice:
Never point or hold the tip of a propette containing an organic liquid facing upwards. Unlike
aqueous solutions, organic liquids escape readily from the propette stem and can suddenly squirt
from the propette if it is held with the tip pointing upwards.
Do not inhale the ethyne vapour deeply! The vapours are easily detectable. Serious respiratory
damage and dulled receptiveness may occur if this rule is not adhered to.
Never bring a flame near any of the organic reagents used in this experiment, including the
calcium carbide powder. They are all flammable. Extinguish the flame of the microburner as soon
as you have finished using it.
It is obvious from this experiment that calcium carbide reacts violently with water. Make sure
that the sample is kept dry at all times and store the powder in a screw top bottle.
Bromine has a high vapour pressure i.e. the vapour above the solution will expand with an
increase in temperature. Bromine solutions must therefore be kept in a cool, dark place in well
stoppered bottles.
The halogens are corrosive. Avoid dropping the bromine solution onto fabrics and skin. Wash
hands thoroughly if any bromine solution makes contact with the skin.
Carbon tetrachloride and the bromine solutions form toxic vapour. Avoid inhaling these vapours
and make sure the experiment is performed in a well ventilated room.
Dispose of all organic waste in the waste container provided, and not down the drain!
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MODEL ANSWERS TO QUESTIONS
Q1. What is the colour of the potassium permanganate/propanone solution?
A1. Pale violet/pink.
Q2. What happens in well F1 when a drop of water falls onto the calcium carbide powder?
A2. A vigorous, rapid reaction occurs. A fizzing sound is heard and there is bubbling
where the water makes contact with the powder.
Q3. What is happening in well F2?
A3. Bubbles appear rapidly in the solution in F2.
Q4. Has the appearance of the potassium permanganate/propanone solution changed?
Describe.
A4. Yes. The potassium permanganate/propanone solution has changed colour from pale
violet to a pale orange-brown.
Q5. Observe the colour of the indicator when added to the residue in well F1.
A5. The indicator colour is purple.
Q6. What has happened to the solution in well F2 after about 5 minutes?
A6. The solution has become colourless and a brown solid has settled at the bottom of
the well.
Q7. Write down a balanced chemical equation to explain the reaction that occurred in well F1
between the calcium carbide and water.
A7. CaC2(s) + 2H2O(l) → C2H2(g) + Ca(OH)2(s)
Q8. Use the answer to question 1 to explain the change in colour of the Universal indicator
solution in well F1.
A8. The calcium hydroxide formed in the reaction is basic and caused the indicator colour
to change to purple.
Q9. While the gas was bubbling into the solution in F2, the following change took place:
KMnO4(aq) → MnO2(s)
Purple
Orange/brown
i.
ii.
iii.
A9.
Explain what type of reaction this is.
Use the partial equation above to describe why the solution in F2 appeared to change when
left for 5 minutes after the reaction was complete.
If a bromine solution was used, write down a balanced chemical equation to show the
reaction occurring in well F2. Why was a colour change observed?
i.
ii.
iii.
This is a redox reaction. The oxidation number of Mn has changed from +7 in
KMnO4 to +4 in MnO2. Mn has gained electrons and has therefore been reduced.
Manganese dioxide is a solid. When the solution was allowed to stand, the solid
brown MnO2 particles sank to the bottom of the well.
The bromine and ethyne are involved in an addition reaction. The bromine adds
across the triple bond of ethyne. A new product (1,2-dibromoethene) is formed.
Further addition of bromine results in the formation of saturated 1,1,2,2 tetrabromoethane. The solution in F2 becomes colourless because there are no
bromine molecules left in solution.
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Q10. Acetylene gas is highly flammable and gives a very high temperature flame when burned
with oxygen. Carbon dioxide and water are produced.
i.
Write down a chemical equation to show the reaction of oxygen with acetylene.
ii.
Name the process mentioned above.
iii.
Identify one industrial use of acetylene gas based on the statement made above.
A10. i.
ii.
iii.
2C2H2(g) + 5O2(g) → 4CO2(g) + 2H2O(l )
Combustion.
It is one of the components of the oxyacetylene torch used for welding and steel
cutting.
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THE SYNTHESIS OF ASPIRIN
TEACHER'S GUIDE AND MODEL ANSWERS
Introduction
The addition of excess water in step 6 is to ensure that any excess acetic anhydride is used up.
The anhydride reacts with water to form acetic acid. This leaves a dilute solution of acetic acid.
Since aspirin is insoluble in cold water, it crystallizes out of solution.
1. Chemicals
All the chemicals required are listed in the worksheet.
Ice is required as well as boiling water - at least enough to fill every student's lunch box or to fill
a communal container eg. a bucket of some sort (see step 3).
Instead of ice, students can use cold water to cool the reaction mixture. This can be tap water
(depending on the ambient temperature) or cold water from a fridge.
2. Apparatus
The vials, propettes, microspatula, microburner, microretort stand, are all components of the
RADMASTE Advanced Microchemistry kit.
The organic comboplate is manufactured from plastic which is resistant to organic solvents and
must be used in this experiment, because the comboplate used with aqueous solutions can be
affected by the organic reagents.
A kettle (or a stove with a pot) may be required to boil the water (see step 2).
3. Hints
This experiment should take about 2 hrs.
Two or three students can share one propette to dispense the concentrated sulphuric acid
because only five drops are used per student.
Hot water bath: Don't fill the container of hot water too full, otherwise, the reaction vials will
topple into the water - make the water depth about 1-2 cm high. The students need only keep their
reaction vials in the water bath for five minutes.
Vacuum filtration: Try to get as much aspirin product out of the reaction vial into the funnel as
possible with as little water as possible. First try to scrape it out and then use water. If not all the
aspirin is retrieved out of the vial after trying to get it out with 0.1 ml water, leave it in the vial. It is
not important to get every last grain of aspirin out of the reaction vial, since no quantitative analysis
is being conducted. Nevertheless, students should still be encouraged to be as accurate as
possible in their experimental method. If a balance is available let the students weigh out their
salicylic acid starting material and their aspirin product. Using too much water to wash out the
crystals from the reaction vial and to wash the product means that the microburner vial will fill up
with water and it will have to be emptied and replaced without spilling any aspirin product from the
funnel. Therefore, guard against using too much water in these steps. It also means that some of
your aspirin product will dissolve.
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4. Cautions
(Taken from Biggs, P. The Physical and Theoretical Chemistry Laboratory: Chemical and Other
Safety Information. Oxford University http://physchem.ox.ac.uk/MSDS/#MSDS).
Acetic anhydride is corrosive and causes severe burns when in contact with the skin. It is
harmful if swallowed or inhaled, and contact with the eyes should be avoided. For this cause, the
entire experiment should be conducted in a well ventilated room or preferably outdoors. Caution
the students to keep their noses far from the reaction vials since inhalation is extremely
unpleasant. Small doses, such as used in this experiment are not harmful, nevertheless, caution
should be maintained. To completely avoid contact with the skin and eyes, it is advisable to use
gloves and safety glasses throughout the duration of the experiment.
Salicylic acid is harmful by inhalation, ingestion and skin absorption. It is advisable to wear
gloves and safety glasses at all times and avoid breathing its dust.
Concentrated sulphuric acid is extremely corrosive, causes serious burns and is highly toxic. It
is harmful by ingestion, inhalation, and through skin contact. Avoid breathing the vapor. Upon
addition of the sulphuric acid to the acetic anhydride/salicylic acid solution, strong, stinging odours
may be evolved.
Ensure that the reaction vial is held far from the face at all times during addition of sulphuric
acid and afterwards.
MODEL ANSWERS
Q1. Write out the line drawings for salicylic acid, acetic anhydride and aspirin.
A1.
Q2. What is the purpose of washing the crystals ?
A2. Washing the crystals removes water soluble impurities that might adhere to the
surface of the crystals.
Q3. Write out a balanced chemical equation for the reaction.
A3.
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Q4. Explain the use of H2SO4 in the experiment
A4. H2SO4 is used as a catalyst. It protonates a carbonyl oxygen atom in the acetic
anhydride molecule, activating the carbonyl group toward nucleophilic attack by the OH group of the salicylic acid. A proton transfer then occurs and the loss of a water
molecule yields a protonated ester. The loss of the proton then yields a free ester
molecule and regenerates the acid catalyst.
O
O
+
H
CH3
CH3
O
H
O
H
+
+
+
CH3
CH3
OH
O
O
O
CH3
CH+3
O
O
H
OH
O
OH
O
O
O
CH3
+
H
O
O
+
OH
H3C
OH
O
Q5. Acetic anhydride is added in excess. This excess, however, can react with the carboxylic acid
group in aspirin. Which step in the procedure guards against this ? Explain.
A5. The addition of excess water in step 6 is to ensure that any excess acetic anhydride is
used up. The anhydride reacts with water to form acetic acid which is unreactive to
aspirin. This leaves a dilute solution of acetic acid. Since aspirin is insoluble in cold
water, it crystallizes out of solution.
EXTENSION QUESTION
Q6. If 1 microspatula of salicylic acid has a mass of approximately 0.2 g, calculate the mass of
aspirin that should be formed.
A6. Molar mass salicylic acid = 138 g/mol
Molar mass aspirin = 180 g/mol
mass of salicylic acid = 0.2 g x 6 microspatula = 1.2 g
nsal = msal/Msal
= (1.2)/138
= 0.0067 mol
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1 mole salicylic acid forms 1 mole aspirin.
masp = nsal x Masp
= 0.0067 x 180
= 1.56 g
Q7. Assuming you obtained 1 g of aspirin, calculate the percentage yield formed.
A7. percentage yield = actual mass of aspirin obtained/theoretical mass of aspirin
obtained x 100
= 1.00g / 1.56g x 100
= 0.64 x 100
=> 64 % yield
Q8. One way to prepare esters is by the reaction of an alcohol with an acid anhydride (as in this
experiment). However, they also may be prepared by the reaction between an alcohol and a
carboxylic acid or an alcohol and an acid chloride. Predict the product of the following
reaction;
A8.
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RECRYSTALLISATION OF CRUDE ASPIRIN AND TEST OF PURITY
TEACHER'S GUIDE AND MODEL ANSWERS
1. Chemicals
All the chemicals required are listed in the worksheet.
2. Apparatus
The vial, propettes, microspatula, microburner, microretort stand, are all components of the
RADMASTE Advanced Microchemistry kit.
The organic comboplate is manufactured from plastic which is resistant to organic solvents and
must be used in this experiment, because the comboplate used with aqueous solutions can be
affected by the organic reagents.
3. Hints
This experiment takes about 45 minutes to perform. However the aspirin product must be left
overnight to dry.
If a solid remains after heating the crude aspirin in the water/methylated spirits mixture, filter the
hot mixture through the funnel, making sure the reception vial is clean. The solid impurities should
remain in the funnel. Remove the impurities, take out the string, and replace it with clean string.
Allow the solution to cool to room temperature and continue with step 7.
Using too much water to wash out the crystals from the reaction vial and to wash the product
means that the microburner vial will fill up with water and it will have to be emptied and replaced
without spilling any aspirin product from the funnel. Therefore, guard against using too much water
in these steps.
4. Cautions
(Taken from Biggs, P. The Physical and Theoretical Chemistry Laboratory: Chemical and Other
Safety Information. Oxford University http://physchem.ox.ac.uk/MSDS/#MSDS).
Ferric chloride is a relatively stable compound. Contact with the skin, however, can cause burns
and it is also harmful if swallowed. Ensure the use of gloves and safety glasses throughout its use.
Working with such small amounts in a well ventilated room with gloves shouldn't cause any
harmful effects. However, still make sure that all safety precautions are observed.
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MODEL ANSWERS TO QUESTIONS
Q1. Why are the crystals dried ?
A1. Some water molecules may remain in the collected crystals and significantly increase
the mass (or change the appearance) of the crystals. The presence of water can also
affect the observed melting point of the crystals. Water molecules, however, may be
removed over time by evaporation (= drying in air). (Note: Since we are not necessarily
measuring the mass of our recrystallised product in this experiment, this step may be
omitted from the experimental procedure. However, because it is a useful piece of
information for students to assimilate and remember, it has been retained).
Q2. Report the melting point of your aspirin product, both the approximate and more accurate
observations. Find out the melting point of pure aspirin. How can we use the actual melting
point to get an idea of the purity of the aspirin product?
A2.
Table 1: Melting Point Determination of Aspirin
T/ o C
m.p determination #1
m.p determination #2
The melting point of aspirin is 138-140oC. Impurities trapped in the aspirin crystal have
an effect on the intermolecular bonds. Thus, they can change the melting point of the
product. By comparing the actual melting point to the literature melting point, we can
determine whether the crystals are pure or not. Not only that, but pure samples of a
substance normally have sharp melting points (about 1-2oC), impure samples of the
same substance melt over a wider range.
Q3. Report the results of your ferric chloride test on your recrystallised aspirin and on pure
aspirin.
A3.
Colour in Ferric Chloride
Pure aspirin
Orange
Crude aspirin
Purple
Q4. Salicylic acid gives a purple colour with ferric chloride. What does the test show about the
presence of salicylic acid in the two aspirin samples?
A4. Salicylic acid is not present in the pure aspirin sample but is present in the crude
aspirin sample.
Q5. Name two possible sources for the presence of the salicylic acid in your recrystallised aspirin.
A5. Unreacted salicylic acid can come from the starting materials. It can also be due to
some decomposition of the aspirin.
Q6. How else could you check the purity of your aspirin product? Explain.
A6. You could compare the Rf values obtained from TLC spots of recrystallised aspirin,
crude aspirin, salicylic acid and commercial aspirin - four spots on one TLC plate. The
aspirin would give a characteristic spot in each case. If there were any salicylic acid in
the recrystallised aspirin, there would be a spot with an Rf value similar to that of the
salicylic acid spot.
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77
THE OXYACETYLENE FLAME
TEACHERS GUIDE AND MODEL ANSWERS
Introduction
Ethyne was previously used as the starting material for the preparation of acetaldehyde, acetic
acid, vinyl chloride and some other chemicals. Today it is exploited mainly for its flammability and is
burned together with propyne and oxygen in oxyacetylene torches for welding and steel cutting. It
is also still used in the preparation of acrylic polymers.
The products formed by burning hydrocarbons depend on the concentration of oxygen and the
temperature. If there is excess oxygen and a high temperature, combustion of ethyne is complete
and the carbon atom is oxidised from -1 in ethyne to +4 in carbon dioxide. This is the highest
oxidation state of carbon:
2C2H2(g) + 5O2(g) →
4CO2(g) + 2H2O(l )
However, if the temperature is not high enough or the concentration of oxygen is low, the carbon
atoms in ethyne are oxidised to an intermediate oxidation state producing carbon (oxidation state
0) and carbon monoxide (oxidation state +2). This is the incomplete combustion of ethyne. It is a
less efficient use of the gas because a smaller amount of oxygen reacts with the carbon and less
heat energy is released during the reaction. Carbon monoxide is also a poisonous gas.
1. Chemicals
All the chemicals required are listed in the requirements table preceding the experiment.
Calcium carbide samples should be as fresh as possible. Older samples may not have the
desired reactivity because water in the atmosphere may have already reacted with the powder.
2. Apparatus
The propettes, syringe, lids, silicone tubing, microspatulas, glass tube and arms of the
microstand are all components of the RADMASTE Microchemistry kits. The organic comboplate® is
manufactured from a material which is resistant to organic solvents and must be used in this
experiment, because the reaction between calcium carbide and water is so vigorous that the
plastic of the conventional comboplate® will crack around the large wells being used.
The teacher must carry out a risk assessment to decide whether gloves and protective glasses
should be worn by the students. The teacher must provide a waste container into which organic
waste can be disposed of.
3. Hints
The teacher should ideally perform the experiment prior to the practical session to assess the
reactivity of the calcium carbide sample. Samples which are old or have been exposed to the
atmosphere will not be as reactive as those that are fresh. Often, it is not the entire sample which
is unreactive but rather the first one or two centimetres of powder close to the surface of the
sample. The teacher should then instruct the students to remove this quantity of powder from the
chemical bottle/s and dispose of it safely. The experimental procedure requires only 1 to 2
microspatulas of calcium carbide powder because the smallest amount of gas will ignite in the
flame of the microburner. If the sample is fresh, only 1 microspatula will be needed. However, if the
reaction with water is poor then a quantity in excess of 1 spatula of powder may be added. Caution
must be exercised here because if too great a quantity of a relatively fresh sample is used, the
reaction in F1 will be too vigorous.
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Make sure that the comboplate® and the microspatula used to add the calcium carbide powder
are both dry, as the powder will react immediately with any water it comes into contact with.
Ensure that well F1 is securely sealed. If the lid is not air-tight, ethyne gas will escape from the
well and the desired flame may not be obtained.
When adding the water from the syringe, it may sometimes happen that the water does not
drop directly onto the powder in the well but collects on the inside of lid 1. For this reason, only
about one drop of water must be added initially. If the water does not immediately fall onto the
calcium carbide powder, the top of the syringe must be tapped gently to force the water into the
well from the inside of the lid. The remainder of the water must then be added slowly from the
syringe.
During the experiment it is normal for the lid to swell slightly, and it may be difficult to remove it
afterwards. The swelling subsides after some time.
The set-up shown in the worksheets has been chosen so that the student does not have to hold
the glass tube in a horizontal position while simultaneously trying to add the water to well F1. The
oxyacetylene flame "bursts" from the end of the glass tube where it remains for as long as ethyne
is produced in F1. The flame becomes smaller and is eventually extinguished as no more ethyne
moves through the tube.
The combustion of ethyne in this experiment is incomplete, shown by the small particles of soot
(carbon) that remain in the air afterwards.
4. Cautions
Ensure that all students are aware of the following safety advice:
Never point or hold the tip of a propette containing an organic liquid facing upwards. Unlike
aqueous solutions, organic liquids escape readily from the propette stem and can suddenly squirt
from the propette if it is held with the tip pointing upwards.
Do not inhale the ethyne vapour deeply! The vapours are easily detectable. Serious respiratory
damage and dulled receptiveness may occur if this rule is not adhered to.
Never bring a flame near any of the organic reagents used in this experiment, including the
calcium carbide powder. They are all flammable. Extinguish the flame of the microburner as soon
as you have finished using it.
It is obvious from this experiment that calcium carbide reacts violently with water. Make sure
that the sample is kept dry at all times and store the powder in a screw top bottle.
Dispose of all organic waste in the waste container provided, and not down the drain!
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MODEL ANSWERS TO QUESTIONS
Q1. Observe the end of the glass tube and note what happens.
A1. A few seconds after adding the water, a bright yellow-orange flame bursts from the
end of the glass tube. The flame continues to burn as ethyne passes through the tube
and then slowly extinguishes as the production of ethyne in F1 ceases. A black smoke
is given off.
Q2. Look upwards and around you. Describe what you see in the air.
A2. There are black particles of soot floating in the air.
Q3. Why is the flame in this experiment referred to as an oxyacetylene flame and not an
acetylene flame?
A3. The acetylene produced in F1 mixes with oxygen in the air as it escapes from the end
of the glass tube. This mixture ignites in the heat from the microburner.
Q4. Complete combustion of a hydrocarbon causes the carbon atoms in the hydrocarbon
molecule to be oxidised to their highest oxidation state, +4. See the experiment which shows
the complete combustion of ethyne.
i.
What are the colours of the products of the complete combustion of ethyne?
ii.
What was the colour of one of the products you noticed after observing the oxyacetylene
flame? Name this product and list its oxidation state.
iii.
Why do you think that the product in ii. above was formed?
A4.
i.
ii.
iii.
Both carbon dioxide and water are colourless.
The soot was black. It was carbon. The oxidation state of carbon is 0.
The combustion of the ethyne was incomplete, probably because the
temperature at which the gas ignited was not high enough for complete
combustion. As a result, the carbon atoms in the ethyne molecules (oxidation
state -1) were not oxidised to their highest oxidation state but rather to zero as
carbon was formed.
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80
ACID-BASE EXTRACTION OF BENZOIC ACID FROM NAPHTHALENE
TEACHER'S GUIDE AND MODEL ANSWERS
Introduction
The following flow chart outlines the procedure.
Mixture
O
OH
+
Benzoic acid
Naphthalene
Ether
SOLUTION
NaOH
Remove ether layer
Dry and evaporate
Ether
aqueous
O
-
O Na
+
Aqueous layer containing
Na salt of Benzoic acid.
1) Wash with ether
2) Add HCl
Benzoic acid
Benzoic acid precipitate
O
Ether
OH
Remove ether layer
Dry and evaporate
Ether
Discard
aqueous
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1. Chemicals
All the chemicals required for this experiment are found in the requirements table preceding the
experiment.
2. Apparatus
The vials, propettes, microspatula are all components of the RADMASTE Advanced
Microchemistry Kit.
The organic comboplate is manufactured from plastic which is resistant to organic solvents and
must be used in this experiment, because the conventional comboplate can be affected by the
organic reagents.
The thermometer is used in the combostill kit.
3. Cautions
(Information from Biggs, P. The Physical and Theoretical Chemistry Laboratory: Chemical and
Other Safety Information. Oxford University http://physchem.ox.ac.uk/MSDS/#MSDS).
Benzoic acid is dangerous if swallowed, it is an eye or respiratory irritant and it may cause
allergic respiratory or skin reactions in some people. Wear safety glasses and gloves when using
it.
Naphthalene, is dangerous if swallowed or inhaled. It can irritate the eyes, skin and respiratory
system and is a possible carcinogen. Wear safety glasses, a lab coat and gloves when using it.
Diethyl ether is extremely flammable and volatile, so keep it way from open flames. It is harmful
by inhalation, ingestion and skin adsoption. So work in a fume hood or a well ventilated room or
even outside.
NaOH is very corrosive, causes severe burns and it may cause serious, permanent harm to the
eyes. It is also very dangerous if ingested. It is harmful by skin contact or by inhalation of dust. It is
advisable to wear gloves, safety glasses and work in a well ventilated room.
Dilute HCl is corrosive. Don't inhale the vapours, or ingest it in any way. The liquid can
cause severe damage to skin and eyes. So use gloves and safety glasses.
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MODEL ANSWERS
Q1. Why do we shake the reaction vial?
A1: To get the reagents (which are in the two different layers) to come into contact with
each other and so react.
Q2. Which layer does the benzoic acid salt go into ?
A2. The lower aqueous layer.
Q3. Why do we add drying agent to the first layer transferred to the comboplate ?
A3. Because there may be a small amount of water in the ether layer. Drying agent reacts
with the water but not with the ether thus removing any remaining water from the
ether.
Q4. What does the word 'decant' mean? In which steps of the experiment, could we have
decanted a liquid?
A4. To decant means to pour the liquid layer away, leaving the solid contained in it behind.
We could have decanted the ether mixture from the drying agent in steps 10 and 16
rather than using a propette. However, the use of a propette is much less messy.
Q5. Which is the ether layer - explain why !
A5. In both cases, the top layer is the ether layer because ether is less dense than water.
Q6. What is the precipitate formed upon the addition of HCl?
A6. Benzoic acid.
Q7. Draw out the line drawings of benzoic acid and naphthalene.
A7.
O
OH
naphthalene
benzoic acid
Q8. Write out a balanced chemical equation for the reaction that occurs in step 5 of the
procedure.
A8.
O
O
-
+
O Na
+
HCl
OH
+
NaCl
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Q9. Write out a balanced chemical equation for the reaction that occurs in step 15 of the
procedure.
A9.
O
O
OH
-
+
NaOH
+
O Na
+
H2O
Q10. After the liquid has evaporated, what substance do you see in well F1 of the comboplate?
And E1?
A10. Naphthalene is in well F1 and benzoic acid is in well E1.
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84
SAPONIFICATION
TEACHER'S GUIDE AND MODEL ANSWERS
Introduction
The hydrolysis of an ester (in this case butter) in basic solution to form a carboxylic acid salt (in this
case soap) is called saponification, after Latin sapo, "soap". It occurs through nucleophilic acyl
substitution. The hydroxide ion adds to the ester carbonyl group to form a tetrahedral intermediate.
An alkoxide ion is then lost to give a carboxylic acid which is then deprotonated to give the
carboxylic acid salt (which is the soap product). This reaction may be represented by the following
equation;
O
O
O
+
HO
-
-
O
C
O
HO
O
-
+
OH
Accounts of the production of soap date back more than 5000 years. Several millennia ago, crude
mixtures of animal fats and alkaline plant ash were used to generate soaps which lathered and
cleaned effectively. The process of making soap hasn't changed much over the years. Most fats
and oils are triesters (ie. the ester molecules of three long chain aliphatic carboxylic acids and a
single glycerol molecule).
carboxylic acid salt (or soap)
These carboxylic acid salts are insoluble in an aqueous solution of NaCl. It is often necessary to
precipitate carboxylic acid salts formed in the saponification by a process called "salting out" with
aqueous sodium chloride. This is because most soaps are insoluble in an aqueous solution of salt.
This explains why it is very difficult to obtain a lather when you wash your hands with soap in sea
water.
The principal ester in butter is an ester of glycerol with butanoic and other acids. When reacted
with NaOH it forms sodium butanoate.
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Action with hard water
Water is called hard when it contains more than 100 parts per million of the salts of such metals as
calcium, magnesium, and iron. When soap is dissolved in hard water, it reacts with these metal
ions to form an insoluble carboxylic acid salt. Only when all the metal ions have reacted with the
soap, is the water "softened", and the remaining soap is able to clean.
1. Chemicals
All the chemicals required for the saponification experiment are found in the requirements table
preceding the experiment.
Ice or (cold water) is required.
2. Apparatus
The vials, propettes, microspatula, microburner, microretort stand, are all components of the
RADMASTE Advanced Microchemistry kit.
The organic comboplate is manufactured from plastic which is resistant to organic solvents and
must be used in this experiment, because the comboplate used with aqueous solutions can be
affected by the organic reagents.
3. Hints
Ice bath: If no ice is readily available, simply pour cold water into a suitable container, eg. the
lunch box, and make the water level about 1-2 cm high.
Vacuum filtration: Make sure all the lids, the silicone tubing and other connections are tightly
sealed. This will ensure more efficient filtration.
Try to get as much soap product out of the reaction vial into the funnel as possible with as little
water as possible. If not all the soap is retrieved out of the vial after 0.2 ml water, leave it in the
vial. It is not important to get every last grain of soap out, since no quantitative analysis is being
conducted.
Using too much water to wash out the product from the reaction vial and to wash the product
means (1) that some of the soap product will dissolve and (2) that the microburner vial will fill up
with water and it will have to be emptied and replaced without spilling any soap product out of the
funnel. Therefore, guard against using too much water in these steps.
When tying the knot in the string, first tie the knot and then cut the ends off.
4. Cautions
(Taken from Biggs, P. The Physical and Theoretical Chemistry Laboratory: Chemical and Other
Safety Information. Oxford University http://physchem.ox.ac.uk/MSDS/#MSDS).
NaOH is very corrosive, causes severe burns and it may cause serious permanent eye damage.
It is also very harmful by ingestion. It is harmful by skin contact or by inhalation of dust. It is
advisable to wear gloves, safety glasses and work in a well ventilated room.
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
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86
MODEL ANSWERS TO QUESTIONS
Q1. Butter contains esters of glycerol with several different carboxylic acids. We refer below to
butyric (butanoic) acid (R = C3) for simplicity, but many of the acids have R= C15-C17.
A1.
!
Write out the chemical formula and the line drawing for glycerol.
!
Write out the chemical formula and line drawing for butyric acid.
H3C
OH
O
CH3CH2CH2COOH
Propose a chemical structure for the glycerol tributyrate ester.
!
H3C
O
O
O
H3C
O
CH3
O
O
Write out a balanced chemical equation for the following reaction and identify the soap.
!
R2
O
HO
O
O
+
3 NaOH
Na
H2O
O
3
R1
O
+
R
HO
O
R3
O
= soap
HO
O
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
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87
!
(optional) Draw out the reaction mechanism for the reaction.
O
O
R4
O
+ HO
R3
O
-
-
R4
O
OH
R3
R3
OH
+
R4
O
-
O
-
H2C
R4 =
R1
O
O
O
R3
R2
O
R4
+
O
-
OH
Q2. Name some other household products that contain fats that could be used in this experiment
instead of butter.
A2. Margarine, olive oil, sunflower oil, bacon grease, peanut butter, vegetable oil - any of
these fats could be used in principle even though some of them are saturated and
some of them are unsaturated fats. This should not make any difference since the
saponification reaction takes place around the -COOR parts of the molecule and does
not involve the breaking of C=C unsaturated bonds.
Q3. Why is the soap layer the upper layer and the aqueous layer the lower layer ?
A3. The organic soap layer is less dense than the aqueous layer, thus it forms the upper
layer. Organic molecules generally form liquids and solids that are less dense that
water.
Q4. What is the colour of the universal indicator paper ? What is the pH ? Explain your
observations.
pH reading with universal
indicator paper
Crude Soap Product
Commercial Soap
Violet, pH ~ 11
light blue, pH ~ 8
Commercial soaps are normally slightly basic (eg. pH = 8). This is because carboxylic
acid salts are weak bases and react with the water to form a low concentration of
hydroxide ion. The more strongly basic nature of the crude soap, however, could be
due the fact that it was produced in a solution of sodium hydroxide, and if it is not
thoroughly washed, some sodium hydroxide may still be present. This poses a
dilemma. Too much water to wash the soap can dissolve some of it. Too little can
cause NaOH impurities to be present in the solid soap.
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
88
Q5. What did you observe when you added the soap to the hard water and soft water ? Explain
your observations.
A5.
Description
Tap Water
Hard Water
The solution turns milky
because the soap molecules
disperse in the water without
forming a precipitate.
The solution remains clear,
but white pieces of solid
float on the surface (see
explanation below).
When soap is placed in hard water, it reacts with the calcium chloride to form calcium
butyrate that is insoluble in water. Only when all the calcium ions have reacted with the
soap is the water softened and the remaining soap molecules can disperse in the water
giving it a milky appearance as in the tap water well. The white solid that floats on top of the
hard water is calcium butyrate (also known as scum). Normally precipitates are more dense
than water and they sink to the bottom of the container. In this case the precipitate is less
dense that water and it floats on top.
Q6. Note what you observe when you add salt to the soap solution, and explain it.
A6. The solution changes from being milky to being clear but with larger pieces of solid
soap floating around. Soap is soluble in water; it forms "micelles" (see question 7)
that scatter the light to make the solution look milky. Soap is insoluble in salt water.
When you add salt to the soap solution, the soap precipitates out.
EXTENSION QUESTIONS
Q7. What is the cause of the cleansing properties of soap? (Hint: look up "micelles" in an organic
chemistry textbook).
A7. Soaps clean because the opposite ends of the carboxylic acid salt are so very
different. The sodium end of the molecule is ionic and therefore, hydrophilic (ie. waterloving). The carbon chain end is non-polar and therefore, lipophilic (ie. fat-loving).
When soaps are dissolved in water, the long carbon chain ends of the soap molecule
cluster together while the sodium hydrophilic parts stick out into the water (see the
figure below). A cluster of such soap molecules is called a micelle. When grease or fat
is placed in soapy water, they are coated by the non-polar 'tails' in the center of the
micelles, thus dispersing the grease or fat molecules. In this way soap is able to clean
fatty stains.
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
89
Q8. Predict the product of the following reaction:
A8.
The UNESCO-Associated Centre for Microscience Experiments
RADMASTE Centre, University of the Witwatersrand, Johannesburg, South Africa
Tel: (+) 27 11 717 4802 Fax: (+) 27 11 403 8733 email: [email protected] website: www.microsci.org.za
90
The UNESCO-associated Centre
for Microscience Experiments
RADMASTE Centre
University of the Witwatersrand
19th Floor, University Corner
Corner Jorissen and Bertha Street
Braamfontein, Johannesburg
South Africa
Private Bag 3
WITS 2050
Tel: (+) 27 11 717 4802
Fax: (+) 27 11 403 8733
email: [email protected]
website: www.microsci.org.za