Download JUNIOR COLLEGE CHEMISTRY DEPARTMENT EXPERIMENT 15

JUNIOR COLLEGE
CHEMISTRY DEPARTMENT
SECOND YEAR PRACTICAL
EXPERIMENT 15
Name: __________________________
Group: _________
Date: ___________
This practical continues the study of the aromatic series of compounds. It will illustrate some
common reactions of a few chosen important aromatic compounds.
You will be working with compounds EE to HH. Always use small samples unless otherwise specified
and take great care to avoid droppers of reagent bottles getting interchanged. Slight contamination can
lead to completely wrong results.
___________________________________________________________________________________
OH
5. The compound EE is phenol :
It consists of colourless deliquecent crystals which turn pink due to atmospheric oxidation.
PHENOL IS A VERY CORROSIVE SUBSTANCE. AVOID ALL CONTACT WITH THE SKIN
AND DO NOT SPILL ANY OF THE SOLID ON THE BENCHES.
Note the characteristic odour of phenol ; this resembles
carbolic
3
Place a few crystals of phenol in a test tube and add about 4 cm of distilled water. Shake well and use
portions of the solution for the following tests.
(a) To a portion of the solution add 2 drops of Universal indicator.
OBSERVATIONS
INFERENCES
Solid phenol dissolves only slightly in
Acidic character is seen. Phenol (PhOH) ionizes
water. Solution has a pH of ~ 5 -6
in water to a higher degree than H2O
PhOH(aq) à H+(aq) + PhO-(aq)
(b) Add some of the phenol solution to a little sodium carbonate solution and warm gently. CARE
OBSERVATIONS
INFERENCES
The phenol dissolves but no gas
Phenol though acidic, is not acidic enough to
release CO2 from CO32- i.e. In acidity H2CO3 >
(effervescence) is seen
PhOH > H2O > ROH
PhOH(aq) + CO32-(aq) à PhO-(aq) + HCO3-(aq).
(c) Add some drops of neutral iron(III)chloride solution to a portion of the solution of phenol.
OBSERVATIONS
INFERENCES
The addition of neutral FeCl3 to phenol
Phenols give a brightly coloured solution with
(EE) gave a bright purple/ violet solution
FeCl3.(Test is used to distinguish between
alcohols i.e. ROH and phenols i.e. PhOH / ArOH
The OH group is an activating group. Explain the meaning of this term.
Expt 15 Page 1
Explanation: .An activating group is one which increases the electron density on the aromatic ring
and as a result the ring becomes more reactive towards electrophiles. (Reactivity is greater than that
of the parent i.e. benzene.)
When aqueous bromine is added to solutions of such compounds, the tribromoderivative is formed
as a white crystalline precipitate.
3
(d) Add about 1 cm of bromine water top a portion of the solution of phenol.
OBSERVATIONS
INFERENCES
The brown colour of Br2(aq) disappears
The decolourisation of bromine water, which is
and a white precipitate is obtained
followed by the formation of a white precipitate,
is indicative of the presence of an activated
(benzene) ring e.g. a phenol, phenylamine or a
derivative of these.
OH
OH
Br
Br
(aq)
+
+ 3Br
2(aq)
Br
3HBr
(s)
(aq)
6. The molecular formula of the compound FF is C7H6O3 ; it has 2 functional groups attached to
the aromatic ring. When carrying out tests (b), (c)(i),(ii) with compound FF it would be useful
to compare the results obtained with those of the tests with compounds DD and EE. KEEP IN
MIND THE FACT THAT FF HAS TWO FUNCTIONAL GROUPS.
(a) Ignite a small portion of FF. Do this by placing some FF on a crucible lid: then heat from
underneath until the solid melts and then apply the flame from above.
OBSERVATIONS
INFERENCES
FF burns with aluminous and very sooty
A highly sooty flame suggests a high degree of
flame
unsaturation – aromatic compound likely.
3
(b) Place some FF in a test tube and add about 2 cm of aqueous sodium carbonate. If nothing
happens in the cold warm gently.
OBSERVATIONS
INFERENCES
The water insoluble FF was first
Effervescence with CO32- confirms the presence of
dissolved by warming. Addition of
an acidic group (This also explains the
Na2CO3(aq) gave a gentle effervescence.
insolubility of FF in cold water – a property of
The gas turned lime-water milky.
aromatic carboxylic acids.
Expt 15 Page 2
(c). Dissolve a small portion of FF in water and to separate portions of the solution add:
(i) a few drops of neutral iron(III)chloride solution.
OBSERVATIONS
INFERENCES
A brightly coloured solution was obtained
Formation of the brightly coloured solution with
when FeCl3 was mixed with a sample of
FF shows that the latter is (also) a phenol i.e. has
FF
an OH group directly attached to the ring.
(ii) a few drops of bromine water.
OBSERVATIONS
INFERENCES
The brown Br2(aq) colour is discharged
Observations indicative of the presence of an
and a white precipitate forms
activated ring i.e. a phenol, phenylamine or one
of their derivatives (Formula here rules out the
amine. See also equation (i) at the end of this
practical)
(d) Heat a small portion of FF with an approximately equal quantity of soda-lime. Note the odour
of the product. CARE.
OBSERVATIONS
INFERENCES
A carbolic smell was produced when FF
Carbolic smell indicates the formation of phenol
was heated strongly with (solid) sodalime
– a result of a decarboxylation reaction. Hence
FF must be a carboxylated phenol. i.e. FF is a
hydroxybenzoic acid
3
(e) Place a small portion of FF in a test tube , add about 1 cm of methanol and a few drops of
concentrated sulphuric acid. Warm the mixture in a water bath for a few minutes. Allow to
cool and then pour the contents of the test tube into some aqueous sodium carbonate in small
beaker.
OBSERVATIONS
INFERENCES
After heating FF in CH3OH + c. H2SO4,
Smell is that for methyl 2-hydroxybenzoate –
and adding the cooled mixture to Na2CO3
a highly characteristic smell. FF must therefore
a strong smell of surgical spirit/Oil of
be 2-hydroxybenzoi c acid
wintergreen/“Vicks” was noted.
Expt 15 Page 3
Suggest a name and structure for FF:
OH
COOH
Name:..2-hydroxybenzoic acid........
In view of the two functional groups present in FF comment on the results obtained in tests:
u (c)(i) - N.B. Keep in mind that both functional groups in FF react with neutral iron(III)chloride
Comment: The deactivating COOH would have given a buff precipitate with FeCl3 while the phenol
gives a strongly coloured solution. FF, with both types of groups, gives the formation of a brightly
coloured solution. The activating group chemistry i.e. that of the OH dominates over that of the
deactivating group, the COOH
u (c)(ii) - N.B. Keep in mind that one of the functional groups present in FF is activating and the other
deactivating.
Comment: For c(ii), ArCOOH + Br2 would give a slow decolourisation reaction while the ArOH gives
an instant decolourisation with the subsequent formation of a white precipitate. F, having both groups,
gives an immediate decolourisation followed by the formation of a white precipitate. Thus the activating
group reaction is, once again, dominant over that of the deactivating group
7. The molecular formula of compound GG is C9H8O2
(a) Ignite a small portion of the solid GG.
OBSERVATIONS
INFERENCES
GG burns with a luminous, very sooty
Degree of sootiness suggests a highly unsaturated
flame
compound – aromatic compound likely
(b) Mix some GG with an approximately equal amount of solid sodium carbonate in a test tube.
then add some distilled water and warm gently IF nothing happens in the cold.
OBSERVATIONS
INFERENCES
When a suspension of GG in water was
Release of CO2 from CO32-.indicates the presence
heated, then mixed with Na2CO3(aq) ,
of an acidic group i.e. COOH/ SO3H (if formula
effervescence was observed. Gas turned
prmits)
lime-water milky
3
(c) Place some GG in a test tube and add about 1 cm of ethanol and a few drops of concentrated
sulphuric acid. Warm the mixture for a few minutes in a water- bath , allow to cool and then
pour the contents of the test tube into some sodium carbonate solution.
OBSERVATIONS
INFERENCES
Expt 15 Page 4
The addition of the cooled mixture of GG
Fruity smell ( from an ester), by reaction with an
+ C2H5OH + H2SO4 to aqueous Na2CO3
alcohol confirms the presence of a carboxylic
gave the release of a fruity smelling
acid group in GG
vapour
(d) Dissolve some GG in hot water and divide the solution into two portions.
(i) to the first portion add a few drops of bromine water. NOTE CAREFULLY WHAT HAPPENS.
OBSERVATIONS
INFERENCES
Br2 is decolourised – mixture goes
Decolorisation suggests either an activated ring
slightly cloudy. White ppt?.
and/ or an unsaturation on a side-chain
(ii) to the other portion add a few drops of sodium carbonate solution followed by 2 drops of dilute
potassium manganate(VII) solution.
OBSERVATIONS
INFERENCES
The purple MnO4- in aq. CO32- turns to
Colour change shows that an oxidisable material
a brown precipitate on the addition of FF
such as an alkene (i.e. an unsaturation on a side
chain) is present
In view of the results obtained in the above tests, complete the following structure of GG:
H
H
C
H
C
H
C
COOH
C
H
COOH
8. The molecular formula of HH is C7H6O2.
(a) Ignite 2 drops of the liquid on a crucible lid.
OBSERVATIONS
INFERENCES
HH burns with a very sooty flame. Its
Sootiness in flame indicates a high degree of
flame is also fairly luminous
unsaturation i.e. HH is likely to be an aromatic
compound
3
(b) Add 1 drop of HH to about 1 cm of neutral iron(III)chloride.
OBSERVATIONS
A highly coloured solution is seen when
INFERENCES
A phenol (or a derivative) is indicated
HH is mixed with FeCl3
3
(c) Add 1 drop of HH about 1 cm of bromine water.
Expt 15 Page 5
OBSERVATIONS
INFERENCES
The bromine instantly decolourised and
Reaction with Br2 suggests and confirms the
a white precipitate was formed
presence of an activated ring e.g. a phenol,
a phenylamine or one of their derivatives.
3
(d) Place 1 cm of 2,4 - D.N.P.H reagent in a test tube and add 1 drop of HH.
OBSERVATIONS
INFERENCES
Mixing 2,4-DNPH with a sample of HH
Formation of a precipitate with 2,4-DNPH
gave an orange/yellow precipitate
confirms that the unknown compound is either
an aldehyde or a ketone i.e. it has a carbonyl
group in its structure
3
(e) Place 1 cm of silver nitrate solution in a test tube and add 1 drop of aqueous sodium hydroxide.
Then add aqueous ammonia dropwise until only a faint trace of precipitate remains. Add 1drop
of HH to the mixture.
OBSERVATIONS
INFERENCES
A silver mirror was produced when HH
HH is a reducing agent since only these can give
was treated with Tollen’s reagent
a silver mirror with Tollen’s. Aldehydes are
reducing agents (and have a carbonyl group as
seen in test (d) above)…(See eqn. (ii) below)..
N.B. RINSE THE TEST TUBE WITH DILUTE NITRIC ACID AFTER YOU FINISH THE
ABOVE TEST.
Suggest possible structural formulae for:
(i) HH :
(ii) The main organic products obtained in reactions (c) and (d) :
OH
H
C
OH
O
Br
O 2N
Br
OH
H
C
N N
H
Name:.
2-hydroxybenzaldehyde.
Br
Expt 15 Page 6
NO2
(i)
OH
OH
COOH
Br
Br2
Br
+
CO2
Br
(ii)
OH
OH
OH
CHO
Br2
Br
COOH
Br
Br2
+ CO2
Br
Expt 15 Page 7