Download OBSERVATIONS

1
Alcohols, Aldehydes, and Ketones
Introduction
Chemical Tests to Distinguish Alcohols, Aldehydes, and Ketones
In class you learned the structural differences between alcohols, aldehydes, and ketones, and to
distinguish between primary (1o), secondary (2o), and tertiary (3o) alcohols. There are a variety
of chemical tests that can be used in the laboratory to distinguish between these compounds.
This is useful when one is working with an unknown compound. In this experiment you will carry
out three different chemical tests (described below) on a variety of known alcohols, aldehydes,
and ketones and observe the results. Then you will use these tests to identify an unknown.
Chromic Acid Test
As you learned in class, 1o alcohols, 2o alcohols, and aldehydes are oxidized by chromic acid
(H2CrO4)—examples of this reaction are shown below (note the colors indicated for reactants
and products). 3o alcohols and ketones cannot be oxidized by chromic acid.
OH
R C
O
H
+
H2CrO4
R
H
1 alcohol
colorless
o
chromic acid
orange
+
H2CrO4
R
R
2 alcohol
colorless
chromic acid
orange
O
C
Cr2O3
new Cr compound
green precipitate
O
H
o
R
+
H
aldehyde
colorless
OH
R C
C
C
+
Cr2O3
R
ketone
colorless
new Cr compound
green precipitate
O
H
aldehyde
colorless
+
H2CrO4
chromic acid
orange
C
R
OH
carboxylic acid
colorless
+
Cr2O3
new Cr compound
green precipitate
What will you see during a chromic acid test?
Chromic acid is orange in solution. When it reacts with an alcohol or aldehyde, it is converted to
a new chromium compound which is green and insoluble. Therefore:
 if chromic acid reacts with a compound (oxidizes it), the orange color will disappear and
you will observe a blue, blue-green, or brown-green precipitate (it may take ~5-10
minutes for the color to fully develop);
 if chromic acid doesn’t react with a compound, the solution will remain golden orange
(there may be some precipitate, but it will not be blue or green).
2
Lucas Test
All alcohols react with hydrochloric acid (HCl) in the presence of ZnCl2 to produce alkyl halides—
an example of this reaction is shown below (note the colors indicated for reactants and
products). Aldehydes and ketones do not react with HCl in this manner.
R OH
o
+
o
any 1 , 2 ,
or 3o alcohol
colorless
HCl
hydrochloric acid
colorless
ZnCl2
R Cl
+
H2O
alkyl halide
insoluble; forms a
precipitate which appears
white or cloudy in solution
What will you see during a Lucas test?
The alcohols you will test are each clear and colorless liquids. HCl is also a clear and colorless
solution. The alkyl halide products of the reaction are insoluble in the solution. Therefore:

If HCl reacts with an alcohol, you will observe cloudiness in the solution due to a small
amount of white precipitate (the insoluble alkyl halide);
 the cloudiness will appear instantly for 3o alcohols
 the cloudiness will appear quickly (within 5 seconds to 30 minutes) for most 2o
alcohols, but sometimes takes longer (so you might not see evidence of this
reaction during the lab period)
 the cloudiness will appear very slowly (will take hours) for 1o alcohols (therefore,
you will not see evidence of this reaction during the lab period)

If HCl doesn’t react with an alcohol, the solution will not become cloudy (it may change
color to a pale yellow, but if no cloudiness is present, the reaction has not occurred).

Aldehydes change color (a shade of gold or brown) and quickly form a thick precipitate,
but this is not due to the formation of an alkyl halide as in the case of an alcohol (a
reaction is occurring for aldehydes, but it is not the reaction shown above). Aldehydes do
not react with HCl in the manner that alcohols do.
Continued on next page
3
2,4-DNP Test
Aldehyes and ketones react with a compound called 2,4-DNP (2,4-dinitrophenylhydrazine) —
examples of this reaction are shown below (note the colors indicated for reactants and
products). Alcohols do not react with 2,4-DNP.
O
C
R
H
aldehyde
colorless
+
2,4-DNP
clear orange
organic product
+
H2O
insoluble yellow, orange, or red
compound; forms a precipitate
O
C
R
R
ketone
colorless
+
2,4-DNP
clear orange
organic product
+
H2O
insoluble yellow, orange, or red
compound; forms a precipitate
What will you see during a 2,4-DNP test?
The compounds you will test are clear, colorless liquids. 2,4-DNP is a clear, orange liquid.
When aldehydes or ketones react with 2,4-DNP, the products are insoluble yellow, orange, or
red-colored compounds. Therefore:
 if 2,4-DNP reacts with a compound, you will observe a yellow/orange/red precipitate;
 if 2,4-DNP doesn’t react with a compound, the solution will remain a clear orange color
(with no cloudiness or precipitate).
4
Alcohols, Aldehydes, and Ketones
Prelab
Name_______________
Identify each of the compounds below as a primary alcohol, secondary alcohol, tertiary alcohol,
aldehyde, or ketone.
CH3
CH3CCH2CH3
CH3CH2CH2 OH
OH
CH3
O
H H H
C
H C C C
CH2CH3
H H OH
OH
OH
CHCH3
CH2 OH
H
CH3CHCH2CH2 OH
CH3
CH3CH2
CH3
O
C
CH2CH3
O
O
C
C
CH2CH3
C
CH3
CH3
OH
H
O
O
C H
CH3
CH2 OH
OH
H
5
Alcohols, Aldehydes, and Ketones
Procedure
1. Choose one of the unknowns to work with—use the same unknown throughout the entire
experiment. Record the number of your chosen unknown on the report sheet.
You will first set up the 2,4-DNP test for your unknown as well as for the following 5 known
compounds: ethanol, 2-butanol, 2-methyl-2-propanol, 2-butanone, and benzaldehyde. On the
report sheet, draw the structures of these 5 compounds and identify each one as an aldehyde,
ketone, or 1o, 2o, or 3o alcohol—do this NOW before beginning the procedure. (The
benzaldehyde structure is done for you.)
2. NOTE: do not mix up the 95% ethanol and pure ethanol (one of the 5 known compounds) in
this experiment!
NOTE: Be very careful not to mix up the droppers from different reagent bottles or to let the tip
of the dropper touch the edge of your test tubes— this will cause cross-contamination and
produce anomalous results!!
Thoroughly clean 6 test tubes as described during the prelab lecture. In the 6 test tubes, set up
the 2,4-DNP test for the 5 known compounds and your unknown compound (one test tube for
each compound). Follow the directions on the next page for setting up the 2,4-DNP test. While
the tubes are sitting for 15 minutes, you can continue with steps 3 and 4. At the end of the 15
minute waiting period, record the results on the report sheet (following the instructions given on
the report sheet) and dispose of the waste as indicated below.
3. Thoroughly clean 6 more test tubes. In the 6 test tubes, set up the chromic acid test for the 5
known compounds and your unknown compound (one test tube for each compound). Follow the
directions on the next page for setting up the chromic acid test. Record the results on the report
sheet (following the instructions given on the report sheet) and dispose of the waste as indicated
below.
4. Thoroughly clean 6 more test tubes. In the 6 test tubes, set up the Lucas test for the 5 known
compounds and your unknown compound (one test tube for each compound). Follow the
directions below for setting up the Lucas test. Record the results on the report sheet and
dispose of the waste as indicated below.
5. Based on the results of the three tests, determine whether your unknown was butanal,
2-pentanone, 4-methyl-2-pentanol, or 2-methyl-2-butanol. Record this on the report sheet and
explain how you arrived at this conclusion.
6
____________________________________________________________________________
2,4-DNP Test
Place 1 mL of 95% ethanol into each of the 6 clean test tubes Add 2 mL of the 2,4-DNP
reagent to each tube. Each tube will contain a different compound to be tested—add 1 drop of
the compound to be tested to the proper tube, as shown below:
Tube 1
1 drop of your unknown
Tube 2
1 drop of pure ethanol
Tube 3
1 drop of 2-butanol
Tube 4
1 drop of 2-methyl-2-propanol
Tube 5
1 drop of 2-butanone
Tube 6
1 drop of benzaldehyde
Mix each tube thoroughly by swirling the test tube or flicking it with your finger. Allow the tubes
to remain undisturbed for at least 15 minutes (it’s OK to leave them longer if you are working on
other tests).
At the end of 15 minutes, refer to the information in the introduction in order to interpret the
results—record the results on the report sheet, following the instructions on the report sheet.
Dispose of the test tube contents in the non-halogenated water-insoluble flammable organic
solvents waste bottle. You may use some acetone from the bottles with red lids near the sinks
in order to wash the waste out of the test tubes and into the waste bottle.
____________________________________________________________________________
Chromic Acid Test
Place 1 mL of acetone into each of the 6 clean test tubes. Then add 1 drop of chromic acid
(H2CrO4) to each tube. Each tube will contain a different compound to be tested—add 1 drop of
the compound to be tested to the proper tube, as shown below:
Tube 1
1 drop of your unknown
Tube 2
1 drop of pure ethanol
Tube 3
1 drop of 2-butanol
Tube 4
1 drop of 2-methyl-2-propanol
Tube 5
1 drop of 2-butanone
Tube 6
1 drop of benzaldehyde
Mix each tube thoroughly by swirling the test tube or flicking it with your finger. Let the tubes sit
undisturbed for about a minute and then check them for signs of a reaction. Refer to the
information in the introduction in order to interpret the results—record the results on the report
sheet, following the instructions on the report sheet.
Dispose of the test tube contents in the hazardous metal waste bottle.
____________________________________________________________________________
7
____________________________________________________________________________
Lucas Test
Place 10 drops of Lucas reagent into each of the 6 clean test tubes. Each tube will contain a
different compound to be tested—add 2 drops of the compound to be tested to the proper tube,
as shown below. AS SOON AS you add drops of the compound to a given tube, watch for the
signs of an immediate reaction (appearance of cloudiness).
Tube 1
2 drops of your unknown
Tube 2
2 drops of pure ethanol
Tube 3
2 drops of 2-butanol
Tube 4
2 drops of 2-methyl-2-propanol
Tube 5
2 drops of 2-butanone
Tube 6
2 drops of benzaldehyde
Mix each tube thoroughly by vigorously swirling the test tube. Place the tubes in a rack and
check them immediately for signs of a reaction—record any immediate reactions on the report
sheet.
Allow the tubes to stand for ~5 minutes, checking occasionally for signs of a reaction in the
tubes that did not react immediately. If any reactions become apparent during this time, record
the amount of time necessary for the reaction on the report sheet.
At the end of 10 minutes, do a final check of the test tubes and record results for any tubes that
took the full 10 minutes to react or that did not react.
Refer to the information in the introduction in order to interpret the results.
Dispose of the test tube contents down the drain.
8
Alcohols, Aldehydes, and Ketones
Report Sheet
Name _____________
Compounds to be Tested
Name
Structure
Aldehyde, Ketone, 1o, 2o, or 3o Alcohol?
Unknown # ____
(to be determined)
(to be determined)
ethanol
2-butanol
2-methyl-2-propanol
2-butanone
O
benzaldehyde
C
H
9
Data
Instructions
2,4-DNP test:
record POSITIVE (reaction occurred) or NEGATIVE (reaction did not occur)
Chromic acid test: record POSITIVE (oxidation reaction occurred)
NEGATIVE (oxidation reaction did not occur)
Lucas test:
Compound
or
record immediate cloudiness;
cloudiness after X minutes; OR
no cloudiness by end of 10 minutes;
ALSO record any dramatic color
change and thick precipitate which may indicate presence of an aldehyde
2,4-DNP Test
Chromic Acid Test
Lucas Test__________
Unknown # ____
ethanol
2-butanol
2-methyl-2-propanol
2-butanone
benzaldehyde
___________________________________________________________________________
List of possible unknowns:
CH3
CH3
O
O
CH3CHCH2CHCH3
CH3CCH2CH3
C
C
H
CH2CH2CH3
CH3
CH2CH2CH3
OH
OH
butanal
2-pentanone
4-methyl-2-pentanol
2-methyl-2-butanol
Identification of Unknown: My unknown is ______________
Explanation of how you were able to identify the unknown—include the results of ALL tests in
your explanation.
10
Alcohols, Aldehydes, and Ketones
Postlab
Name_______________
1. Under each compound that can be oxidized by chromic acid, write “chromic.”
2. Under each compound that will react with 2,4-DNP, write “2,4-DNP.”
3. Under each compound that will instantly produce a small amount of white
precipitate/cloudiness with Lucas reagent, write “Lucas instant.”
4. Under each compound that will produce a small amount of white precipitate/cloudiness with
Lucas reagent within about 5 seconds to 30 minutes, write “Lucas 5s-30min.”
Some compounds will have more than one thing written under them.
CH3
CH3CCH2CH3
CH3CH2CH2 OH
OH
CH3
O
H H H
C
H C C C
CH2CH3
H H OH
OH
OH
CHCH3
O
CH2 OH
H
CH3CHCH2CH2 OH
CH3
CH3CH2
CH3
C
CH2CH3
O
O
C
C
CH2CH3
C
CH3
CH3
OH
H
O
O
C H
CH3
CH2 OH
OH
H