Chemical Tests on Alcohol and Carbonyl Compounds
Introduction
In this experiment you will run several chemical tests on known compounds which are
mainly alcohols or carbonyl compounds. The alcohols are either primary, secondary, or tertiary,
depending upon how many other carbon atoms are bonded to the carbon bonded to the OH group.
The carbonyl compounds are either aldehydes or ketones. Once you have tested the known
compounds you will run the tests on an unknown compound which may be either one of the knowns
or a different alcohol or carbonyl compound. You can then determine by analogy the information the
tests tell you about the unknown.
An Introduction to the Tests:
2,4-Dinitrophenylhydrazine Test:
Carbonyl compounds can be distinguished from alcohols using the 2,4-dinitrophenylhydrazine (2,4-DNP) test. Aldehydes and ketones react to form a precipitate but alcohols do not
react. The color of the precipitate can vary in color from yellow to red depending on the degree of
conjugation in the compound. The precipitate is called a 2,4-dinitrophenylhydrazone. The reactions
involved are the following:
O2N
O2N
O
OH
C
R
H
R'
H2N
NH
R
NO2
C
N
R'
N
NO2
H
O2N
R and R' can be H, alkyl or aryl groups.
H
R
C
N
N
NO2
H2O
R'
Conjugation occurs when an organic compound contains a series of alternating single and double
bonds. Thus, an aldehyde or ketone is conjugated if the carbon in the carbonyl group is single
bonded to a second carbon that is double bonded to a third carbon. An example is benzaldehyde:
H
O
C
When testing your known and unknown compounds it is important to note both the color and time it
takes to form a precipitate.
Chemical Tests on Alcohol and Carbonyl Compounds, Page 1
The Lucas Test:
Primary, secondary, and tertiary alcohols can be differentiated from each other using two
tests: the Lucas test and the chromic acid test. The Lucas reagent is a mixture of hydrochloric acid
and zinc chloride. It will react with tertiary alcohols immediately, with secondary alcohols within
five to ten minutes, but it will not react at all with primary alcohols. The reactions are as follows:
R
R'
C
R
HCl
OH
ZnCl2
R'
R"
C
OH
H2O
3° Alcohol
H2O
2° Alcohol
R"
R
R' CH OH
HCl
ZnCl2
R' CH2 OH
HCl
ZnCl2
R
R'
CH Cl
No reaction.
1° Alcohol
You can tell that the reaction is taking place when the solution becomes cloudy. The alkyl halide is
not soluble in water so it forms a second immiscible phase, which is the cloudiness. The rates for the
reactions are different because the stabilities of the intermediate carbocations are different. Tertiary
alcohols form very stable tertiary carbocations, secondary alcohols form secondary carbocations of
intermediate stability, and primary alcohols form very unstable primary carbocations. The Lucas test
only works for alcohols with fewer than six carbon atoms because only these alcohols dissolve well
enough in the Lucas reagent. This test should only be run on samples which you have previously
shown to be alcohols.
The Chromic Acid Test:
The chromic acid reagent is a mixture of potassium dichromate and sulfuric acid. It reacts
with aldehydes and with primary and secondary alcohols, but not with ketones or with tertiary
alcohols. Primary alcohols are oxidized to aldehydes which are further oxidized to carboxylic acids.
Secondary alcohols are oxidized to ketones which do not react further. Thus, it isn’t necessary to rely
on the timing of the Lucas test to distinguish between secondary and tertiary alcohols. In addition,
you can distinguish between aldehydes and ketones. The reactions are:
R'
3
R
C
R'
OH
2 H2CrO4 + 3 H2SO4
3 R C O
Cr2(SO4)3 + 8 H2O
2° Alcohol
H
3
R CH2 OH
2 H2CrO4 + 3 H2SO4
3 R C O
Cr2(SO4)3 + 8 H2O
1° Alcohol
H
3
R
C
OH
O
2 H2CrO4 + 3 H2SO4
Cr2(SO4)3 + 8 H2O
3 R C
O
Aldehyde
Chemical Tests on Alcohol and Carbonyl Compounds, Page 2
The Tollens’ (or Silver Mirror) Test:
The Tollens’ reagent is a mixture of the silver ion and ammonia in basic solution. It can be
used to distinguish between aldehydes and ketones or alcohols since aldehydes are easily oxidized
while the others are not. When aldehydes are oxidized with the silver ion, the silver is reduced to
silver metal which plates out as a shiny mirror on the inside of a test tube. The reaction is:
H
R
C
O
O
2 Ag(NH3)2 + 3 OH
R
2 H2O + 2 Ag0 + 4 NH3
C
O
Aldehyde
R'
R
C
O
2 Ag(NH3)2 + 3 OH
No Reaction.
Ketone
The Iodoform Test:
The iodoform reagent is a mixture of iodine and potassium iodide dissolved in water. If a
methyl group is adjacent to a carbonyl or a hydroxyl group that can be oxidized to a carbonyl it will
react to form iodoform, CI3H, which is a yellow precipitate. The reactions are:
OH
O
R CH CH3
+ I2 + 2 OH
R
C
+ 2 H2O + 2 I
CH3
Alcohol
O
R
C
O
CH3
+ 3 I2 + 3 OH
R
C
+ 3 H2O + 3 I
CI3
Ketone
O
R
C
O
CI3
+ OH
R
+ CHI3
C
O
Formation of Iodoform
Chemical Tests on Alcohol and Carbonyl Compounds, Page 3
Procedure:
Wear safety goggles and gloves for the entire experiment.
2,4-Dinitrophenylhydrazine Test: (This reagent is a suspected carcinogen;)
Run this test on all known and unknown compounds. Place 6 to 7 drops of 2,4-DNP reagent
in one of the depressions of a spot plate. Add one drop of the compound being tested. Stir with a
glass stirring rod. Record whether a precipitate forms or not. If a precipitate forms record its color
and the amount of time it takes to form. Dispose of the products in the proper disposal container.
Chromic Acid Test: (This reagent is a suspected carcinogen.)
Run this test on all known and unknown compounds. Use caution since this is a hazardous
chemical and has a tendency to splatter on mixing. Place five to six drops of acetone on a spot plate
followed by one drop of the liquid known or unknown. Stir with a glass stirring rod. Add one drop of
chromic acid reagent. Stir and note if there is a color change. To assure that the acetone does not
interfere with the test results because of contamination, run a blank test with acetone. (This is a test
with the acetone and the chromic acid, but without any known or unknown compound.)
Lucas Test:
Don’t do this test until you have determined which compounds are alcohols. Run the test
only on alcohols. Place about five drops of the alcohol to be tested in a micro test tube. Add to this
about twenty drops of the Lucas reagent. Shake the mixture and allow the solution to stand. Note the
time required for the solution to become cloudy and use this information to classify the alcohol as
primary, secondary, or tertiary.
Tollens’ (Silver Mirror) Test:
Don’t do this test except on compounds that you have already shown to be aldehydes or
ketones. For each compound you want to test, place 1.0 mL of silver nitrate solution in a micro test
tube. Add one drop of sodium hydroxide to the test tube. A brown precipitate will form. Add
concentrated ammonia drop by drop with shaking until the precipitate dissolves. (This should take a
few drops.) Add one drop of the sample to be tested, stir, and allow the mixture to stand for about 10
minutes at room temperature. If no silver mirror has formed, place the tube in a 40 °C water bath for
about 5 minutes. Record the results and dispose the products in the proper disposal container.
Iodoform Test: (Iodine is one of the reagents used in this test. It is highly toxic and causes burns.
Run this test on all of the known and unknown compounds. For each compound you want to
test place 2 drops of the compound that you are testing in a separate micro test tube. Add 5 drops of
aqueous 10 % potassium hydroxide. Heat the test tubes in a 50–60 °C water bath and add dropwise
with the KI/I2 reagent until the solution is dark brown. (This should take about 20 drops.) Then add
10 % potassium hydroxide dropwise with stirring until the brown color fades. Let the tubes sit in the
hot water bath for an additional two minutes, then allow the solutions to cool and note whether a
yellow precipitate of CHI3 has formed.
Chemical Tests on Alcohol and Carbonyl Compounds, Page 4
Name ____________________________________
Alcohol and Carbonyl Lab Report
Predictions: Before you start experimental work predict the results for the known compounds. Enter
your predictions in ink, then have your instructor initial your results.
Compound
2,4-DNP
Results (Color
& Time)
Chromic Acid
Test (Color
Change)
Lucas Test
(Time to turn
cloudy)
Tollens’ Test
(Silver Mirror;
yes or no)
Iodoform Test
(Yellow
precipi-tate; yes
or no)
1. acetone
2. acetophenone
3. tert-butyl
alcohol
4. benzaldehyde
5. benzophenone
6. butanal
7. 1-butanol
8. 2-butanol
9. 2butanone
10. cyclohexanone
Chemical Tests on Alcohol and Carbonyl Compounds, Page 5
Results: Record your observations as you do the tests and indicate what the tests indicate about each
compound.
Compound
2,4-DNP
Results (Color
& Time)
Chromic Acid
Test (Color
Change)
Lucas Test
(Time to turn
cloudy)
Tollens’ Test
(Silver Mirror;
yes or no)
Iodoform Test
(Yellow
precipi-tate; yes
or no)
1. acetone
2. acetophenone
3. tert-butyl
alcohol
4. benzaldehyde
5. benzophenone
6. butanal
7. 1-butanol
8. 2-butanol
9. 2butanone
10. cyclohexanone
Unknown #
_____
Chemical Tests on Alcohol and Carbonyl Compounds, Page 6
Name
Problems:
1. What functional group is present in your unknown?
2. Summarize the test results that led you to your conclusion. You should include in your
summary reasons why you did or did not perform specific tests on your unknown
3. What other structural features did these tests reveal about the structure of your unknown?
(e.g.primary alcohol or methyl ketone.)
4. Write equations for each reaction with your unknown that occurs.
Chemical Tests on Alcohol and Carbonyl Compounds, Page 7
5. Draw a chemical structure for each of the known compounds.
6. Did any of your compounds give incorrect test results? If so, list them.
Chemical Tests on Alcohol and Carbonyl Compounds, Page 8
5. Complete the following reactions. If no reaction occurs write N. R. to indicate that.
a.
HO
ZnCl2
+ HCl
H3C
b.
O
+ Ag(NH3)2 + OH
c.
OH
CH3
CH
CH
+ I2 + OH
H3C
CH3
d.
O
O2N
HN
NO2
C
H
+
H2N
e.
O
+
CH 3
H 2CrO 4 +
H 2SO 4
H
Chemical Tests on Alcohol and Carbonyl Compounds, Page 9