QUALITATIVE TESTS FOR IDENTIFICATION OF CARBON-CARBON
UNSATURATION AND ALCOHOL FUNCTIONS (12/23/2009)
In this experiment we will examine two reagents (bromine in carbon tetrachloride and aqueous
potassium permanganate) which are widely used for qualitative identification of carbon-carbon
unsaturation and three reagents (chromic acid, hydrochloric acid, and hydrochloric-zinc chloride)
which are widely used for qualitative identification of alcohols.
Please handle these reagents with care and avoid contact with skin. Bromine causes chemical
burns, permanganate causes stubborn to remove stains, and chromic acid and Lucas reagent are
very strongly acidic solutions.
Reagents for Detection of Unsaturation
Bromine in Carbon Tetrachloride
This reagent is commonly used to identify the presence of olefinic or acetylenic functions. The
red bromine color of this reagent is discharged when the unsaturated function reacts with the
bromine. This reagent should be used in conjunction with the aqueous permanganate reagent to
avoid erroneous conclusions. Erroneous results with the bromine reagent may occur for two
reasons. Firstly, not all olefinic or acetylenic compounds take up bromine. When electron
withdrawing groups are attached to the unsaturated carbon atoms the uptake of bromine is
considerably slowed and in extreme cases completely stopped.
C 6H5CH
CH2
+
C6H5CH
CHCO2H
fast
Br2
+
Br2
slow
C6H5CHBrCH2Br
C 6H5CHBrCHBrCO2H
Secondly, some other functionalities discharge the bromine color but with the evolution of
hydrogen bromide. That is to say, they undergo bromine substitution instead of bromine
addition. Compounds which undergo substitution extremely rapidly are phenols, enols, and
anilines. Thus, a positive test for unsaturation is one that discharges the bromine color without
the evolution of hydrogen bromide.
Procedure: Add 1 drop of the liquid (or the tip of a microspatula of a solid plus 3 drops of
ethanol) to a small test tube and then add a 5% (0.3 M) solution of bromine in carbon
tetrachloride drop by drop (with agitation) until the bromine color remains. Run this test on (1)
cyclohexene, (2) cyclohexane, (3) ethanol, and (4) cinnamic acid and record your observations.
For the cinnamic acid, let the test tube sit and check it periodically during the remainder of the
lab period to see how long it takes for the bromine color to disappear.
Aqueous Potassium Permanganate
The purple permanganate color of this reagent is discharged by olefinic and acetylenic functions.
This is known as the Baeyer test for unsaturation. In cold (room temperature or below) dilute
aqueous solutions the main products from the reaction of an olefin with permanganate are the
corresponding vicinal diol and manganese dioxide (brown solid). Acetylenes produce the
corresponding diones. If heated these initial products undergo further oxidation, leading
eventually to carbon chain cleavage with the formation of carboxylic acids. This test is very
general and gives positive tests with those unsaturated compounds that do not decolorize
bromine in carbon tetrachloride.
RCH
CHR + KMnO4 + 2 H 2O
RCH
CHR + MnO2 + 2 KOH
OH OH
RCH
CHR
further
oxidation
2 RCO 2H
OH OH
Two cautions are in order when using the Baeyer test. Firstly, aldehydes give positive tests
because they are readily oxidized to carboxylic acids by permanganate. Secondly, although pure
alcohols do not react within 5 minutes, they often contain trace impurities that are easily oxidized
and give a slight reaction. Likewise alkanes often contain trace amounts of olefins as impurities
and may give a slight reaction. For these reasons decolorization of a drop or two of the reagent
should not be accepted as a positive test.
Procedure: Add 4 drops of the liquid (or the tip of a microspatula of a solid) to be tested to 1
mL (~20 drops) of ethanol in a small test tube. Then add a 1% potassium permanganate solution
drop by drop with shaking until a dark precipitate forms or the purple color of the permanganate
persists. Run this test on the same compounds used in the bromine test above and record your
observations. If decolorization doesn't occur in ~1 minute, allow the mixture to stand for 5
minutes with occasional shaking.
Perform both the above tests with the two unknowns and report your observations and
conclusions.
Reagents for Detection and Classification of Alcohols
Chromic Acid
This reagent provides a rapid method for the detection of alcohol functions and also allows for
distinguishing primary and secondary alcohols from tertiary alcohols. When a solution of the
alcohol in acetone is treated with several drops of the chromic acid solution, the orange-red color
of the chromic acid is discharged and a green to blue opaque suspension of chromium(III) sulfate
is formed. Primary and secondary alcohols react within 2 seconds but tertiary alcohols do not.
Tertiary alcohols will eventually react when allowed to stand for longer periods of time in the
strongly acidic solution. Aldehydes give a positive tests, but olefins, acetylenes, amines, ethers,
and ketones give negative tests within 2 seconds.
RCH 2OH
+
H2CrO4
+
H2SO4
RCO 2H
+
Cr2(SO 4) 3
+
H2O
R 2CHOH
+
H2CrO4
+
H2SO4
R2CO
+
Cr2(SO 4) 3
+
H2O
Procedure: Add 1 drop of the liquid to be tested to 1 mL of reagent grade acetone in a small test
tube. Then add 1 drop of the chromic acid reagent and note the result within 2 seconds. A
positive test is for the orange color of the chromic acid to be replaced by a green to blue opaque
suspension. Run this test on (1) 1-butanol, (2) 2-butanol, and (3) 1,1 –dimethylethanol (tbutanol) and record your observations.
Hydrochloric Acid-Zinc Chloride (Lucas Reagent)
Only tertiary alcohols and alcohols that can form carbocations of equal stability to tertiary
carbocations react with concentrated hydrochloric acid to produce alkyl chlorides. However,
alcohols are more reactive towards Lucas reagent than with concentrated hydrochloric acid. The
zinc chloride is a potent Lewis acid and forms such a strong Lewis acid-base complex with the
hydroxyl group that it becomes a better leaving group than even water. Thus, Lucas reagent
allows for distinguishing primary, secondary, and tertiary alcohols from one another. Primary
alcohols are essentially inert towards Lucas reagent at ordinary temperatures, but secondary and
tertiary alcohols are converted into their alkyl chlorides at easily discernible different rates by
Lucas reagent. Because a positive test depends upon the appearance of the alkyl chloride as a
second liquid phase, the test is clearly limited to alcohols that are soluble in the reagent. This
usually limits the test to monofunctional alcohols with less than 6 carbons and to certain
polyfunctional alcohols such as glycols. Tertiary alcohols give positive tests in less than 1
minute, whereas secondary alcohols begin to show milky emulsions within 5 minutes with a
distinct layer visible in about 10 minutes.
H
R 3COH
HCl
R 3C
O
H
H2O
R3C + Cl
R 3CCl
Cl
H
R 2CHOH
ZnCl 2
Cl
R2CH
O
ZnCl
HOZnCl
R 2CHCl
H
R 3COH
ZnCl 2
R 3C
O
ZnCl
Cl
HOZnCl
R 3C + Cl
R 3CCl
Procedure: To 3 drops of the alcohol in a small test tube add 1 mL of Lucas reagent
which has been warmed to ~25 oC. Using a gloved finger, stopper the tube and shake. Allow
the mixture to stand and note the time required for the formation of the alkyl chloride, which
appears as a milky emulsion or a second layer. Run this test on (1) 1-butanol, (2) 2-butanol, (3)
1,1 –dimethylethanol (t-butanol) and record your observations. Repeat this procedure using
concentrated hydrochloric acid in place of the Lucas reagent and record your observations.
Perform the tests for detection and classification of alcohols with the two unknowns given by
your TA and report your observations and conclusions. Record the unknown code in you lab
notebook.
Lab Notebook Preparation
Prior to lab, prepare tables similar to the example provided for each of the five tests that are to be
performed during the lab period. These will serve for all of your data and observation entries.
Conclusions should also be entered directly into the tables and turned in by the end of the lab
period you complete the experiment in. Some points, which are normally assigned to the postlab
exercise have been redistributed to these conclusions. The experiment’s Postlab Exercise is due
one week following the completion of the experiment.
NOTE: Some of the reactions can (or should be) observed immediately; others should be
observed over a period of time.
WASTE DISPOSAL INFORMATION
1.
Detection of Unsaturation Tests
a. Br2 in CCl4 TEST: Discard ALL test solutions in Halogenated Waste Container.
b. KMnO4 (aq) TEST: Test solutions containing Cyclohexene & Hexane go into NON-halogenated
Waste Container.
2. DETECTION & CLASSIFICATION OF ALCOHOLS
Use sink for disposal (with plenty of running water).
Name:__________________________________ Section:________ Date:_____________
POSTLAB EXERCISE FOR: QUALITATIVE TESTS FOR IDENTIFICATION OF CARBON-CARBON
UNSATURATION AND ALCOHOL FUNCTIONS (12/23/2009)
>>Due the lab following the completion of the experimental portion of the notebook (30 pts.
TOTAL). Please answer the questions on this form. <<
SUMMARY (21pts)
The summary is in the form of conclusions for each of the tests performed and (should be)
included in your notebook’s experimental section.
1. Conclusions for the unsaturation tests (3pts/test = 12 pts.).
2. Conclusions for the alcohol tests (2pts/test = 10 pts.).
QUESTIONS (9pts)
Based upon the background information provided on the above qualitative tests and your
observations, answer the following two questions on this form and include turn them in
with your lab notebook pages.
1. (3pts) A compound that is known to be one of the three shown below gives a negative
bromine test, but a positive permanganate test. Which is the correct structure?
OH
CH3CH2
H
C
CH3CH2CH2C(CH 3) 2
CH2CH3
B
H
C
C
H
A
HO2C
C
CO2H
H
C
2. (6pts) An alcohol gives a positive Lucas test, but does not react with concentrated
hydrochloric acid. Would you expect this alcohol to give a positive or negative chromic acid
test? Explain.