Qual Test Exp - Chemistry
Qual Test Exp - Chemistry
Qual Test Exp - Chemistry
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
QUALITATIVE TESTS FOR IDENTIFICATION OF CARBON-CARBON<br />
UNSATURATION AND ALCOHOL FUNCTIONS (Rev’d 2/16/2004)<br />
In this experiment we will examine two reagents (bromine in carbon tetrachloride and<br />
aqueous potassium permanganate) which are widely used for qualitative identification of<br />
carbon-carbon unsaturation and three reagents (chromic acid, hydrochloric acid, and<br />
hydrochloric-zinc chloride) which are widely used for qualitative identification of<br />
alcohols.<br />
Please handle these reagents with care and avoid contact with skin. Bromine causes<br />
chemical burns, permanganate causes stubborn to remove stains, and chromic acid and<br />
Lucas reagent are very strongly acidic solutions.<br />
Reagents for Detection of Unsaturation<br />
Bromine in Carbon Tetrachloride<br />
This reagent is commonly used to identify the presence of olefinic or acetylenic functions.<br />
The red bromine color of this reagent is discharged when the unsaturated function reacts<br />
with the bromine. This reagent should be used in conjunction with the aqueous<br />
permanganate reagent to avoid erroneous conclusions. Erroneous results with the bromine<br />
reagent may occur for two reasons. Firstly, not all olefinic or acetylenic compounds take<br />
up bromine. When electron withdrawing groups are attached to the unsaturated carbon<br />
atoms the uptake of bromine is considerably slowed and in extreme cases completely<br />
stopped.<br />
C 6 H 5 CH CH 2 + Br<br />
fast<br />
2 C 6 H 5 CHBrCH 2 Br<br />
slow<br />
C 6 H 5 CH CHCO 2 H + Br 2 C 6 H 5 CHBrCHBrCO 2 H<br />
Secondly, some other functionalities discharge the bromine color but with the evolution of<br />
hydrogen bromide. That is to say, they undergo bromine substitution instead of bromine<br />
addition. Compounds which undergo substitution extremely rapidly are phenols, enols,<br />
and anilines. Thus a positive test for unsaturation is one that discharges the bromine color<br />
without the evolution of hydrogen bromide.<br />
Procedure: Add 1 drop of the liquid (or the tip of a microspatula of a solid plus 3 drops of<br />
ethanol) to a small test tube and then add a 5% (0.3 M) solution of bromine in carbon<br />
tetrachloride drop by drop (with agitation) until the bromine color remains. Run this test<br />
on (1) cyclohexene, (2) cyclohexane, (3) ethanol, and (4) cinnamic acid and record your<br />
observations. For the cinnamic acid, let the test tube sit and check it periodically during<br />
the remainder of the lab period to see how long it takes for the bromine color to disappear.
Aqueous Potassium Permanganate<br />
The purple permanganate color of this reagent is discharged by olefinic and acetylenic<br />
functions. This is known as the Baeyer test for unsaturation. In cold (room temperature or<br />
below) dilute aqueous solutions the main products from the reaction of an olefin with<br />
permanganate are the corresponding vicinal diol and manganese dioxide (brown solid).<br />
Acetylenes produce the corresponding diones. If heated these initial products undergo<br />
further oxidation, leading eventually to carbon chain cleavage with the formation of<br />
carboxylic acids. This test is very general and gives positive tests with those unsaturated<br />
compounds that do not decolorize bromine in carbon tetrachloride.<br />
RCH CHR + KMnO 4 + 2 H 2 O<br />
RCH<br />
CHR<br />
+<br />
MnO 2<br />
+ 2 KOH<br />
OH<br />
OH<br />
RCH<br />
OH<br />
CHR<br />
OH<br />
further<br />
oxidation<br />
2 RCO 2 H<br />
Two cautions are in order when using the Baeyer test. Firstly, aldehydes give positive<br />
tests because they are readily oxidized to carboxylic acids by permanganate. Secondly,<br />
although pure alcohols do not react within 5 minutes, they often contain trace impurities<br />
that are easily oxidized and give a slight reaction. Likewise alkanes often contain trace<br />
amounts of olefins as impurities and may give a slight reaction. For these reasons<br />
decolorization of a drop or two of the reagent should not be accepted as a positive test.<br />
Procedure: Add 4 drops of the liquid (or the tip of a microspatula of a solid) to be tested<br />
to 1 mL (~20 drops) of ethanol in a small test tube. Then add a 1% potassium<br />
permanganate solution drop by drop with shaking until a dark precipitate forms or the<br />
purple color of the permanganate persists. Run this test on the same compounds used in<br />
the bromine test above and record your observations. If decolorization doesn't occur in ~1<br />
minute, allow the mixture to stand for 5 minutes with occasional shaking.<br />
Perform both the above tests with the two unknowns and report your observations and<br />
conclusions.<br />
Chromic Acid<br />
Reagents for Detection and Classification of Alcohols<br />
This reagent provides a rapid method for the detection of alcohol functions and also allows<br />
for distinguishing primary and secondary alcohols from tertiary alcohols. When a solution<br />
of the alcohol in acetone is treated with several drops of the chromic acid solution, the<br />
orange-red color of the chromic acid is discharged and a green to blue opaque suspension<br />
of chromium(III) sulfate is formed. Primary and secondary alcohols react within 2 seconds<br />
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 test,<br />
but olefins, acetylenes, amines, ethers, and ketones give negative tests within 2 seconds.<br />
RCH 2 OH + H 2 CrO 4 + H 2 SO 4 RCO 2 H + Cr 2 (SO 4 ) 3 + H 2 O<br />
R 2 CHOH + H 2 CrO 4 + H 2 SO 4 R 2 CO + Cr 2 (SO 4 ) 3 + H 2 O<br />
Procedure: Add 1 drop of the liquid to be tested to 1 mL of reagent grade acetone in a<br />
small test tube. Then add 1 drop of the chromic acid reagent and note the result within 2<br />
seconds. A positive test is for the orange color of the chromic acid to be replaced by a<br />
green to blue opaque suspension. Run this test on (1) 1-butanol, (2) 2-butanol, and<br />
(3) 2-methyl-2-propanol and record your observations.<br />
Hydrochloric Acid-Zinc Chloride (Lucas Reagent)<br />
Only tertiary alcohols and alcohols that can form carbocations of equal stability to tertiary<br />
carbocations react with concentrated hydrochloric acid to produce alkyl chlorides.<br />
However, alcohols are more reactive towards Lucas reagent than with concentrated<br />
hydrochloric acid. The zinc chloride is a potent Lewis acid and forms such a strong Lewis<br />
acid-base complex with the hydroxyl group that it becomes a better leaving group than<br />
even water. Thus Lucas reagent allows for distinguishing primary, secondary, and tertiary<br />
alcohols from one another. Primary alcohols are essentially inert towards Lucas reagent at<br />
ordinary temperatures, but secondary and tertiary alcohols are converted into their alkyl<br />
chlorides at easily discernible different rates by Lucas reagent. Because a positive test<br />
depends upon the appearance of the alkyl chloride as a second liquid phase, the test is<br />
clearly limited to alcohols that are soluble in the reagent. This usually limits the test to<br />
monofunctional alcohols with less than 6 carbons and to certain polyfunctional alcohols<br />
such as glycols. Tertiary alcohols give positive tests in less than 1 minute, whereas<br />
secondary alcohols begin to show milky emulsions within 5 minutes with a distinct layer<br />
visible in about 10 minutes.<br />
H<br />
R 3 COH HCl R 3 C O<br />
H<br />
H 2 O<br />
R 3 C<br />
+ Cl R 3 CCl<br />
Cl<br />
R 2 CHOH ZnCl 2<br />
Cl<br />
R 2 CH<br />
H<br />
O<br />
ZnCl<br />
HOZnCl<br />
R 2 CHCl<br />
R 3 COH<br />
ZnCl 2<br />
R 3 C<br />
H<br />
O<br />
ZnCl<br />
HOZnCl<br />
R 3 C<br />
+ Cl R 3 CCl<br />
Cl<br />
Procedure: To 3 drops of the alcohol in a small test tube add 1 mL of Lucas reagent
which has been warmed to ~25 o C. Using a gloved finger, stopper the tube and shake. Allow<br />
the mixture to stand and note the time required for the formation of the alkyl chloride,<br />
which appears as a milky emulsion or a second layer. Run this test on (1) 1-butanol, (2) 2-<br />
butanol, (3) 2-methyl-2-propanol and record your observations. Repeat this procedure<br />
using concentrated hydrochloric acid in place of the Lucas reagent and record your<br />
observations.<br />
Perform the tests for detection and classification of alcohols with the two unknowns and<br />
report your observations and conclusions.<br />
Lab Notebook Preparation<br />
Prior to lab, prepare tables similar to the example provided for each of the five tests that<br />
are to be performed during the lab period. These will serve for all of your data and<br />
observation entries. Conclusions should also be entered directly into the tables and turned<br />
in by the end of the lab period you complete the experiment in. Some points, which are<br />
normally assigned to the postlab exercise, have been redistributed to these conclusions.<br />
The experiment’s Postlab Exercise is due one week following the completion of the<br />
experiment.<br />
NOTE: Some of the reactions can (or should be) observed immediately; others should be<br />
observed over a period of time.<br />
WASTE DISPOSAL INFORMATION<br />
1. Detection of Unsaturation <strong>Test</strong>s<br />
a. Br 2 in CCl 4 TEST: Discard ALL test solutions in Halogenated Waste Container.<br />
b. KMnO 4 (aq) TEST: Discard ALL test solutions in NON-halogenated Waste Container.<br />
2. DETECTION & CLASSIFICATION OF ALCOHOLS<br />
Use sink for disposal (with plenty of running water).
Name:_______________________________Section:________ Date:___________<br />
POSTLAB EXERCISES FOR: QUALITATIVE TESTS FOR IDENTIFICATION OF CARBON-<br />
CARBON UNSATURATION AND ALCOHOL FUNCTIONS (Rev’d 2/16/2004)<br />
‣ Due in lab one week following the scheduled experiment. Please answer questions<br />
on this form.<br />
SUMMARY (21pts)<br />
The summary is in the form of conclusions for each of the tests performed and<br />
(should be) included in your notebook.<br />
1. Conclusions for the unsaturation tests (3pts/test = 12pts).<br />
2. Conclusions for the alcohol tests (3pts/test = 9pts).<br />
QUESTIONS (9pts)<br />
Based upon the background information provided on the above qualitative tests and<br />
your observations answer the following two questions on this form.<br />
1. (3pts) A compound that is known to be one of the three shown below gives a negative<br />
bromine test, but a positive permanganate test. Which is the correct structure<br />
OH<br />
CH 3 CH 2<br />
H<br />
HO 2 C<br />
H<br />
CH 3 CH 2 CH 2 C(CH 3 ) 2<br />
H<br />
C<br />
C<br />
CH 2 CH 3<br />
H<br />
C<br />
C<br />
CO 2 H<br />
A B C<br />
2. (6pts) An alcohol gives a positive Lucas test, but does not react with concentrated<br />
hydrochloric acid. Would you expect this alcohol to give a positive or negative<br />
chromic acid test <strong>Exp</strong>lain.