Functional Group Tests
2,4 Dinitrophenylhydrazine Test
Shows positive test for:
Aldehydes and ketones
Reactions:
Reacts with the carbonyl group of aldehydes and ketones
How to perform the test:
Five drops of the compound to be tested are mixed with 5 drops of the
dinitrophenylhydrazine reagent (an orange solution) in 2 ml of ethanol and the tube
shaken. If no positive test is observed immediately, the mixture should be allowed to
stand for 15 minutes.
A positive test is indicated by:
The formation of a yellow, orange or orange-red precipitate.
The KMnO4 Test
Shows positive test for:
Aldehydes, alkenes and phenols
Reactions:
Aldehydes are oxidized to carboxylic acids
Alkenes are oxidized to glycols
Functional Group Tests
Monohydroxy phenols do not yield stable products but dihydroxyl phenols can be
oxidized to quinones
How to perform the test:
Three drops of the compound to be tested are dissolved in 2 ml of water or aqueous
ethanol. 2% KMnO4 solution (a purple solution) is added drop wise and the solution is
shaken.
Indications of a positive test:
The purple color of the KMnO4 solution disappears and a precipitate of MnO2 is formed.
A negative test (left) and a positive test (right)
Lucas Test - distinguishes between 1o, 2o, and 3o alcohols
Lucas' reagent is a solution of saturated and anhydrous zinc chloride in concentrated
hydrochloric acid. This solution is used to classify alcohols of low molecular weight. The
reaction is a substitution in which the chloride replaces a hydroxyl group.
Lucas Test for Secondary and Tertiary Alcohol
Functional Group Tests
Alcohols 1-Butanol, 2-Butanol, t-Butyl alcohol.
Procedures
To 0.2 mL or 0.2 g of the unknown in a test tube add 2 mL of the Lucas reagent at room
temperature. Stopper the tube and shake vigorously, then allow the mixture to stand. Note the
time required for the formation of the alkyl chloride, which appears as an insoluble layer or
emulsion.
The Lucas reagent is already prepared for you.
Positive test
Appearance of a cloudy second layer or emulsion
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3o alcohols: immediate to 2-3 minutes
2o alcohols: 5 -10 minutes
1o alcohols: no reaction
Functional Group Tests
Complete the following table by indicating the functional groups present on each compound
and its water solubility (soluble, borderline, insoluble). You can review the structure of each
compound by clicking on its name. If you have access to a printer, printing this page would be
helpful.
compound
bp (oC)
propanal
49
4-methyl benzaldehyde
204
ethanol
78
1-hexanol
158
t-butanol
82
1-butanol
117
acetone
56
cyclohexene
83
phenol
182
methylcyclohexane
101
functional group
water solubility
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Functional Group Tests
pentane
36
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Post Questions:
Consider the two functional group tests below.
a. Tollen’s Test - gives a positive result for aldehydes but not for ketones
b. Lucas Test - distinguishes between 1o, 2o, and 3o alcohols.