Advices For Studying Organic Chemistry

O Cr

R

C

R

R

O

O

O OH

R

C

R

R

H

+ − O Cr

O

O

OH+ H+ + H 2 O

3°Alcohol This chromate ester cannot
undergo elimination of H 2 CrO 3

12.4E A CHEMICAL TEST FOR PRIMARY AND SECONDARY ALCOHOLS

1. The relative ease of oxidation of 1° and 2° alcohols compared with the difficulty
   of oxidizing 3° alcohols forms the basis for a convenient chemical test.

1. 1° and 2° alcohols are rapidly oxidized by a solution of CrO 3 in aqueous sulfuric
   acid.


2. Chromic oxide (CrO 3 ) dissolves in aqueous sulfuric acid to give a clear orange
   solution containing Cr 2 O 7 2– ions.


3. A positive test is indicated when this clear orange solution becomes opaque and
   takes on a greenish cast within 2 s.

Clear orange solution Greenish opaque solution

CrO 3 /aqueous H 2 SO 4 Cr3+ and oxidation products

RCH 2

or

R CH

R

+

OH

OH

i) This color change, associated with the reduction of Cr 2 O 7 2– to Cr3+, forms

the basis for “Breathalyzer tubes” used to detect intoxicated motorists.

In the Breathalyzer the dichromate salt is coated on granules of silica gel.

ii) This test not only can distinguish 1° and 2° alcohols from 3° alcohols it also

can distinguish them from other compounds except aldehydes..

12.4F SPECTROSCOPIC EVIDENCE FOR ALCOHOLS

1. Alcohols give rise to O–H stretching absorptions from 3200 to 3600 cm–1 in