Organic Chemistry

850 CHAPTER 20 More About Oxidation–Reduction Reactions

SOLUTION TO 7a The six-membered ring indicates that the compound can be synthe-

sized by means of a Diels–Alder reaction.

20.2 Oxidation of Alcohols

Oxidation is the reverse of reduction. For example, a ketone is reducedto a secondary

alcohol, and the reverse reaction is the oxidationof a secondary alcohol to a ketone.

A reagent that is often used to oxidize alcohols is chromic acid which is

formed when chromium trioxide or sodium dichromate is dis-

solved in aqueous acid. These reactions are easily recognized as oxidations because

the number of bonds in the reactant decreases and the number of bonds

increases.

Primary alcohols are initially oxidized to aldehydes by these reagents. The reaction,

however, does not stop at the aldehyde. Instead, the aldehyde is further oxidized to a

carboxylic acid.

Notice that the oxidation of either a primary or a secondary alcohol involves re-

moval of a hydrogen from the carbon to which the OH is attached. The carbon bearing

the OH group in a tertiary alcohol is not bonded to a hydrogen, so the OH group can-

not be oxidized to a carbonyl group.

Chromic acid and the other chromium-containing oxidizing reagents oxidize an al-

cohol by first forming a chromate ester. The carbonyl compound is formed when the

chromate ester undergoes an E2 elimination (Section 11.1).

CH 3 CH 2 CH 2 CH 2 OH CH 3 CH 2 CH 2 CH

O

a primary alcohol an aldehyde

CH 3 CH 2 CH 2 COH

O

a carboxylic acid

H 2 CrO 4 further

oxidation

[]

OH

OH

O

O

O

CH 3 CH 2 CHCH 3

OH

CHCH 2 CH 3

CH 3 CH 2 CCH 3

secondary alcohols ketones

CrO 3

H 2 SO 4

Na 2 Cr 2 O 7

H 2 SO 4

H 2 CrO 4

CCH 2 CH 3

C¬H C¬O

(CrO 3 ) (Na 2 Cr 2 O 7 )

(H 2 CrO 4 ),

ketone secondary alcohol

reduction

oxidation

H 2

Raney Ni

O O

H ∆ HOH

+

3-D Molecules:

Chromic acid;

Chromium trioxide;

Sodium dichromate

a chromate ester

Cr OH

O

O

HO Cr OH

H

+

O

O

HO Cr OH

H

O

+

+

O

O

RCH 2

RCH 2 OH H 2 O B

Cr OH H 2 CrO 3

HB+

H

O

O

O

RCH RCHRCH O+

HB + H 2 O

an E2 reaction

mechanism for alcohol oxidation by chromic acid

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