Organic Chemistry

812 CHAPTER 19 Carbonyl Compounds III

Consequently, a successful Claisen condensation requires an ester with two

and an equivalent amount of base rather than a catalytic amount of base.

When the reaction is over, addition of acid to the reaction mixture reprotonates the

ester anion. Any remaining alkoxide ion that could cause the reaction to re-

verse would also be protonated.

PROBLEM 24

Give the products of the following reactions:

a. b.

PROBLEM 25

Explain why a Claisen condensation product is not obtained from esters such as ethyl

benzoate and ethyl 2-methylbutanoate.

19.15 The Mixed Claisen Condensation

A mixed Claisen condensationis a condensation reaction between two different es-

ters. Like a mixed aldol addition, a mixed Claisen condensation is a useful reaction

only if it is carried out under conditions that foster the formation of primarily one

product. Otherwise, a mixture of products that are difficult to separate will be formed.

Primarily one product will be formed if one of the esters has no (and

therefore cannot form an enolate) and the other ester is added slowly so that the ester

without is always in excess.

A reaction similar to a mixed Claisen condensation is the condensation of a ketone

and an ester. Because the of a ketone are more acidic than those of an

ester, primarily one product is formed if the ketone and the base are each added slow-

ly to the ester. The product is a Because of the difference in acidities of the

primarily one condensation product is obtained even if both reagents

have

ethyl acetate

excess

condensation of a ketone and an ester

O

++CH 3 C OCH 2 CH 3 CH 3 CH 2 OH

1. CH 3 CH 2 O−


2. HCl

O O

O

CCH 3

add slowly a -diketone

a-hydrogens.

a-hydrogens,

b-diketone.

a-hydrogens

a-hydrogens

a-hydrogens

1. CH 3 CH 2 O−


2. HCl

CH 3 CHCH 2 COCH 2 CH 3

O

CH 3

1. CH 3 O−

CH 3 CH 2 CH 2 COCH (^3) 2. HCl
O
b-keto
a-hydrogens

1. CH 3 CH 2 O−


2. HCl

O

O C OCH 2 CH 3

O

C

O

CHCOCH 2 CH 3

add slowly excess

CH 3 CH 2 CH 2 COCH 2 CH 3 +

+ CH 3 CH 2 OH

CH 2 CH 3

a mixed Claisen condensation

CH COCH 3 CH 3 CH 2 OH

O O

R

C− COCH 3 RCH 2 C

O O

R

RCH 2 C

HCl

+ CH 3 CH 2 O− +