A Guidebook to Mechanism in Organic Chemistry

Claisen Ester Condensation

CHgCOjEt

Cf &CH,C0 2 Et

OEt (XXXVII)

+ EtOH

Me—C

O

II

Me—C—CHCOjEt

O

II

Me—C—CH» • COjEt

(XXXIX) +

HOEt

+ (XXXVIII)

eOEt

Normally a gram equivalent of sodium is employed as a source of

the sodium ethoxide catalyst required, but only a little ethanol need

be added initially as more is liberated as soon as the reaction starts,

with the formation of the carbanion (XXXVII). This adds to the

carbonyl group of a second molecule of ester, folio wgd^by the expulsion

of EtO^0 to yield the j8-keto ester (XXXVIII), whiclyf finally conv^ed

into the carbanion (XXXIX), i.e. the 'sodio-derivative' (hence the

need for the employment of a whole gram equivalent of sodium). The

formation of (XXXIX) is an essential feature of the reaction for it

helps to drive the equilibrium (i) over to the right; this is made necessary

by the fact that the carbanion (XXXVII) is not as highly stabilised as,

for examPle,^^^j^u, and is consequently more reluctant to

form. This is reflected in the fact that RaCH-C0 2 Et does not
undergo the reaction in the presence of EtO^0 despite the fact that it
has an a-hydrogen atom and so could form a carbanion, because the
product RjCH-CO-CRvCOgEt cannot be converted to a carbanion
such as (XXXIX) and so fails to drive the equilibrium over to the
right. m
Such esters can however be made to condense satisfactorily in the
presence of very strong bases such as Ph 3 CeNae, for here the initial
carbanion formation is essentially irreversible:

R,CHCO,Et+Ph 3 Ce -* R,CCO,Et+Ph,CH

G 177