A Guidebook to Mechanism in Organic Chemistry

Acid Catalysed Reactions

does not play any significant part in the amide formation. Both

transesterification and amide formation from the ester are reversible'

unlike alkaline ester hydrolysis, as the carboxylate anion is not

involved.

The reactions of acid chlorides show a number of resemblances to

the nucleophilic displacement reactions of alkyl halides, proceeding

by uni- and bi-molecular mechanisms, the actual path followed being

markedly affected by the polarity and ion-solvating ability of the

medium (cf. p. 60) as well as by the structure of the substrate. The

reactioafcafiacid anhydrides are in many ways intermediate between

those of acyl halides, in which the group that is ultimately expelled

shows a considerable readiness to be lost as an anion, and esters in

which the leaving group normally requires assistance for its ultimate

displacement.

(ii) Acid catalysed reactions

Esters also undergo acidic hydrolysis, initial protonation being

followed by nucleophilic attack by H 2 0; acyl-oxygen cleavage is

again observed:

O OH ^ OH

II II® I

R—C—OEt ^=±= R—C—OEt ^=±: R—C—OEt

L

H^O*

i» v.

H,6^

11

O OjH OH

II -H& \* —EtOH I ©

R—C =F=i R—C® ^=i= R—CVpEt

I I l^H

OH HO HO

Unlike alkaline hydrolysis, the overall reaction is experimentally
reversible and esters are commonly made by protonation of the
carboxylic acid followed by nucleophilic attack of ROH, an excess of
the latter normally being employeTj so as to displace the ?= in the
desired direction, Esters, R-CO-OR, also undergo ester exchange
with R'OH under these conditions. Esters of tertiary alcohols,
however, have been shown by lsO labelling experiments to undergo
alkyl-oxygen cleavage on hydrolysis