A Guidebook to Mechanism in Organic Chemistry

Acid Catalysed Reactions

place. It is found however, that if the acid is dissolved in concentrated

H 2 S0 4 and the resultant solution poured into cold methanol, ready

esterification takes place. Similarly the methyl ester, which is highly

resistant to acid hydrolysis under normal conditions, may be

reconverted to the acid merely by dissolving it in concentrated

H 2 S0 4 and then pouring this solution into cold water.

The clue to what is taking place is provided by the fact that dissolving

2,4,6-trimethylbenzoic acid in H 2 S0 4 results in a four-fold depression

of the latter's freezing point due to the ionisation:

RC0 2 H + 2H 2 S0 4 ^ R • 0=0 + H 3 Oe + 2HS0 4 e

The resultant acylium ion (LII) would be expected to undergo

extremely ready nucleophilic attack, e.g., by MeOH, and as -C=0

has a linear structure

attack by MeOH can take place at right angles to the plane of the
benzene ring and is thus not impeded by the two flanking methyl
groups. The same acylium ion is obtained on dissolving the methyl
ester in HjS^ and this undergoes equally ready attack by H 2 0 to
yield the acid. Benzoic acid itself and its esters do not form acylium
ions under these conditions, however. This is probably due to the fact
that whereas protonated benzoic acid can stabilise itself by delocalisa­
tion (LIII),

(LII)

O

H

c

(LII)

HO, OH HQ OH

C

(LIII)