On the relative leaving group abilities of imidazole and the substituted phenoxide ions. The effect of the acyl group

Rate coefficients for the imidazole‐catalysed hydrolysis of substituted phenyl acetates, chloroacetates and dichloroacetates have been determined in 40% (v/v) acetonitrile‐water. It is shown that the increasing electron‐withdrawal by the acyl group makes the general base‐catalysed nucleophilic reaction of imidazole detectable beside the uncatalysed nucleophilic attack of the amine. This is attributed to the acyl group‐induced change of the partitioning ratio of imidazole and the ester leaving group. A kinetic evidence is obtained of a change in the rate‐limiting step with increasing imidazole concentration for the imidazole‐catalysed reactions of 4‐chloro‐ and 4‐methoxyphenyl dichloroacetates.