The nucleophilic displacement of alpha-substituents of diphenylacetic acid and its methyl ester by MeOH under a variety of experimental conditions allowed us to establish that the reaction occurs rapidly in one of the following structural combinations: (a) carboxylate anion and a good leaving group; (b) carboxylic group and a less reactive leaving group; (c) methyl ester and proton-activated, poor leaving group. The importance of internal hydrogen bonding (case b) was the governing factor in the outcome of the reaction of hydroxydiphenylacetic acid (3a) and its methyl ester 3b with acetyl chloride. The key intermediate in substitution at the alpha-carbon is though to be a carbocation stabilized both by the attached phenyl groups and, to a smaller extent, by the carboxyl group.
