Intramolecular reaction and intramolecular catalysisd̊ II. Mechanism of intramolecular catalysis in hydrolysis of 2-(2'-benzimidazolyl)phenyl acetate by benzimidazolyl group

As a model for acetyl-α-chymotrypsin, 2-(2'-benzimidazolyl)phenyl acetate was synthesized in a good yield and its hydrolytic kinetics has been studied in buffer solutions at 45'C within a wide pH range (0.65–8.80). The measured pseudo-first-order rate constants (k_obs) at different pH values conform well with the data derived from the proposed theoretical model. It is shown that in an acidic solution of medium strength (pH = 2.0–3.0), the ester is hydrolyzed via intramolecular general acid catalysis by benzimidazolium cation; while at pH values near neutrality (pH = 5.5–7.0), this ester is hydrolyzed via intramolecular general base catalysis by neutral benzimidazole. These findings are discussed and the activation parameters of the hydrolysis reaction were also determined.