Methylimidazole-Catalyzed Ester Hydrolysis: Nonlinear Kinetics

Article abstract of DOI:10.1021/jo00283a015

The rate constants for acyl transfer from some p-nitrophenyl esters (of trimethylacetate, -butyrate, and -acetate) and from phenyl acetate to imidazole and to the various monomethylimidazole derivatives were examined over a wide range of amine concentration (0.03-3 M).These are all effective nucleophilic catalysts for ester hydrolysis.At low amine concentrations (with limiting ester) the rate constants for imidazole-catalyzed and 1-methylimidazole-catalyzed hydrolysis are nearly identical (2-methylimidazole is ca. 3 times smaller and 4-methylimidazole is ca. 3 times larger with the p-nitrophenyl esters).At high concentrations, however, the reactivities of the methylimidazoles differ markedly from that of imidazole.The observed pseudo-first-order rate constants appear to be an essentially linear function of imidazole concentration (up to ca. 0.5 M imidazole) but show a nonlinear dependence on methylimidazole concentration.With increasing methylimidazole concentration the reaction varies from first order to zero order in catalyst concentration.Ester hydrolysis in thus inhibited at high methylimidazole concentrations relative to the reaction with imidazole.Various explanations for this phenomena are explored, and results are interpreted in terms of a possible noncovalent complexation between the methylimidazoles and the ester.