Imidazole catalyses in aqueous systems. IV. bimolecular and michaelis-menten-type catalyses of a phenyl ester hydrolysis by some hydrophobic imidazole Derivatives

Catalytic hydrolyses of p-Acetoxybenzoic acid by several imidazole derivatives were conducted at 15-40C in aqueous systems using a pH-stat. Imidazole compounds containing the naphthalene ring catalyzed the hydrolysis following the Michaelis-Menten kinetics as in enzymatic reactions, while catalysis by less hydrophobic imidazoles followed the second-order kinetics. Enhancement of the overall catalytic efficiency in the Michaelis-Menten pathway was estimated to be ten times at most as compared with the bimolecular pathway. The thermodynamic and activation parameters were obtained at 15-40C for the Michaelis-Menten pathway. The binding of catalyst and substrate was characterized by small positive enthalpy changes (4H +3.5 kcal/ mol) and large positive entropy changes (AS +28 eu), indicating that the complex formation was ascribable to hydrophobic interactions. The intra-complex product formation was characterized by not unreasonable 4H values and extraordinarily large negative 4S ( 50 eu) values. The unexpectedly small rate enhancement in the Michaelis-Menten pathway was attributed to the unfavorable 4S' term, which suggested that the structure of the Michaelis complex was quite different from that of the transition state of the product formation.