Imidazole Catalyses in Aqueous Systems. IV. Bimolecular and Michaelis-Menten-Type Catalyses of a Phenyl Ester Hydrolysis by Some Hydrophobic Imidazole Derivatives

Abstract

Abstract Catalytic hydrolyses of p-acetoxybenzoic acid by several imidazole derivatives were conducted at 15–40°C 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 15–40°C for the Michaelis-Menten pathway. The binding of catalyst and substrate was characterized by small positive enthalpy changes (ΔH≅+3.5 kcal/ mol) and large positive entropy changes (ΔSu≅+28 eu), indicating that the complex formation was ascribable to hydrophobic interactions. The intra-complex product formation was characterized by not unreasonable ΔH≠ values and extraordinarily large negative ΔS≠ (≅−50 eu) values. The unexpectedly small rate enhancement in the Michaelis-Menten pathway was attributed to the unfavorable ΔS≠ term, which suggested that the structure of the Michaelis complex was quite different from that of the transition state of the product formation.