Effective Charge Development in Ester Hydrolysis Catalyzed by Cationic Micelles

Article abstract of DOI:10.1021/jo00294a009

Reaction of hydroxide ion with substituted phenyl laurates is catalyzed by micelles containing CTAB (cetyltrimethylammonium bromide).The observed rate constants for ester consumption obey a Michaelis-Menten-like rate law, k_obsd = (k_OHK_eq + k_OH^cat)/(K_eq + ), where k_OH^cat is the second order rate constant for reaction of hydroxide ion with micelle-bound ester, K_eq is a partition coefficient for dissociation of ester from the micelle-bound ester, and k_OH is the second order rate constant for reaction of free ester with hydroxide ion.The value of the parameter K^eq is independent of the nature of the substituent on the ester.The ratio k_OH^cat/K_eq represents the free energy change from ester in aqueous solution to the transition state of the ester reaction in the micellar pseudophase.The dependence of log k_OH^cat/K_eq on the pK_a of the leaving phenol obeys a linear Bronsted equation with β_1g = -0.51, and log k^OH obeys a linear Bronsted equation with β_1g = -0.56.Since both Bronsted parameters refer to catalyzed and uncatalyzed reactions starting from the same standard state (aqueous solution), the effective charge in the transtition state of both reactions is almost the same.The microscopic medium is thus similar for both cases, consistent with the COOAr moiety's residing in an aqueous-like region of the pseudophase in the catalytic process.