Catalysis and inhibition of the alkaline hydrolysis of benzocaine and analogs by a cationic surfactant.

Abstract

The first-order rate constants for the alkaline hydrolysis of ethyl benzoate and ethyl p-nitro- and p-aminobenzoates (benzocaine), catalyzed by CTAB, a cationic surfactant, were measured at 25°in the presence of sodium hydroxide and sodium bromide. Although the alkaline hydrolysis of esters is generally accelerated by cationic micelles, that of the three benzoates tended to be inhibited by CTAB micelles under the following conditions : (i) at high CTAB concentrations, (ii) at high concentrations of sodium hydroxide, (iii) at high concentrations of an added salt (including buffer components), (iv) at high ester concentrations, and (v) for compounds possessing a small value of the Hammett substituent constant. These effects were explained quantitatively on the basis of a theory in which the total counterion concentration is assumed to remain constant regardless of the concentrations of CTAB, sodium bromide, and sodium hydroxide, and in which the association constant for a CTAB micelle and an ester, the degree of counterion binding to CTAB micelles, the ion-exchange constant, and a relative rate constant are taken as parameters.