Ethyl Cyclohexanone-2-carboxylate in Aqueous Micellar Solutions. 1. Ester Hydrolysis in Cationic and Nonionic Micelles

Abstract

Hydrolysis rates for the ester of ethyl cyclohexanone-2-carboxylate (ECHC) were measured in aqueous dioxane mixtures and in aqueous micellar solutions of cationic, nonionic, and mixed cationic−nonionic detergents. The following cationic surfactants were used: dodecyltrimethylammonium bromide (DTABr), tetradecyltrimethylammonium bromide (TTABr), tetradecyltrimethylammonium chloride (TTACl), tetradecyltrimethylammonium acetate (TTAAc), and cetyltrimethylammonium bromide (CTABr). Poly(ethylene oxide)-9-dodecyl ether (C12E9) and poly(ethylene oxide)-20-cetyl ether (C16E20) were used as the nonionic surfactants. The kinetic profiles of the reactions were investigated as a function of surfactant concentration. In aqueous strong mineral acid (HCl or HBr) solutions of the cationic surfactants DTABr, TTABr, TTACl, and CTABr, the reaction was greatly inhibited at surfactant concentrations above the critical micelle concentration; the same behavior was observed for the influence of the nonionic surfactants and for the mixed surfactants DTABr + C12E9 analyzed in an equimolar mixture of both surfactants. Micellar effects were analyzed by means of the pseudophase model. When the hydrolysis reaction was studied in aqueous buffered solutions of acetic acid−acetate, the shape of the observed rate constant versus surfactant concentration (of TTACl and TTABr) profiles depended on surfactant concentration. At low surfactant concentration, first-order rate constants pass through maxima with increasing surfactant concentration followed by a gradual but steady decrease in the rate as the surfactant concentration increases further. By contrast, the first-order rate constant increases throughout the whole concentration range of TTAAc used. Micellar effects observed for the reaction in aqueous buffered solutions were analyzed qualitatively or quantitatively by means of the pseudophase ion exchange (PPIE) model. In every experimental situation, the reaction at the micellar interface is not negligible; however, the second-order rate constant in this region is lower than that measured in water.