FUNCTIONAL MICELLES .8. MICELLAR CATALYSIS OF THE HYDROLYSIS OF SUBSTITUTED PHENYL CARBOXYLATES BY IMIDAZOLE CONTAINING CATIONIC SURFACTANTS

Micellar catalysis of dimethyl[2-(4-imidazolyl)ethyl]octadecylammonium chloride (2) and dimethyl [(4-imidazolyl) methyl]octadecylammonium chloride (la) has been investigated for the hydrolysis of substituted phenyl esters of carboxylic acids (5). The catalysis was observed to occur through the rapid formation and decomposition of the acylimidazole intermediate for both neutral and anionic forms of 2 and la (pH 7-9). Observed Michaelis-Menten type kinetics for the acylation step were analyzed to obtain catalytic rate constants (km) and the apparent association constants (Kapp/N) separately. The effect of acyl chain length and the substituent effects on the leaving phenoxy group were then examined with respect to the km and Kapp/N values. Qualitatively, the Kapp/N values appear to be accounted for in terms of a hydrophobic binding force between the substrate and micelle. However, a case has been observed which suggests a more complex binding process involving multistep equilibria than explained simply by such hydrophobic force. For the km values, a linear relationship was found between log km and ~ constants, which suggests a stabilization of transition state due to an electrostatic interaction between the positive charge of the catalyst ammonium group and the negative charge of the leaving phenoxy group. The temperature effects on the micellization of 2 and on the km and Kapp/N values of p-methyl- and p-nitrophenyl hexanoates were also examined. The resulting thermodynamic parameters for the binding process (Kapp/N) were found to be widely different for both magnitude and sign with respect to the two esters. © 1979, American Chemical Society. All rights reserved.