Dipolar Micelles. 5. Micellar Effects on the Hydrolysis of Neutral and Charged Esters

The specific base catalyzed hydrolyses of positively charged and neutral esters in three betainelike micelles of structures I—III, and five dipolar micelles IV-VIII, have been studied. These comprised nine different esters; C₁₀H₂₁N⁺(CH₃)₂ZBr^- Z = 2-(p-nitrobenzoyloxy)ethyl (CPNBA), Z = 3-(p-nitrobenzoyloxy)propyl (HCPNBA), Z = 3-(2,4-dinitrophenyloxycarbonyl)propyl (DNPDE⁺); p-nitrophenyl acetate (PNPA); p-nitrophenyl hexanoate (PNPH); p-nitrophenyl decanoate (PNPD); 2,4-dinitrophenyl acetate (DNPA); 2,4-dinitrophenyl decanoate (DNPD); and decyl p-nitrobenzoate (DPNBA). Study has shown that the second-order rate coefficients of PNPA, PNPH, and PNPD enhance with increasing concentration of premicelle aggregates (subunits) and decrease in the presence of micelles. The betainelike micelles exhibit inhibitory effect on rates of hydrolyses of most substrates included in this study. The inhibitory efficiency was found to depend on the positions of both the reaction site and the carboxylate anion of the zwitterionic micelle. It is suggested that proximity of microenvironmental factors affects primarily the course of hydrolyses on the micellar surface.