Dipolar Micelles. 6. Catalytic Effects of Betaine-Like Micelles on the Hydrolysis of Substituted Phenyl Esters

The catalytic effects of five betaine-like micelles on the hydrolysis of substituted phenyl esters were measured. The micellaric catalysts are of the general structure [CH₃(CH₂)_nN+(CH₃)₂(CH₂)_mCOOH]Br_, where n = 9, m = 1 (I); n = 15, m = 1 (IA); n = 9, m = 2 (II); n = 9, m = 3 (III); and n = 15, m = 3 (IIIA); compound IV, n = 0, m = 1, is also included in this study for comparison purposes. The following esters were examined: 2, 4-dinitrophenyl acetate (OPNA); 2,4-dinitrophenyl hexanoate (OPNH); 2,4-dinitrophenyl decanoate (OPND); 4-nitrophenyl decanoate (PND); 3-nitrophenyl decanoate (MND); 2, 5-dinitrophenyl decanoate (OMND). The catalytic rate constants (kn) and the nucleophilic catalysis by hydroxide ion (K_oH) were determined. From the pH-rate profiles of OPNH in I and in IV, it is observed that in spite of the similarities in the pK_a values of I and IV the catalytic reactivity of the latter is inferior to that of I. Therefore, it is possible to conclude that the catalytic efficiency of I stems from the micelle-substrate intracomplex reaction, which is not the case in IV. The isotope effect of 1.1 determined for OPND in micelle I indicates that catalysis occurs via nucleophilic attack by the carboxylate anion. The plot of log ^KoH against log kn in micelle III for esters possessing various leaving groups is nonlinear. Esters having good leaving groups, as in OMND and OPND, fall on a line of slope 1, and the value of the deuterium isotope effect [K_n(H₂O)/ Kn(D₂O)] equals 1.15. On the other hand, for esters bearing poor leaving groups, as in MND and PND, the respective values of the slope and the deuterium solvent isotope effect are 2.2 and 1.95. Therefore, it is suggested that in micelle III the catalytic hydrolysis of OMND and OPND follows the nucleophilic route, whereas that of MND and PND approaches the general-base type route.