Changes in the relative contribution of specific and general base catalysis in cationic micelles. The cyclization of substituted ethyl hydantoates

The rate-surfactant profiles for the HO-- and AcO--catalyzed ring closure o f two ethyl hydantoates, E2 and E3, to hydantoins with three cetyltrimethyl ammonium salts (CTAX, X = Br-, Cl-, or AcO-) are measured in 0.02 and 0.2 M acetate buffers 50% base with starting pH 4.65. Marked accelerations assoc iated with large pH increases are found in 0.02 M buffered CTAOAc. Smaller accelerations and smaller pH changes are observed in 0.2 M buffered CTAOAc and CTACl. From these profiles, the micellar rate constants for the specifi c base- and general base-catalyzed reactions, k(2,m)(HO-) and k(2,m)(AcO-), respectively, of E2 and E3 are obtained separately. The resulting values o f k(2,m)(/kw), E2/E3 rate constant ratios, and kinetic solvent isotope effe cts, KSIEs, are consistent with a strong predominance of the HO- reaction i n the dilute buffer, while in the more concentrated buffer, specific and ge neral catalysis compete for the two substrates. This result is in sharp con trast with that observed in water in which the reaction of E2 is almost exc lusively specifically catalyzed. The increase in the general base-catalyzed pathway for E2 is attributed not to an increase in the rate constant for t his pathway in micelles but to a smaller decrease than that for the specifi c catalysis (k(2,m)/(kw), = 0.2 and 0.4 for the specific and general cataly sis, respectively). The different responses of the rate constants to the mi cellar media are interpreted as a larger effect of the interfacial polarity on the specific than on the general catalysis. The apparent contradiction between the rate constant decreases and the marked accelerations in micella r media is discussed in terms of pH changes, i.e., [HO-] changes, and of ac etate inclusion via ion exchanges at micellar interfaces.