| Abstrakt: |
The rate-surfactant profiles for the HO(-)- and AcO(-)-catalyzed ring closure of two ethyl hydantoates, E2 and E3, to hydantoins with three cetyltrimethylammonium 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 associated 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 specific base- and general base-catalyzed reactions, and, respectively, of E2 and E3 are obtained separately. The resulting values of k(2,m)/k(w), E2/E3 rate constant ratios, and kinetic solvent isotope effects, KSIEs, are consistent with a strong predominance of the HO(-) reaction in the dilute buffer, while in the more concentrated buffer, specific and general catalysis compete for the two substrates. This result is in sharp contrast with that observed in water in which the reaction of E2 is almost exclusively specifically catalyzed. The increase in the general base-catalyzed pathway for E2 is attributed not to an increase in the rate constant for this pathway in micelles but to a smaller decrease than that for the specific catalysis (k(2,m)/k(w) = 0.2 and 0.4 for the specific and general catalysis, respectively). The different responses of the rate constants to the micellar 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 micellar media is discussed in terms of pH changes, i.e., [HO(-)] changes, and of acetate inclusion via ion exchanges at micellar interfaces. |
