Catalysis of the methoxyaminolysis of phenyl acetate by a preassociation mechanism with a solvent isotope effect maximum

General-acid catalysis of the reaction of methoxyamine with phenyl acetate by the proton, carboxylic acids, and ammonium ions follows a nonlinear Bronsted curve. This curve agrees with the expected enforced preassociation mechanism of catalysis. The stronger acids, including the proton, follow a Bronsted slope of ..cap alpha.. = 0.16 weaker acids react with partially rate-limiting proton transfer to the addition intermediate T/sup + -/, and the weakest acids follow a steeper Bronsted slope approaching ..cap alpha.. = 1.0. There is no decrease in the rate constant for catalysis by chloroacetic acid with increasing viscosity in water-glycerol mixtures; a decrease is observed for the reaction of methylamine with p-tolyl acetate catalyzed by acetate buffers, which is believed to proceed by a diffusion-controlled trapping mechanism. A sharp maximum in the solvent isotope effect at pK/sub HA/ = 6.8 confirms the kinetically significant proton-transfer step in the intermediate region near ..delta..pK = 0. The decrease with stronger acids represents a decrease in the isotope effect for this proton-transfer step, which is largely rate limiting for acids of pK/sub a/ = 4-7, but the decrease with weaker acids can be explained by the change to rate-limiting diffusional separation of T/sup +/ and A/sup -/. Two explanations are offered for the decreased isotope effect with increasing acid strength. (1) There is a sharp change to an asymmetric structure of the transition state for the very rapid proton-transfer step, as suggested by Melander and Westheimer. (2) There is a shift to a rate-limiting change in solvation that occurs immediately either before or after the proton-transfer step with stronger acids. It is possible to fit the observed Bronsted curve and isotope effect maximum with calculated rate constants that are based on a rate law and estimated rate constants for the steps of the latter mechanism.