Journal of the American Chemical Society p. 572 - 580 (1981)
Update date:2022-08-11
Topics:
Cox, Michael M.
Jencks, William P.
General-acid catalysis of the reaction of methoxyamine with phenyl acetate by the proton, carboxylic acids, and ammonium ions follows a nonlinear Broensted curve.This curve agress quantitavely with the behavior expected for the enforced preassociation mechanism of catalysis that was predicted for this reaction.The stronger acids, including the proton, follow a Broensted slope of α=0.16 that represents rate-limiting amine attack assisted by hydrogen bonding, weaker acids react with partially rate-limiting proton transfer to the addition intermediate T+/-, and the weakest acids follow a steeper Broensted slope approaching α=1.0 that represents rate-limiting separation of the protonated intermediate T+.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 pKHA = 6.8 confirms the kinetically significant proton-transfer step in the intermediate region near ΔpK = 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 pKa = 4-7, but the decrease with weaker acids can be explained by the change to rate-limiting diffusional separation of T+ and A-.Two explanations are offered for the decreased isotope effect with increasing acid strengh. (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 Broensted 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.
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