Characterization of Transition-States by Isotopic Mapping and Structure Reactivity Coefficients - Vibrational Analysis Calculations for the General-Base-Catalyzed Addition of Alcohols to Acetaldehyde

by Murray, Christopher James; Webb, Tim

General-base catalysis of the breakdown of acetaldehyde hemiacetals represents equilibrium ionization of the hemiacetal CH3CH(OH)OR to form the hemiacetal anion, CH3CH(O-)OR, followed by rate-determining general-acid catalysis of the cleavage of the hemiacetal anion to form acetaldehyde and ROH as described in the previous paper. Solvent deuterium isotope effects for the kinetically equivalent reverse reaction, general-base-catalyzed addition of alcohols ROH to acetaldehyde, have been modeled by using vibrational analysis calculations. Two independent weighting parameters that describe the bond orders for the catalyst O-H bond (W(OH)) and the C-O bond between the central carbon and oxygen atoms (W(CO)) were allowed to vary independently between 0 and 1 to generate isotopic maps as a function of transition-state structure. The effect of coupling of proton transfer to heavy-atom motion on the magnitude of the solvent kinetic isotope effect k(H2O)/k(D2O) was investigated by varying the relative ratios of several interaction force constants f(ij) that couple two internal stretching coordinates. This process generates models of the reaction-coordinate motion for decomposition of the transition state to reactants and products. It is concluded that a reaction coordinate with essentially equal contributions of proton and heavy-atom motion is most consistent with the experimental isotope effects (k-p(B))H2O/(k-p(B))D2O = 1.7 +/- 0.5 for general-base catalysis of the addition of alcohols to acetaldehyde.

Journal of the American Chemical Society
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1520-5126; 0002-7863