Transition State Structural Features for Anilide Hydrolysis from β-Deuterium Isotope Effects

Article abstract of DOI:10.1021/ja00317a045

The hydrolysis in basic aqueous solution of p-NO2C6H4NHCOCH3 and p-NO2C6H4NHCOCD3 at 30 deg C from -> = 0.002 to 2.31 M generates observed isotope effects k₀^H/k₀^D that begin in the least basic solution at 0.967 +/- 0.011, pass through a maximum of around 0.98 at -> about 0.03 M, and then fall to 0.933 +/- 0.020 in the most basic solution.This phenomenon arises from a base-dependent mixture of rate-limiting transition states.At the lowest base concentrations, decomposition of the uninegative tetrahedral adduct, with k₂^H/k₂^D = 0.94 +/- 0.02, dominates.In the intermediate range, decomposition of the dinegative adduct, with k₃^H/k₃^D = 1.00 +/- 0.02, becomes important.In the most basic solutions, nucleophilic attack of hydroxide on the substrate (k₁^H/k₁^D = 0.94 +/- 0.01) assumes the major role.These isotope effects are consistent with quasi-tetrahedral transition states for formation and decomposition of the uninegative adduct and with rate-limiting trapping of diffusional separation of -, ArNH^-> for decomposition of the dinegative adduct.The observed isotope effect at -> = 0.208 M seems to show an anomalously large dependence on temperature, changing from 0.944 +/- 0.016 at 3.8 deg C to 0.977 +/- 0.019 at 50 deg C, which generates on an Arrhenius model A_H/A_D = 1.17 and ΔE_a = 117 cal mol^-1.The data are shown, however, also to be fully consistent with a different model: a shift in limitation of the rate away from the K₁ transition state toward the k₃ transition state , as the temperature is raised, and with simple zero-point-energy origins for the individual isotope effects.