KINETIC ALPHA-SECONDARY DEUTERIUM-ISOTOPE EFFECTS FOR O-ETHYL S-PHENYL BENZALDEHYDE ACETAL HYDROLYSIS

The rate of hydrolysis of O-ethyl S-phenyl benzaldehyde acetal at 25 °C in 20% dioxane-80% water is independent of; pH over the range pH 6-12; kobsd= 1.9 X 10-7 s-1. Under more acidic conditions, the rate increases linearly with the activity of the hydra ted proton; k2= 2.95 X 10-2 M-1 s -1. The kinetic α secondary deuterium isotope effect for acid-catalyzed hydrolysis of O-ethyl S-phenyl benzaldehyde acetal, measured at 25 °C in 20% aqueous dioxane containing 0.05 M HC1, is kH/kD= 1.038 ± 0.008, a value consistent with a transition state in which the C-S bond is stretched rather little. In contrast, the corresponding isotope effect for the pH-independent hydrolysis of this substrate, measured at 42.5 °C in 20% dioxane, is 1.13 ± 0.02, a value consistent with complete C-S bond cleavage in the transition state and rate-determining diffusion apart of the ion-pair formed as the initial intermediate, in accord with the suggestion of Jensen and Jencks. © 1979, American Chemical Society. All rights reserved.