Isotope effect in hydrogen atom abstraction from substituted phenols by polyvinyl acetate radicals in vinyl acetate: evidence for tunnelling

Abstract

Rate constants have been determined for the hydrogen atom abstraction from 72 substituted phenols and 33 of their deuterated analogues by polyvinyl acetate radicals at 50°C. The reactions were carried out in vinyl acetate medium which forms hydrogen bonds with the reaction centres. The magnitude of the isotope effect varies considerably and shows maxima as a function of both the reactivity of the light compounds and the sum of electrophilic substituent constants, Σσ⁺. Several k_H/k_D values are equal to or smaller than, while others are significantly higher than, the semi-classical limit, which could be accounted for by the zero-point energy differences in the initial state. These data indicate evidence for tunnelling. In some cases the measurements were carried out between 30–70°C, but the poor accuracy of the Arrhenius parameters does not allow us to obtain independent support for the above conclusion. It is pointed out that the mechanism allowing for tunnelling implies that the hydrogen atom is abstracted within the hydrogen bond.