Kinetic isotope effects. Part 5.—Bromination of acetophenone-d3and inter-relationship between deuterium and tritium isotope effects

Abstract

Measurements are reported on the bromination of acetophenone-d3 in alkaline media and over a temperature range. Taken in conjunction with the previously reported rates of deprotonation and detritiation, it is possible to draw certain conclusions concerning Swain's equation for relating deuterium and tritium isotope effects. It is suggested that the relationship may be employed for predicting the possible magnitude of secondary isotope effects in some reactions although it is unlikely to provide a sensitive means of probing possible quantum mechanical contributions where the latter are small. For a number of proton transfer reactions for which the transition states differ markedly from one another the relationship holds well. Measurements on the rates of ionization of acetone at 25° are in good agreement with those obtained previously. In this case the transition state is more symmetrical than for a number of acetophenones. Consequently the tunnelling contribution is higher and when a statistical factor of six is inserted for the detritiation results is sufficient to lower the correlation factor from the value of 1.44 to 1.12.