Kinetic isotope effects. Part 2.—Rates of abstraction of hydrogen and tritium from acetophenone and some para- and meta-substituted acetophenones in alkaline media

Abstract

Measurements are reported on the kinetics of the base-catalyzed ionization of acetophenone and a series of para- and meta-substituted acetophenones, some of which cover a temperature range. The bromination and detritiation results at 25° yield curved Hammett plots and the magnitude of the isotope effect can be correlated with the pK of the corresponding benzoic acids (the higher values being associated with the weaker acids), and the rates of ionization. The activation energy data are in line with Wiberg's prediction that “in general, in any series of related reactions, the reaction with the lowest activation energy will have the lowest isotope effect”. The ratio of the Arrhenius factor (A_T/A_H) is in all case except one close to the theoretical limits set for a three-centre reaction and for para-methoxyacetophenone the value (14) is considerably higher. Curvature of the Arrhenius plot for this compound is also obtained.