Kinetic isotope effects. Part 4.—Bromination and detritiation of some ortho-substituted acetophenones in alkaline media

Abstract

Measurements are reported on the rates of bromination and detritiation of a series of ortho-subsitituted acetophenones in aqueous alkaline media at 25°, and of ortho-methylacetophenone over a temperature range. The rates, which are higher than those for the corresponding meta- and para-substituted compounds, are discussed in relation to the acid strengths of the corresponding benzoic acids. The iostope effects are lower than for the meta- and para-substituted acetophenones, with the exception of ortho-methylacetophenone, for which the activation energy difference E_T–E_H(3.4 kcal/mole), the A-factor ratio (3 A_T/A_H= 16), and a curved Arrhenius plot for the deprotonation results at the lowest temperature reflect the importance of proton tunnelling.