Studies in decarboxylation. Part V. Kinetic isotope effects in the gas-phase thermal decarboxylation of 2,2-dimethyl-4-phenylbut-3-enoic acid

Abstract

2,2-Dimethyl-4-phenylbut-3-enoic [²H]acid shows a kinetic isotope effect of 2·87 ± 0·23 at 559·2°K and 2·81 ± 0·18 at 543·0°K. There is no deuterium exchange before decarboxylation. For the [¹⁴C]acid the effect is 1·035 ± 0·010 at 551·0°K. These observations, together with the effect of substituents on the rate of decarboxylation described in Part IV, require that all the bonds in the transition state are either making or breaking simultaneously.