Evaluation of the Activation Energies of Radical Substitution Reactions in the Gaseous Phase. II. An Approximate Formula with Two Constants

Abstract

Abstract The following approximate formula for evaluating the activation energies of radical substitution reactions has been derived from the formula proposed in a previous paper: Ea=Di−εDf(1+δDi) where Di and Df are the bond dissociation energies of the initial and the final bonds. The calculated values are in good accordance with the observed values within ±2.0 kcal/mol. The empirical formulae, proposed by Evans-Polanyi and Semenov et al., between the activation energy and the reaction heat have been derived from this approximate formula. The Hammett-type rule has also been observed to hold in radical substitution reactions, and the substituent constants and the reaction constants have been determined by the use of the formula. A good correlation between the bond dissociation energy of the initial molecule and the radical reactivity index, delocalizability, based on the quantum chemical standpoint, has also been observed.