Modeling reductive dehalogenation with quantum chemically derived descriptors.

Abstract

Existing models for the reductive dehalogenation reaction under environmentally relevant conditions use Hammett and Taft coefficients as descriptors. Drawbacks of these descriptors are the limited possibilities for interpretation in terms of reaction mechanisms, and the limited availability of these descriptors for more "exotic' substituents. Therefore, in this study new descriptors are tested, using semi-empirical molecular orbital calculations. These descriptors are based on the energetic and electronic properties of the reaction sites and should be able to account for the systematics of the rate constants in a better way than substituent coefficient models. This approach is expected to give reliable estimates of the rate constants even for compounds containing less common structural features. Several relationships for a series of halogenated aromatics are presented here, relating the experimental rate constants to, among others, the calculated activation energy of the rate limiting step in the reductive dehalogenation process. Results show that semi-empirical molecular orbital descriptors are capable of describing the reaction kinetics within a homologous series of compounds. All descriptors can be explained for in terms of reaction mechanisms, thus corroborating the hypothesis about mechanisms taking place in the environment.