HAMMETT REACTION CONSTANTS FOR THE ELECTROREDUCTION OF SUBSTITUTED CHLOROBENZENES AND BROMOBENZENES - EFFECT OF SOLVENT DYNAMICS ON THE 2-ELECTRON IRREVERSIBLE REDUCTIVE CLEAVAGE

The half-wave potentials, corresponding to irreversible two-electron t ransfers, were determined for the electroreduction of substituted chlo ro- and bromobenzenes in a number of organic solvents. In each solvent they obey the Hammett equation, and the reaction constants rho, found for each series are solvent dependent. Since the first electron trans fer is the rate-determining step for the overall process and the irrev ersible half-wave potentials depend on the heterogeneous rate constant s, it is shown that the rho values obtained are linear functions of th e logarithms of the solvent longitudinal relaxation times. This is the first example indicating the role of solvent relaxation dynamics in i rreversible electrode processes accompanied by bond cleavage. Differen ces in the Hammett behaviour obtained and that described by us previou sly for the reduction of substituted iodobenzenes are discussed on the basis of a mechanism of electrode processes.