ELECTRON-TRANSFER AND BOND BREAKING - EXAMPLES OF PASSAGE FROM A SEQUENTIAL TO A CONCERTED MECHANISM IN THE ELECTROCHEMICAL REDUCTIVE CLEAVAGE OF ARYLMETHYL HALIDES

A systematic investigation of the kinetics of the electrochemical reduction of a series of 11 arylmethyl halides in acetonitrile and N,N'-dimethylformamide reveals a striking change in the reductive cleavage mechanism as a function of the energy of the pi* orbital liable to accept the incoming electron. With ring-substituted nitrobenzyl chlorides and bromides, a stepwise mechanism involving the intermediacy of the anion radical takes place. When slightly less electron-withdrawing substituents, such as nitrile or ester groups, are involved, the reaction occurs via a concerted electron transfer-bond breaking mechanism. This is also observed with the unsubstituted benzyl chloride and bromide as well as with 9-anthracenylmethyl chloride. The main factor then governing the thermodynamics and kinetics of the reductive cleavage is then the dissociation energy of the bond being broken. There is an excellent agreement between the predictions of the recently developed model of dissociative electron transfer and the experimental data both in terms of the quadratic character of the activation-driving force relationship and the magnitude of the intrinsic barrier. In this connection, procedures utilizing the difference in cyclic voltammetric peak potential between two compounds bearing the same nucleofugal group to estimate the difference in their bond dissociation energies are discussed.