Dimerization of electrochemically generated ion radicals: mechanisms and reactivity factors

Electrohydrodimerization of activated olefins, unlike other families of compounds, has given rise to vigorous controversies as to the radical–radical character of the coupling of the anion radicals as originally proposed. Careful examination of the facts shows that the original proposals were correct. radical dimerizations, the reactivity is governed by the interplay between three factors, bond formation, coulombic repulsion, and solvation. The latter factor is essential to counteract coulombic repulsion, making exergonic or slightly endergonic, a reaction that would be strongly endergonic in the gas phase. Solvation plays also a central role in the kinetics of the reaction. Thanks to the ensuing energy gain, the activation energy may be very small. The payback is however a strong negative contribution of solvation to the entropy of activation making the reaction much slower than the diffusion limit, unlike what happens with simple neutral radicals. These various features are illustrated with the example of the electrodimerization of acrylonitrile in a polar solvent such as N,N-dimethylformamide or acetonitrile, with the help of quantum chemical calculations.