Concerted Proton−Electron Transfer: Effect of Hydroxylic Additives on the Reduction of Benzophenone, 4-Cyanobenzophenone, and 4,4′-Dicyanobenzophenone

The electrochemical reduction of the title compounds proceeds in two steps in N,N-dimethylformamide. The first step is a one-electron reduction to form the anion radical of the benzophenone, a reaction which is affected by the presence of hydroxylic additives such as water, ethanol, or methanol because of hydrogen-bond formation between the additive and the anion radical. Formation constants for these complexes have been evaluated for both protiated and deuterated versions of the additives. It was found that the formation constants for the protiated additives were about 20% larger than for the deuterated additives. For benzophenone, the second step of reduction in the presence of water proceeds by a concerted proton−electron transfer reaction with the electron passing from the electrode to the hydrogen-bonded complex with proton transfer from water to the oxygen atom of benzophenone anion radical. This conclusion is supported by data that rule out alternative mechanisms, by the small value of the apparent value of the transfer coefficient and by agreement between experimental and calculated reorganization energies for the process. On the other hand, the second stage of reduction of 4-cyanobenzophenone proceeds by initial electron transfer followed by proton transfer from water to the dianion. The overall mechanism includes formation of 1:1, 1:2, and 1:3 dianion:water complexes with the proton transfer occurring within the 1:3 complex. 4,4′-Dicyanobenzophenone shows little evidence for proton transfer to the dianion, but the formation of the dianion:water complexes is important.