Electrochemical reactions of organic compounds in liquid ammonia. I. Reduction of benzophenone

The electroreduction of benzophenone in liquid ammonia was studied by cyclic voltammetry. In anhydrous solutions, the reduction occurs in two reversible one-electron steps producing the anion radical and the dianion. In the presence of alcohol, the first reduction is unaffected and the second reduction wave becomes irreversible and is shifted toward positive potentials, following qualitatively the criteria for a rapid reaction of the dianion with the proton donor. iquid ammonia is an amphiprotic, very weakly L acidic, slightly ionized solvent in which solvated electrons are stable. Its unique properties have been exploited by organic chemists to achieve numerous reductions of unsaturated compounds by alkali metals dissolved in liquid ammonia. * The overall reaction can be written NH8 R + 2M + 2 H A + R H z + 2M+ + 2A(1) where R is the reducible compound, M the alkali metal, and HA a proton source (generally an alcohol) and is known as the Birch reduction in the case of the aromatic hydrocarbons or simply reduction by dissolved metals for the other compounds. The overall reaction shown in eq 1 occurs in several steps. It is generally accepted2 that the first step is the formation of solvated electrons followed by the reversible reaction of these to produce anion radicals.