Quantitative evaluation of the mechanism of electroreduction of benzoyl cyanides.

The mechanism of reduction of benzoyl cyanide, 6, p-methoxybenzoyl cyanide, 7, and p-chlorobenzoyl cyanide, 8, has been studied in acetonitrile (6 and 7), N,N-dimethylformamide (6), and acetonitrile containing water (all three compounds). The reaction proceeds by initial reduction to form the anion radical followed by dimerization to produce an intermediate dianion, the dianion of the dicyanohydrin of benzil. The latter loses cyanide to give the anion of the monocyanohydrin of benzil, which undergoes two parallel reactions: expulsion of cyanide to give the corresponding benzil and rearrangement to the monoanion of mandelonitrile benzoate. The addition of water brings about an increase in the dimerization rate constant and an associated increase in the amount of benzil that is produced. The standard potentials for the initial reduction step have been evaluated, and their dependence on the substituent is discussed. The dimerization rate constants have also been evaluated.