L. Nadjo, J. M. Sevéant
Mar 1, 1971
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Journal
Journal of Electroanalytical Chemistry
Abstract
Abstract The reduction of a wide series of p- and m-substituted benzophenones and 2-substituted fluorenones is studied in dimethylformamide. With the exception of the p-Cl, m-Br and p-Br benzophenones, the first reduction process consists simply in the formation of a stable anion radical. It is thus possible to correlate, in this series, characteristic potentials having a definite thermodynamical meaning with the Hammett constants. The behaviour of the halo-derivatives of benzophenone cited is different as indicated by the Hammett plot. A reductive cleavage of the carbon-halogen bond is observed leading to the unsubstituted benzophenone. The subsequent evolution of the species initially formed is not the same in the conditions of a preparative-scale electrolysis, where two electrons are exchanged, as in polarographic and voltammetric conditions where only one electron is exchanged. In the last case, the experimental observations are rationalized by assuming a reaction sequence composed of an expulsion of the halide ion from the anion radical initially generated followed by the abstraction of an hydrogen atom from the solvent. Other possible reduction mechanisms are discussed. Assuming the hydrogen abstraction mechanism, it is shown, using linear sweep voltammetry for the kinetic analysis, that the dehalogenation step is presumably the rate determining one. Considering the second reduction process, i.e. the reduction of the benzophenone initially formed, it is shown that an electron exchange reaction in solution interferes in the e.c.e. type mechanism. The possible kinetic influence of this reaction on the polarization patterns is discussed.