RCl + e → R^• + Cl^- (1)

RCl + e = RCl^•- (2a)

RClR^•- → R^• + Cl^- (2b)

Factors deciding on such transition were intensively examined [3]. In the present work a transition of a mechanism is analyzed for results obtained at the same conditions in different solvents. In benzonitrile at glassy carbon or mercury electrodes the stepwise process was found with the reversible electron transfer (2a) followed by the bond cleavage (2b) and the resulting radical R^• undergoes the disproportionation giving a two electrons transfer for the overall process. In acetonitrile and N,N-dimethylformamide (DMF) the one electron process was established, indicating the H atom abstraction by the radical R^• from solvent molecules. A detail analysis of potential dependence of the apparent transfer coefficient α = (RT/F) dlnk/dE, obtained from convoluted voltammetric curves in DMF and CH₃CN, indicates the concerted process. According to the theory of the dissociative electron transfer [3] a slope dα/dE = 12.06/ΔG_o^‡ is inversely proportional to the intrinsic activation barrier, ΔG_o^‡. A low slope dα/dE = 0.14 V^-1 found for the concerted process is in agreement with the high intrinsic activation barrier, which includes a contribution form the bond dissociation energy (ca. 2.9 eV [2]). The transition from the stepwise process in C₆H₅CN to the concerted one in DMF and CH₃CN can be explained by an acceleration of the bond cleavage in the radical anion (2b) due to a stronger solvation of the final product Cl^- by CH₃CN and DMF.

[1] D. Kuck, Pure Appl. Chem. 2006, 78, 749.

[2] J. S. Jaworski, M. Cembor, D. Kuck, Electrochim. Acta 2006, in press.

[3] S. Antonello, F. Maran, Chem. Soc. Rev. 2005, 34, 418.

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