Polar Radicals. 15. Interpretation of Substituent Effects on the Mechanism of Electrolytic Reduction of the Carbon-Halogen Bond in Series of Substituted Benzyl Halides

Article abstract of DOI:10.1021/jo01313a031

The electrochemical polarographic reduction of a series of substituted benzyl chlorides and benzyl bromides showed half-wave potentials which gave excellent Hammett linear free-energy correlations for each series.These correlations were obtained using ?^- substituent constants.The correlation with ?^- suggests that the potential-determining electrochemical process involves carbon-halogen bond breaking or, alternatively, radical anion intermediate formation.The Hammett ρ values for each correlation were calculated; their magnitudes, Cl>Br, were in the inverse order of their reactivity toward chemical reduction.As in the case of trialkyltin hydride reduction, both ρ values were positive.The results of the polarographic electroreduction of benzyl iodide and benzylmercuric iodide confirmed the previously reported mechanism for reduction;namely, that the iodide underwent reaction with mercury to form benzylmercuric iodide, which was subsequently reduced, as was benzylmercuric iodide itself, by a process having two distinct polarographic waves.The half-wave potentials for the second wave (the wave at the most negative potential) for series of substituted benzyl iodides and benzylmercuric iodides were both found to give excellent Hammett correlation, having positive ρ values.The correlations were both obtained by using ?^- substituent constants.The success of the use of these substituent constants was taken as a confirmation of the suggestion, previously made, that the second step in the electroreduction was the reduction of the benzylmercuryl radical, and suggests that the process is concerted with carbon-mercury bond rupture.