Polarographic Reduction of Aldehydes and Ketones. IV. Linear Free Energy Treatment of Substituted Benzophenones
Reduction of thirty nine meta- and para-substituted benzophenones at the dropping mercury electrode was investigated. Substituent effects on half-wave potentials were determined for the one-electron wave of the protonized form in acidic media, for the two-electron wave at pH 9·3 and for the wave in sodium hydroxide solutions that corresponds predominantly to a one-electron reduction. For correlations the Hammett equation and an equation relating meta with corresponding para values derived earlier were used. The deviations from the latter equation as well as the use of constants \(\sigma _{p - x}^ - \) in the Hammett plot has enabled some conclusions to be drawn about the electronic structure of the radicals involved. Even when the correlations of the half-wave potentials with Hammett substituent constants show only a limited precision, they allow the conclusion that half-wave potentials are controlled by substituent I and M effects transmitted through the benzene nucleus.