Polarographic reduction of aldehydes and ketones. Part X. Recombination of carbanions of ω-cyanoacetophenone

Abstract

Reduction of the keto-form of ω-cyanoacetophenone takes place at a more positive potential than that of the enol or the carbanion-enolate. During polarographic electrolysis the equilibrium is perturbed and the keto-form is generated by recombination of the ambident anion with proton donors. The rate of this recombination volume reaction governs polarographic limiting currents in alkaline media. From polarographic measurements the rate constant for the recombination rate was calculated to be 2 × 10⁸ l. mole^–1 sec.^–1. l.r., n.m.r., and u.v. spectra in aqueous solutions of ω-cyanoacetophenone indicate that less than 15% and probably only ca. 1% of the enol form is present. Owing to the rapid establishment of the equilibria between the keto-form and the ambident anion, dynamic methods, such as bromine-titration or polarography, cannot be used for measurement of the enol content. Reports of unusual solvent effects which involve α-cyanoketones are doubtful. In acidic media polarographic reduction of ω-cyanoacetophenone (which always results in the reduction of the CO group) is complicated by protonation of the carbonyl group preceding the electrode process proper.