Electroreduction of Aromatic Oximes: Diprotonation, Adsorption, Imine Formation, and Substituent Effects

Aromatic oximes are reduced in aqueous solution in a four-electron process. The reducible species in the pH
range 5−8 is a diprotonated form of the oxime. This species is generated in the course of electrolysis in the
vicinity of the electrode surface from the adsorbed neutral form of the oxime. The reduction is initiated by
a cleavage of the N−O bond. The diprotonation facilitates the reduction process by the preformation of
OH2+ as a good leaving group and by a positive charge on the azomethine nitrogen. Diprotonation has been
proven based on shapes of i = f(pH) plots, by observed shifts of half-wave potentials with pH and by
comparison with the reduction of nitrones. Some observed deviations from theoretical i = f(pH) plots were
attributed to the role of adsorption on the rate of protonation. Adsorption is also responsible for dips on some
of the i−E curves. Adsorption plays a role at concentrations as low as 1 × 10-5 M, when the electrode
surface is still not fully covered. This indicates that catalyzed protonation occurs on islets of adsorbed materials.
At pH 2−5 the studied oximes in the vicinity of the electrode are predominately present in a protonated
form, which is less strongly adsorbed. In this pH range the protonation takes place in a homogeneous reaction
layer of the electrode. It yields a monoprotonated form, which is reduced. The separation of two two-electron
waves observed for some oximes in acidic media serves as an experimental proof of the formation of imines
as reduction intermediates. This separation is caused by the differences in pKa values of protonated forms of
oximes and imines. The effects of substituents in the para position on the benzene ring are characterized by
correlation with the Hammett substituent constant σx. This has been proven at pH 1.5 for substituted
benzaldehyde oximes and at pH 5.0 for substituted acetophenone oximes.