Study of the reduction mechanism of the N-oxides of pyridine monocarboxylic acids at mercury electrodes

Abstract

The reduction mechanisms of the N-oxides of picolinic, nicotinic and isonicotinic acids were studied by dc and differential pulse polarography and voltammetry over a wide pH range. Tafel slopes and reaction orders were obtained at potentials corresponding to the foot of the waves. In addition, the UV absorption spectra of these N-oxides in strongly acidic solutions were recorded. From the variations of the absorbances with the pH, the acid dissociation constants of both the N-oxide and the carboxyl groups were obtained. The reduction products of the processes corresponding to the waves appearing at less negative potential were the corresponding carboxylic acids (picolinic, nicotinic and isonicotinic respectively). Polarographic and voltammetric results indicate that adsorption of the N-oxides occurs on the electrode. The results indicate that the protonated form of the N-oxide group is reduced in acidic media, whereas the unprotonated form is reduced at pH &>; 9. In both cases the reduction pathway is similar to that found for nicotinamide N-oxide.