A contribution to the elucidation of the reduction mechanism of thioisonicotinamide on mercury electrodes

Abstract

The electroreduction of thioisonicotinamide has been studied in aqueous media in the pH range 4 to 12 by d.c. and DP polarography and linear-sweep cyclic voltammetry. Tafel slopes and reaction orders were obtained from I-A curves traced at potentials corresponding to the foot of the polarographic wave. The protonation o K of the pyridinic nitrogen was obtained from spectroscopic (UV) measurements, having a value of 3.67. In the pH range 4 to 8, the reduction pathways correspond to an ECE process. The first part of the mechanism consists of a two-electron reduction of the thioisonicotinamide protonated in the heterocyclic nitrogen (which is formed in the electrode from the unprotonated species); the rate-determining step (rds) is an elimination reaction of SH 2 and NH 3 to give a mixture of intermediates which are further reduced through a two-electron process to a mixture of 4-aminopyridine and 4-mercaptopyridine. In basic media (pH > 9) the overall reduction corresponds to a two-electron process. The mechanism consists of a reversible two-electron reduction of the unprotonated reactant and the rds is a protonation reaction taking place in the reaction layer.