A CONTRIBUTION TO THE ELUCIDATION OF THE REDUCTION-MECHANISM OF THIOISONICOTINAMIDE ON MERCURY-ELECTRODES

The electroreduction of thioisonicotinamide has been studied in aqueou s 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 obtai ned from I-E curves traced at potentials corresponding to the foot of the polarographic wave. The protonation pK of the pyridinic nitrogen w as obtained from spectroscopic (UV) measurements, having a value of 3. 67. In the pH range 4 to 8, the reduction pathways correspond to an EC E process. The first part of the mechanism consists of a two-electron reduction of the thioisonicotinamide protonated in the heterocyclic ni trogen (which is formed in the electrode from the unprotonated species ); the rate-determining step (rds) is an elimination reaction of SH2 a nd NH3 to give a mixture of intermediates which are further reduced th rough a two-electron process to a mixture of 4-aminopyridine and 4-mer captopyridine. In basic media (pH > 9) the overall reduction correspon ds to a two-electron process. The mechanism consists of a reversible t wo-electron reduction of the unprotonated reactant and the rds is a pr otonation reaction taking place in the reaction layer.