Electroreduction of 6-chloro-2-pyridinecarboxylic acid on mercury electrodes

This paper presents polarographic and voltammetric studies on the electroreduction of 6-chloropicolinic acid (6-chloro-2-pyridinecarboxylic acid), 6ClP, on mercury electrodes. The studies were performed in the acidity range 1 M H2SO4 to pH 8. Above the last pH value no signals were obtained. In dc polarography, a reduction wave was observed, appearing at more negative potentials as the pH was increased, being the limiting current pH-dependent. In DP polarography two peaks were observed. The pK values corresponding to the acid–base equilibria in which 6ClP is involved are obtained being −1.84 (–COOH group) and 3.28 (pyridinic nitrogen). Kinetic parameters such as Tafel slopes and electrochemical reaction orders have been determined at the potentials corresponding to the foot of the dc waves, as well as characteristic parameters of the DP polarograms. From these results, together those obtained by cyclic voltammetry, reaction pathways are proposed. Below pH 3.3 the species protonated at the heterocyclic nitrogen is reduced, being the rate-determining step, r.d.s., a protonation placed after two reversible two-electron transfers in which proton-donors other than the H+ ion are involved. The recombination of the carboxylate anion with the H+ is the reaction responsible of the decrease in the limiting current of the wave at higher pH values. Above pH 4 the r.d.s. is the second one-electron transfer, which is irreversible. In both cases, the hydrated aldehyde obtained is reduced only partially, due to the slow rate of the dehydration reaction.