Reductive cleavage of chlorine from 6-chloronicotinic acid on mercury electrodes

This paper presents polarographic (direct current, dc, and differential pulse, DP) and voltammetric (linear-sweep cyclic voltammetry) studies on the electroreduction of 6-chloronicotinic acid (6CNA) on mercury electrodes. In order to obtain the dissociation constants of 6CNA, UV–vis spectra were recorded as a function of pH. pK values of −0.80 ± 0.05 (–COOH group) and 3.2 ± 0.1 (pyridinic nitrogen) were obtained. The electrochemical studies were performed in the acidity range 6 M H2SO4 to pH 8. Above the last pH value no signals were obtained. Electrolysis made at potentials corresponding to the limiting current of the first wave indicates that there is a reductive cleavage of chlorine from the molecule. This was confirmed by dc and DP polarografic results and also by voltammetric results. Kinetic parameters such as Tafel slopes and electrochemical reaction orders have been determined at potentials corresponding to the foot of the waves. From these results, together with those obtained by cyclic voltammetry, a reaction pathway is proposed, in which the rate-determining step of the process is the release of a chloride ion from the radical formed after the uptake of a H+ ion and an electron.

► Dissociation constants (as pK) of 6 chloronicotinic acid (6CNA) obtained by UV–vis spectroscopy: −0.80 ± 0.05 (–COOH group) and 3.2 ± 0.1 (pyridinic nitrogen). ► Electrolysis of 6CNA evidenced the reductive cleavage of chlorine from the molecule. ► Kinetic parameters (Tafel slopes and reaction orders) determined at the foot of the waves. ► Reduction pathways have been proposed.