Electrochemical reduction of N-methyl nicotinic acid at a mercury electrode

The electrochemical reduction of N-methyl nicotinic acid (MNA) in aqueous solutions has been studied, over a wide pH range, on the basis of DC and DP polarographic data and by cyclic voltammetry. The influence of the pH, reactant concentration and drop time on different polarographic and kinetic parameters such as the limiting current, half-wave potentials, Tafel slopes and reaction orders with respect to MNA and the H+ ion was studied for the electroreduction processes involved. The process corresponding to the first wave observed in an acid medium, pH < 4, involves a competition between the dimerization of the radical yielded in the first electron transfer and the protonation and reduction of this radical to yield the corresponding hydrated aldehyde. This interpretation accounts for the variations of iL and E1/2 with the pH and the reactant concentration, for the reaction order with respect to MNA and for the oxidation peak observed by cyclic voltammetry. The second reduction wave, at more negative potentials, results from the reduction of the radical yielded in the first electron transfer to give an RH– anion, which undergoes a rate-determining protonation step. Only one wave, with the typical features of a dimerization process involving a radical–substrate coupling followed by a disproportionation step to yield the final dimer and a molecule of reactant, is observed in neutral and basic media. The anodic peak, observed throughout the pH range by cyclic voltammetry, is attributed to the oxidation of the dimer, as is the case with similar compounds.