Nicotinamide and N'-Methylnicotinamide: Electrochemical Redox Pattern

The mechanisms for the electrochemical reduction of nicotinamide (3‐carbamoylpyridine) and N'‐methylnicotinamide [3‐(N‐methylcarbamoyl)‐pyridine] in aqueous media, as well as those for reduction and oxidation of intermediate and final products, have been investigated. Both compounds involve the same redox pattern of two successive one‐electron additions. The initial step involves simultaneous addition of an electron and a proton to form a free radical (only an electron in the pH range where the compound is protonated), which rapidly dimerizes to an apparent 6,6' species; oxidation of the free radical can be seen at pH 9. At more negative potential, the nicotinamide is reduced to an apparent 1,6‐dihydropyridine species (the dimer is stable to reduction in the available potential range); the two polarographic waves begin to merge above pH 8 and are completely merged by pH 12. E1/2 for both electron‐transfer processes becomes more negative with increasing pH between pH 4 and 9. The rate constants for dimerization of the initially produced free radicals are 1.8 × 106 and 4.9 × 106 liter mol−1 sec−1 for nicotinamide and N'‐methylnicotinamide, respectively, at 30° and pH 9. Adsorption of both compounds and their reduction products at the mercury‐solution interface is negligible. Both dimer and dihydropyridine species are oxidized to the original nicotinamide but at considerably more positive potentials than those necessary for their formation.