The mechanism of electrochemical reduction of N-haloamides in acetonitrile: trapping of intermediate amide anions and father-son protonation

Article abstract of DOI:10.1139/v82-168

The electrochemical reduction of N-haloamides (ZCONRX) in acetonitrile involves two consecutive one-electron transfers and generates the amide anions.Attempts to trap the intermediate amidyl radical resulting from first electron transfer were unssuccesful.In the case of the N-halo-N-hydroamides (R=H), the amide anion formed at the electrode abstracts a proton from an incoming N-halo-N-hydroamide molecule to give the parent amide and the conjugate base of the N-haloamide (father-son protonation).Thus, half of the N-halo-N-hydroamide reaching the electrode is reduced, the other half being converted to its conjugate base.In acetonitrile-LiClO4 (0.2 M) containing 0.2percent water, the conjugate base is reducible and polarograms therefore show two waves (irreversible and diffusion controlled) of equivalent intensities due respectively to the reduction of the N-halo-N-hydroamide and to the reduction of its conjugate base.In the case of the N-chloro-N-alkyl(aryl)amides (X=Cl, R=alkyl or aryl), the amide anion abstracts a proton from the medium to give the parent amide and anionic species that react with the starting N-chloroamide regenerating the amide anion.Hence, the coulometric results are low (0.5 F/mol).However, in the presence of acetic acid, the reduction consumes 2 F/mol as expected.The N-alkylamide anion has been trapped by N-alkylation and N-acylation.The voltammograms of N-chloro-N-alkyl(aryl)amides show multiple waves on mercury and platinum due to passivation-adsorption phenomena but a single wave on vitreous carbon.