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

Abstract

The electrochemical reduction of N-haloamides (ZCONRX) in acetonitrile involves two consecutive one-electron transfers and generates the amide anion. Attempts to trap the intermediate amidyl radical resulting from the first electron transfer were unsuccessful. 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 0.2% water, the conjugate base is reducible and the polarograms therefore show two waves (irreversible and diffusion controlled) of equivalentintensities 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 [Formula: see text] 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.