Organic electrode processes: generalized reduction mechanisms

Abstract

PHILIP J. ELVING. Can. J. Chem. 55,3392 (1977). Prototype formulations are considered for the reaction pathways commonly encountered for the electrolytic reduction of organic compounds in aqueous and nonaqueous media on the basis that the essential feature is severance of a chemical bond which requires transfer into the reaction site of one or two electrons; multiple electron processes result from instability at the applied potential of initially produced species or their chemically altered products. To obtain electrical neutrality, uptake of electrons may have to be accompanied by that of a cation, generally a proton. Pertinent factors in the path followed in the overall electrode process may include a sequence of free radical or radical anion going to dimeric or carbanion species (depending on the relative rates of chemical and charge-transfer steps), possible interaction of intermediate products with their progenitors, proton donor activities, medium properties (e.g in respect to rates of chemical reactions accompanying charge-transfer and control of the chemical states of reactants and intermediates), stereochemical factors inherent in reactant and intermediate species, and chemical interaction with the electrode material. Application of the general mechanistic considerations are illustrated by various types of electrolytic reductions: aromatic hydrocarbon, nitrogen heterocycle, carbon-halogen bond, ketone carbonyl group, and beta halogenated and/or unsaturated acids. PHILIP J. ELVING. Can. J. Chem. 55,3392 (1977). Des formules modkles sont considerees pour des chemins rkactionnels souvent rencontres dans la reduction tlectrolytique de composks organiques en milieu aqueux et non aqueux. On se base sur le fait que la rupture du lien chimique exige le transfert d'un ou deux Blectrons au site rtactionnel. Les processus multi-electroniques resultent de I'instabilite au potentiel appliquk des espkces initialement produites ou de leurs produits transformks. Afin d'obtenir une neutralitk Blectrique, le transfert d'un Blectron doit &tre accompagne par celui d'un cation, gknkralement un proton. Les facteurs appropriks dans ce chemin rkactionnel suivi, tout au long du processus prenant place aux klectrodes, peut impliquer : (a) la sequence d'un radical libre ou d'un radical anionique donnant des dim&res ou carbanions (dkpendant des vitesses relatives des Btapes chimiques et celles de transfert de charge) ; (b) I'interaction possible des intermkdiaires avec les produits initiaux, (c) des activitks proton-donneur; (d) des propriktes de milieu (i.e. par rapport aux reactions chimiques accompagnant le transfert de charge ainsi que du contrBle des Btats chimiques des rkactifs et intermkdiaires); (e) des facteurs stkrtochimiques propres aux rtactifs et intermkdiaires; (f) l'interaction chimique avec I'klectrode. L'application des considkrations mkcanistiques generales est illustrte par diffkrents types de reduction tlectrolytique, un hydrocarbure aromatique, un hkterocycle contenant un atome d'azote, un lien carbone-haloghe...

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