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The first step following the initial formation o f the anion radical i n the electrochemical reduction o f aromatic halides i s the cleavage o f the C-X bond leading to the neutral Are radical. The latter species undergoes three concurrent reactions: Hatom abstraction f r o m the solvent and electron transfer at the electrode and/or f r o m the initial anion radical. The quantitative analysis o f this threefold competition allows one to predict the effect o f the intrinsic (rate constants) and operational (concentration, stirring rate, cell geometry) parameters. I t i s the basis on which the results o f deuterium incorporation by deuterated water or solvent can be used as a tool to investigate the reaction mechanism and to determine the characteristic rate constants. This is illustrated by the study o f the chloro, bromo, and iodo derivatives of three aromatic residues (9-anthryL I-naphthyl, and 4-cyanophenyl) in MezSO and acetonitrile. It i s shown that the proposed reduction mechanism i s indeed followed in each case and the degree o f competition between the three concurrent steps can be evaluated. Using the values obtained independently (by cyclic voltammetry or redox catalysis) for the cleavage rate of the anion radicals o f the chlorocompounds, the correspondi n g rates for the bromo and iodo derivatives are determined as well as the rates o f H -a t o m transfer t o the three aromatic radicals and the magnitude of the corresponding deuterium isotope effects.

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