Medium Effects on the Reductive Cleavage of the Carbon−Halogen Bond in Methyl and Methylene Halides

Abstract

The thermodynamics and kinetics of dissociative electron transfer to a series of small aliphatic halocarbon molecules, including CH3Br, CH2Br2, CH3I, and CH2I2, are investigated. Such studies are of general interest because of the involvement of some of these compounds in the catalytic ozone depletion cycle, their toxicity, and their cancerogenic effects once present in the environment. Because of fairly large solubilities of these compounds in water, comparative kinetic experiments could be performed in N,N‘-dimethylformamide (DMF) and in aqueous solutions. As expected, cathodic reduction of these molecules on glassy carbon in DMF takes place at quite negative potentials (ca. −2 to −3.2 V vs Fc/Fc+), and their kinetics follow predictions of the dissociative electron transfer (DET) model (Savéant, J.-M. J. Am. Chem. Soc. 1987, 109, 6788). In contrast, similar experiments performed in aqueous solutions revealed many orders of magnitude faster kinetics of electron transfer to CH3I, CH2I2, and CH2Br2 molecules with the corresponding overpotentials about 1 V lower than those predicted on the basis of the DET model.