Iron porphyrin and cysteine mediated reduction of ten polyhalogenated methanes in homogeneous aqueous solution: Product analyses and mechanistic considerations

Reductive dehalogenation reactions of polyhalogenated C-1- and C-2-compound s are presently of particular interest because of the potential applicabili ty of such processes in the treatment of wastes as well as in remediation a pproaches to removing such compounds from contaminated soils and aquifers. In this context, it is not only important to know the reaction kinetics of a specific compound with relevant reductants but also the type of product(s ) formed under given conditions. In this study we have identified reaction intermediates as well as the final products of the reduction of 10 polyhalo genated methanes (PHMs) by an iron porphyrin in the presence of cysteine. C ysteine was chosen for two reasons: (i) as bulk electron donor and (ii) as an aqueous organic compound exhibiting functional groups (i.e., -NH2, -SH) that may undergo reactions other than just hydrogen abstraction with reacti ve intermediates such as radicals and carbenes. The data obtained support o ur hypothesis postulated in an earlier kinetic study that the initial and r ate-determining step in the reduction of PHMs by the iron porphyrin is a di ssociative one-electron transfer. Furthermore, it is shown that in fast con secutive reactions involving primarily the mercapto group of cysteine, all compounds were completely dehalogenated. Except for the fluorine containing compounds, the carbon of a given PHM was quantitatively recovered as N-for mylcysteine. In the case of the fluorinated compounds, carbene intermediate s could be trapped, which reacted further to some unidentified product(s), possibly including carbon monoxide. Finally, it is shown that the reduction of tetrahalomethanes by cysteine as sole reductant leads predominantly to the formation of the corresponding haloforms, suggesting that, in this case , the reaction occurred primarily by an X-philic dissociative two-electron transfer. The results of this study offer an interesting perspective for a fast complete dehalogenation of PHMs by using a very reactive one-electron donor (i.e., a reactive iron species) in the presence of organic matter exh ibiting reduced sulfur groups.