Iron porphyrin and mercaptojuglone mediated reduction of polyhalogenated methanes and ethanes in homogeneous aqueous solution

Reduction rate constants of seventeen polyhalogenated methanes and ethanes were measured in aqueous solutions containing bulk reductants and the electron-transfer mediators iron porphyrin or juglone (5-hydroxy-1,4-naphthoquinone). Rate constants Varied 4 orders of magnitude in the case of reaction with iron porphyrin and 7 orders of magnitude in the reaction with mercaptojuglone, an addition product of juglone and hydrogen sulfide. For the iron porphyrin system the results support a reaction mechanism in which one electron is transferred in an outer-sphere process to the polyhalogenated alkane with bond breakage in the transition state. In the juglone system two competing reaction mechanisms are proposed: an outer-sphere electron transfer to the polyhalogenated alkane and an S(N)2 reaction at the halogen. The kinetic results are directly applicable to environmental systems containing common reductants such as natural organic matter and reduced iron species, providing a "fingerprint" of reactivity for examination of reactive species and rate-limiting steps in those systems.