1,1,2,2-Tetrachloroethane reactions with OH^-, Cr(II), granular iron, and a copper-iron bimetal: Insights from product formation and associated carbon isotope fractionation

Despite widespread implementation of zero-valent iron remediation schemes, the manner and order of chemical bond cleavage in iron-mediated organohalide transformations remains imperfectly understood. We present insights from carbon isotope fractionation for the dehalogenation of 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) and 1,1,1-trichloroethane (1,1,1-TCA) by various reactants. Elimination of HCl by OH^- gave isotope fractionation in 1,1,2,2-TeCA of ε = -25.6‰, KIE_C = 1.02 to 1.03 per carbon center, consistent with a concerted (E2) mechanism. In contrast, 1,1,1-TCA reduction by Cr(II), Fe(0), and Cu-plated iron (Cu/Fe) resulted in ε = -13.6‰ to -15.8‰ indicating the initial involvement of a single C-Cl bond (KIE _C ≈ 1.03). 1,1,2,2-TeCA reduction by Cr(II), Fe(0), and Cu/Fe yielded ε = -18.7‰, -19.3‰, and -17.0‰, respectively. In the two latter cases, depletion of the minor product TCE by 26‰ indicated its formation via nonreductive dehydrohalogenation. The major 1,1,2,2-TeCA reduction products, cis- and trans-DCE, differed by 2.3‰ ± 1.0‰ in Cr(II) systems, but were equivalent in Fe(0) and Cu/Fe systems. In contrast, the ratio of cis-DCE to trans-DCE concentration was 2.5 for reduction with Cr(II) and Fe(0), but ∼3.8 with Cu/Fe. Complementary isotope and concentration data therefore suggest differences in the transition state geometry and/or reaction intermediates in each reductant system.