| Abstract
| - Despite widespread implementation of zero-valent ironremediation schemes, the manner and order of chemicalbond cleavage in iron-mediated organohalide transformationsremains imperfectly understood. We present insightsfrom carbon isotope fractionation for the dehalogenationof 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) and 1,1,1-trichloroethane (1,1,1-TCA) by various reactants. Eliminationof HCl by OH- gave isotope fractionation in 1,1,2,2-TeCAof ε = −25.6‰, KIEC = 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 initialinvolvement of a single C−Cl bond (KIEC ≈ 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 lattercases, depletion of the minor product TCE by 26‰ indicatedits formation via nonreductive dehydrohalogenation. Themajor 1,1,2,2-TeCA reduction products, cis- and trans-DCE,differed by 2.3‰ ± 1.0‰ in Cr(II) systems, but wereequivalent in Fe(0) and Cu/Fe systems. In contrast, theratio of cis-DCE to trans-DCE concentration was 2.5 forreduction with Cr(II) and Fe(0), but ∼3.8 with Cu/Fe.Complementary isotope and concentration data thereforesuggest differences in the transition state geometry and/or reaction intermediates in each reductant system.
|
