Pathways of chlorinated ethylene and chlorinated acetylene reaction with Zn(0)

To successfully design treatment systems relying on reactions of chlorocarbons with zero-valent metals, information is needed concerning the kinetics and pathways through which transformations occur. In this study, pathways of chlorinated ethylene reaction with Zn(0) have been elucidated through batch experiments. Data for parent compound disappearance and product appearance were fit to pseudo-first-order rate expressions in order to develop a complete kinetic model. Results indicate that reductive beta-elimination plays an important role, accounting for 15% of tetrachloroethylene (PCE), 30% of trichloroethylene (TCE), 85% of cis-dichloroethylene (cis-DCE), and 95% of trans-dichloroethylene (trans-DCE) reaction. The fraction of PCE, TCE, trans-DCE, and cis-DCE transformation that occurs via reductive elimination increases as the two-electron reduction potential (E-2)for this reaction becomes more favorable relative to hydrogenolysis. In the case of PCE a nd TCE, reductive elimination gives rise to chlorinated acetylenes. Chloroacetylene and dichloroacetylene were synthesized and found to react rapidly with zinc, displaying products consistent with both hydrogenolysis and reduction of the triple bond. Surface area-normalized rate constants (k(SA)) for chlorinated ethylene disappearance correlate well with both one-electron (E-1) and two-electron (E-2) reduction potentials for the appropriate reactions. Correlation with E-2 allows prediction of the distribution of reaction products as well as the rate of disappearance of the parent compound.