Electrolytic reduction of low molecular weight chlorinated aliphatic compounds: Structural and thermodynamic effects on process kinetics

A series of chlorinated low molecular weight alkanes and alkenes was transformed electrolytically using a porous nickel cathode at potentials from -0.3 to -1.4 V (versus standard hydrogen electrode). Kinetics were first- order with respect to the concentration of the halogenated targets. The dependence of the first-order rate constants on cathode potential followed thee Butler-Volmer equation, modified to account for mass transfer resistance to reaction. The masstransfer-limited rate constant for reaction of all species was about 1.55 L m^-2 min^-1. Log-transformed reaction rate constants for reduction of chlorinated alkanes, derived via experiments at the same cathode potential (E(c) = -1.0 or -1.2 V vs SHE), were linearly related to carbon-halogen bond enthalpies, as expected based on a physical model that was developed from transition state theory. The chlorinated ethenes reacted much faster than predicted from bond enthalpy calculations and the alkane-based correlation, suggesting that alkenes are not transformed via the same mechanism as the chlorinated alkanes. Dihaloelimination was the predominant pathway for reduction of vicinal polychlorinated alkanes. For chlorinated alkenes and geminal chlorinated alkanes, sequential hydrogenolysis was the major reaction pathway.