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 the Butler-Volmer equation, modified to account for mass transfer resistance to reaction. The mass-transfer-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-hased correlation, suggesting that alkenes are not-transformed via the same mechanism as the chlorinated alkanes. Dihalo-elimination was the predominant pathway for reduction of vicinal polychlorinated alkanes. For chlorinated alkenes and geminal chlorinated alkanes, sequential hydrogenolysis was the major reaction pathway.