Mechanism and Regioselectivity of the Electrochemical Reduction in Polychlorobiphenyls (PCBs): Kinetic Analysis for the Successive Reduction of Chlorines from Dichlorobiphenyls

摘要

The regioselective electrochemical reduction ofthree different dichlorobiphenyls is analyzed using quantum chemical calculations and convolution potential sweep voltam-metry. The heterogeneous rate constants of C-CI bond reductions of each of the dichlorobiphenyls are estimated. The mechanism of the electrochemical reduction is confirmed by estimating the intrinsic barrier from the experimental data on transfer coefficients as well as theoretical calculations involving bond length and potential energy diagrams. The reductions follow the Marcus-Hush quadratic activation driving force relation barring the meta-Cl of the 3,4-dichlorobiphenyI which obeys Butler- Volmer kinetics. The first electron transfer step is rapid in comparison with the bond-breaking, implying the stepwise reduction for all the dichlorobiphenyls studied here. High performance liquid chromotagraphy analysis of the products of the bulk electrolysis confirms the order of the reduction in dichlorobiphenyls viz. the ease of reduction follows the order ortho-Cl > para-Cl > meta-Cl.