Published: December 05, 2011 r2011 American Chemical Society 655 dx.doi.org/10.1021/jp2066474 | J. Phys. Chem. C 2012, 116, 655–664 ARTICLE pubs.acs.org/JPCC Mechanism and Regioselectivity of the Electrochemical Reduction in Polychlorobiphenyls (PCBs): Kinetic Analysis for the Successive Reduction of Chlorines from Dichlorobiphenyls A. Muthukrishnan, †,§ Vadim Boyarskiy, ‡ M. V. Sangaranarayanan,* ,† and Irina Boyarskaya ‡ † Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India ‡ Department of Chemistry, St. Petersburg State University, Universitetskii pr. 26, St. Petersburg, 198504 Russia b S Supporting Information 1. INTRODUCTION Polychlorobiphenyls (PCBs) constitute a major environmen- tal pollutant. PCBs are employed for diverse purposes such as coolants for transformers, capacitors, pesticide extenders, cutting oils, reactive flame retardants, lubricating oils, hydraulic fluids, adhesives, wood floor finishes, paints, dedusting agents, water- proofing compounds, casting agents, vacuum pump fluids, fixatives in microscopy, surgical implants, etc. The decomposi- tion of PCBs is mandatory since PCBs are highly toxic for humans and animals. Hence diverse techniques have been developed for the degradation of PCBs and among them, the following deserve mention: (a) thermal oxidative degradation at high temperatures; 1 (b) hydrogenation of PCBs using bimetallic surfaces; 2 (c) cobalt-catalyzed methoxycarbonylation; 3 (d) photo- chemical decomposition; 4,5 (e) biochemical degradation of PCBs using bacteria; 6,7 and (f) decomposition of polychlorobi- phenyls using supercritical water containing NaOH into phenol, biphenyl, and CO 2 . 8 The conventional chemical methods of decomposition of polychlorobiphenyls such as reduction, hydro- genation, and dechlorination have also been known. 2,5,9À12 It must be emphasized that among diverse methods of removal of pollutants, electrochemical decomposition constitutes the safest and most effective method. Furthermore, selective electroche- mical dechlorinations can be achieved. One of the earliest analyses for the electrochemical reduc- tion of the polychlorobiphenyls and polychloronapthalenes consists of employing the technique of interrupted sweep voltammogram. 13 Alternately, the presence of aromatic radical- anion mediators such as biphenyl and naphthalene enhance the rate of the reduction of polychlorobiphenyls. 14 The electroche- mical decomposition of 1-chloronapthalene using naphthalene radical anion mediator provides complete dechlorination. 15 The regioselective electrochemical decomposition of 2,4-dichlorobi- phenyls and a preliminary kinetic and mechanistic analysis using convolution potential sweep voltammtery (CPSV) have been recently carried out. 16 The electrochemical reduction of polychlorobiphenyls in- volves the dissociation of the CÀCl bonds. The CPSV method is a simple and effective technique for the elucidation of the reaction mechanism. 17 The decomposition of each CÀCl bond in polychlorobiphenyls follows one of the two different mechan- isms viz (i) stepwise and (ii) concerted (Scheme 1). The relative rates of the two steps of the reduction (electron transfer vs the bond breaking step) determine the mechanism of the reduction. If the bond breaking step is slow relative to the electron transfer, the reaction follows the stepwise mechanism, and analogously, if the bond breaking step is faster, the concerted mechanism is inferred. The neutral radical formed after dissociation of chloride Received: July 13, 2011 Revised: November 30, 2011 ABSTRACT: The regioselective electrochemical reduction of three different dichlorobiphenyls is analyzed using quantum chemical calculations and convolution potential sweep voltam- metry. The heterogeneous rate constants of CÀCl bond reduc- tions of each of the dichlorobiphenyls are estimated. The mecha- nism 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 MarcusÀ Hush quadratic activation driving force relation barring the meta-Cl of the 3,4-dichlorobiphenyl which obeys ButlerÀVolmer 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.