Reactivity of Substituted Chlorines and Ensuing Dechlorination Pathways of Select PCB Congeners with Pd/Mg Bimetallics

Conflicting accounts occur on the reactivity of substituted chlorines and the ensuing dechlorination pathway of PCBs undergoing catalytic hydrodechlorination (HDCl). In order to understand these relationships, intermediates and dechlorination pathways of carefully selected 17 congeners were investigated with reactive Pd/Mg systems that bring about their rapid and complete dechlorination. The preferential site of electrophilic attack and its mechanistic aspects were interpreted in terms of steric, inductive, and resonance stabilization. The trends for electrophilic substitution were consistently p- > m- > o-positions indicating that more toxic "coplanar" PCB congeners were easily reduced. The dechlorination rates and pathways were influenced both by inductive effect of Cl that likely governs the stability of the intermediate arenium ion and by steric effects primarily effecting the adsorption step (especially for the o-congeners). Electrophilic attack occurred preferentially on the less substituted phenyl ring in absence of steric effects. A distinct correlation between rate of HDCl and the degree of chlorination was not observed, rather it depended on positions of Cl with respect to the biphenyl bond, and the dominance between counteracting factors of deactivation by subsequent chlorinations and improvement in probability of dechlorination through increased number of Cls.