MECHANISM OF PENTACHLOROETHANE DEHYDROCHLORINATION TO TETRACHLOROETHYLENE

The dehydrochlorination of pentachloroethane to tetrachloroethylene was investigated to gain insight into mechanisms of hexachloroethane reduction as well as structure-reactivity relationships for polyhalogenated alkanes. Although the absence of deuterium exchange excludes the possibility of an (E1CB)R mechanism, several factors suggest the transition state possesses considerable carbanion character: the reaction is insensitive to buffer catalysis, exhibits a moderately large solvent kinetic isotope effect, and only displays a neutral mechanism at low pH. Though our results cannot rule out a stepwise (E1CB)I or (E1CB)ip sequence, we believe CHCl2CCl3 reacts via a concerted mechanism based on a comparison of its dehydrohalogenation kinetics with proton-exchange rates for CHCl3 and CHCl2CF3. Pentachloroethane reported in the reduction of hexachloroethane to tetrachloroethylene is unlikely to result from carbanion protonation. Rather, it may be diagnostic of free-radical reduction mechanisms. Because pentachloroethane reacts relatively rapidly, future studies of hexachloroethane reduction should consider whether pentachloroethane represents a reaction intermediate instead of dismissing it as a minor side product.