Carbon-chlorine bond cleavage in chlorofluoroethanes on Pd(111)

The kinetics of carbon-chlorine bond cleavage on a Pd(lll) surface have been investigated using several dichloroethanes with varying fluorine content (CF3CFCl2, CH3CFCl2, CH2FCFCl2, CH3CHCl2, CH2ClCH2Cl). Preliminary results show that the dechlorination rates in these molecules exhibit trends that are similar to those observed in catalytic hydrodechlorination over supported Pd catalysts. Fluorination of the 1,1-dichloroethanes decreases the rate constant for dechlorination. The presence of only one chlorine atom on each carbon atom (CH2ClCH2Cl vs. CH3CHCl2) also reduces the rate constant for dechlorination. A Hammett correlation using field substituent parameters was established for the rate of dechlorination in an attempt to probe the nature of the transition state to carbon-chlorine bond cleavage ([RC ... Cl]double dagger). The dechlorination rate constant for 1,1-dichloroethanes is slightly reduced with increasing field effects (increasing fluorination) and the Hammett correlation reveals a reaction constant of rho = -1.0 +/- 0.2. The interpretation of this result is that there is little polarization of the C-CI bond in the transition state for dechlorination, which is consistent with a transition state that is early in the reaction coordinate.