CARBON-CHLORINE BOND-CLEAVAGE IN CHLOROFLUOROETHANES ON PD(111)

The kinetics of carbon-chlorine bond cleavage on a Pd(lll) surface hav e been investigated using several dichloroethanes with varying fluorin e content (CF3CFCl2, CH3CFCl2, CH2FCFCl2, CH3CHCl2, CH2ClCH2Cl). Preli minary results show that the dechlorination rates in these molecules e xhibit trends that are similar to those observed in catalytic hydrodec hlorination over supported Pd catalysts. Fluorination of the 1,1-dichl oroethanes decreases the rate constant for dechlorination. The presenc e of only one chlorine atom on each carbon atom (CH2ClCH2Cl vs. CH3CHC l2) also reduces the rate constant for dechlorination. A Hammett corre lation 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). Th e dechlorination rate constant for 1,1-dichloroethanes is slightly red uced with increasing field effects (increasing fluorination) and the H ammett 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 coor dinate.