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

Abstract

The kinetics of carbon–chlorine bond cleavage on a Pd(111) 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 ($\ddag$). 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 ρ = -1.0 ± 0.2. The interpretation of this result is that there is little polarization of the C–Cl bond in the transition state for dechlorination, which is consistent with a transition state that is early in the reaction coordinate.