Dechlorination of Fluorinated 1,1-Dichloroethanes on Pd(111)

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Abstract

The kinetics of C−Cl bond cleavage have been investigated by studying the dissociative adsorption of four fluorinated 1,1-dichloroethanes (CH3CHCl2, CH3CFCl2, CH2FCFCl2, CF3CFCl2) on a Pd(111) surface. The apparent rate constants (kapp) and activation energies (Eapp) for dissociative adsorption on Pd(111) show a systematic trend that depends on the fluorine content of the molecule. Fluorination decreases the rate constant for dissociative adsorption, an effect that is identical to the trend in activity observed for catalytic hydrodechlorination of fluorinated chloromethanes over supported Pd catalysts. Measurement of the desorption energies (Edes) of the dichloroethanes on Pd(111) have shown that increasing the amount of fluorine in the molecule decreases the desorption energy. The focus of this investigation has been the determination of the effect of fluorination on the intrinsic activation barrier to C−Cl bond cleavage (EC-Cl). The intrinsic barrier to C−Cl cleavage was determined for each dichloroethane by adding the desorption energy and the apparent activation energy for dissociative adsorption (EC-Cl = Edes + Eapp). A linear free energy relationship was established for the rate of dechlorination in an attempt to probe the nature of the transition state to C−Cl bond cleavage ([RC···Cl]‡). The intrinsic dechlorination barrier remains unchanged as the degree of fluorination in the molecule is varied. The interpretation of this result is that the electron distribution between the carbon and the chlorine atoms in the transition state for C−Cl bond cleavage is similar to that of the reactant. This is consistent with a homolytic transition state that is early in the reaction coordinate.

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