Solvent effects in the hydrodechlorination of 2,4-dichlorophenol over pd/Al₂O₃

Solvent effects in the liquid phase (0.1 MPa; 303 K) hydrodechlorination (HDC) of 2,4-dichlorophenol have been established over Pd/Al₂O₃. In the absence of secondary reactions, catalyst deactivation, and transport limitations, a stepwise HDC yields 2-chlorophenol and phenol, where product selectivity was insensitive to the nature of the solvent. In contrast, the initial HDC rates exhibited a marked dependence on the reaction medium and increased in the order: benzene < THF < n-hexane < cyclohexane < alcohols < water. Higher rates result from the concomitant effect of an increase in the dielectric constant (e{open}) and a decrease in the molar volume (v) of the solvent, where the major (ca. 80%) contribution is due to e{open}. We attribute this response to the increased solvent capacity to stabilize the arenium intermediate at higher/lower e{open}v, an effect that extends to reaction in water + organic combinations. We provide, for the first time, a reliable quantification of solvent effects that can be potentially applied to other catalytic hydrogenolysis systems.