Liquid-phase hydrodehalogenation of substituted chlorobenzenes over palladium supported catalysts

The liquid-phase hydrodehalogenation of substituted chlorobenzenes has been carried out over palladium supported on AlPO4–SiO2. The hydrodehalogenation of the p-chlorobromobenzene undergo a consecutive reaction starting with the CBr hydrogenolysis. For the other substituted chlorobenzenes, the reaction proceeds through the hydrogenolysis of the CCl bond. The reaction rate obtained for the substrates with electron-withdrawing substituents is lower than that of chlorobenzene while for the substrates with electron-releasing groups it is higher. This is related to a modification of the adsorption strength of the molecule and the surface reaction trough the inductive and mesomeric effects of the substituents. The Taft equation is used to correlate the hydrodechlorination catalytic activity of the p-substituted chlorobenzenes with inductive and mesomeric parameters of the substituents. These results seem to indicate that the reaction proceeds through an electrophilic attack.