Facile Hydrodehalogenation with Hydrogen and Pd/C Catalyst under Multiphase Conditions

A multiphase system consisting of a hydrocarbon solvent, a strong alkaline solution, and a quaternary onium salt, in the presence of a Pd/C catalyst with hydrogen at atmospheric pressure, allows the rapid and progressive displacement of the chlorine atoms from polyhalogenated benzenes. The onium salt in this case constitutes a third liquid phase in which the reaction takes place. At 50 "C, 1,2,4,5-tetrachlorobenzene is reduced to benzene in 30 min using a Pd/Cl molar ratio of 1/130 Halogenated compounds are partitioned between the hydrocarbon solution and the liquid phase of the phase-transfer agent; rapid removal of HC1 adsorbed on Pd/C is effected by neutralization with the alkaline solution. The enhancement of the reaction rate compared with the known methods might be attributed to the facile adsorption of Hz by the catalyst under the reaction conditions. Different reaction rates result in the reduction of the three isomeric chlorotoluenes, whether the reaction is carried out in the presence or absence of the onium salt.