FACILE HYDRODEHALOGENATION WITH H-2 AND PD/C CATALYST UNDER MULTIPHASE CONDITIONS .2. SELECTIVITY AND KINETICS

Hydrodehalogenation of polyhalogenated aromatics with Pd/C catalyst carried out in the presence of a quaternary onium salt follows zero-order kinetics in the substrate and first-order kinetics in the Pd/C catalyst; the related rate constants were determined for o-, m- and p-bromotoluenes, o-, m- and p-chloroalkylbenzenes (methyl, ethyl, and propyl derivatives), and other aryl halides. Reaction rates, depending on the aromatic to be reduced, may be strongly enhanced by the presence of quaternary onium salts: the isomeric chloroethylbenzenes were reduced 50 times faster when operating in the presence of Aliquat 336 (1). Also the hindered 2-chloro-m-xylene easily yielded m-xylene. The cocatalyst onium salts operate by being adsorbed on the Pd/C surface, as shown when kinetic constants are reported by varying the onium salt amount: classical Langmuir adsorption isotherms are observed. The presence of the onium salt may also influence selectivity in the reduction of isomeric aryl halides: when 1 is present, p-dichlorobenzene reacts in diethyl ether at 20 degrees C, 5-fold slower than the ortho isomer; whereas the reduction rates of the two compounds are almost the same in its absence.