Abstract
Polyhalogenated benzenes, when reacted with hydrogen at atmospheric pressure or sodium hypophosphite and in the presence of Pd/C catalyst in a multi-phase system constituted by a hydrocarbon solvent, a strong alkaline aqueous solution and a quaternary onium salt, allow the rapid and progressive replacement of all the chlorine atoms. Thus, operating with hydrogen at 50°C, 1, 2, 4, 5-tetrachlorobenzene yields benzene after 50 min in 99% yield and using a Pd/Cl ratio of 1/120. The presence of an alkaline medium and a phase-transfer agent is synergic. The enhancement of the reaction rate, compared with the already known methods, has been attributed to the enhanced absorption of hydrogen by the catalyst surtace, to partition of halogenated compounds between the hydrocarbon solution and the liquid phase of the phase-transfer agent and to the rapid removal of HC1 absorbed on Pd/C by the alkaline medium. The reaction is effective also with deactivatedP-methoxyaryl halides.
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