Selectivity in hydrodehalogenation of polychloro- and polybromobenzenes under multiphase conditions

The competitive hydrodehalogenation of isomeric o-, m- and p-dichloro (and dibromo) benzenes with Hz at atmospheric pressure and Pd/C or Ni-Raney carried out in a multiphase system (organic solvent and 50% KOH aq. solution) is influenced by the presence of a bulky quatemary onium salt where both the catalytic activity and the selectivity change in relation to the halogen to be removed and the metal catalyst used. In particular, the Ni-Raney catalyst becomes effective in the reduction only if an onium salt is added. Similarly, the onium salt effects the catalytic hydrodehalogenation of 1,2&trichloro- and 1,2,4- tribromobenzene.