Catalytic interconversion of alkyl halides by gas-liquid phase-transfer catalysis
High halogen exchange conversions are achieved when a gaseous mixture of alkyl halides (chlorides, bromides, iodides) is passed over a solid bed consisting of porous inorganic supports bearing a phasetransfer catalyst under gas-liquid phase-transfer catalysis (g.l.-p.t.c.) conditions. The process is catalytic since the bed undergoes no changes once it reaches operating conditions. For example, a methylene dichloride and bromoethane mixture is converted into all the halogen-exchange products, and their statistical distribution at equilibrium depends on the original ratio of the halogens in the organic reagents. Catalytic activity is high: 200 ml of such a mixture can be converted in 1 h by passage through 200 g of alumina coated with 10% tetrabutylphosphonium bromide. The catalytic process is promoted by the halide anions present as Q+X– in the liquid phase constituted by the molten catalyst and as Na+X– in the solid inorganic support; the halide anions partition themselves between the liquid and solid phases as a function of their respective affinities. This catalysis depends on the diffusion, partition, and adsorption of the alkyl halides between the gaseous, liquid, and solid phases, as well as on their intrinsic nucleophilic reactivity. Mechanistic and industrial applicability are discussed.