Continuous conversion of alcohols into alkyl halides by gas–liquid phase-transfer catalysis (G.L.–P.T.C.)
A solution of a primary alcohol (propan-1-ol, butan-1-ol, and pentan-1-ol) in an aqueous halogen acid (hydrochloric or hydrobromic acid) when passed in the gaseous state over a solid porous bed (silica gel) supporting a catalyst, affords the corresponding alkyl halide. The reagent mixture is introduced continuously into a column maintained at 170 °C and the product is collected at the outlet. Since the process is catalytic, the bed is not consumed during the reaction. The reaction by-products depend on the type of catalyst used: Lewis acids (ZnCl2, AlCl3) lead to large quantities of alkenes and isomeric halides, while under typical g.l.–p.t.c. conditions, with a phosphonium salt as the catalyst, only the corresponding dialkyl ether, in addition to the primary alkyl halide, is obtained. The yield of the ether can be reduced progessively to zero by increasing the contact time between the reaction mixture and the catalytic bed.