Continuous-flow, gas phase synthesis of 1-chlorobutane (1-bromobutane) from 1-butanol and aqueous HCl (HBr) over silica-supported quaternary phosphonium salt

The continuous-flow syntheses of 1-chlorobutane and 1-bromobutane were achieved in the gas phase starting from 1-butanol, at 130-170 degrees C and atmospheric pressure. An aqueous mixture of 1-butanol and commercial hydrochloric or hydrobromic acids (37 and 48%, respectively) was fed into a plug-flow catalytic reactor loaded with zinc chloride (ZnCl2: 5 and 15 wt%) or a phosphonium salt (n-Bu4P+Br-: 5 and 15 wt%), both supported on silica gel. Overall yields in butyl halides ranged from 87 to 30%, with weight hourly space velocity (WHSV) up to 0.53 h(-1). The onium salt allowed a high reaction selectivity, since no transposition products were observed. In the presence of a molten phase composed of the onium salt, a S(N)2 nucleophilic substitution takes place. The reaction mechanism is discussed.