Facile, efficient, and environmentally friendly alpha- and aromatic regioselective chlorination of toluene using KHSO5 and KCl under catalyst-free conditions

Abstract

Toluene was subjected to room-temperature chlorination with the KHSO5-KCl system in organic solvent-water mixtures. In a typical reaction, the amounts of toluene, KHSO5, and KCl were 1.0, 5.0 and 7.5 millimoles, respectively. The reaction mixtures, consisting of an organic solvent (2.50 mL) and water (1.50 mL), were rather multiphase like slurry. Depending on the nature of the organic solvent present in the solvent mixtures, either alpha-chlorination or aromatic chlorination of toluene was favoured. The chlorinated solvents CH2Cl2, CHCl3, and CCl4 favoured the alpha-chlorination, yielding >81% benzyl chloride selectivity in 38-59% toluene conversions in 1 hour. The water miscible polar solvents C2H5OH, CH3OH, and CH3CN favoured the aromatic chlorination, yielding 61-86% chlorotoluene selectivity in >98% toluene conversions in 1 hour. Reactions carried out in all solvent mixtures yielded only chlorinated toluene products. The method presented here for the regioselective chlorination of toluene meets the goals of green chemistry as well in a number of ways: (a) catalyst-free conditions; (b) nontoxic, green, safe, stable, and inexpensive reagents; (c) low environmental impact; (d) no heating or cooling; (e) simple and cheap operation; and (f) high regioselectivity.