Facile, efficient, and environmentally friendly α- 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 α-chlorination or aromatic chlorination of toluene was favoured. The chlorinated solvents CH₂Cl₂, CHCl₃, and CCl₄ favoured the α-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.