Abstract
The aluminum chloride (AlCl3)–facilitated electrophilic aromatic substitution (EAS) reaction of benzene (PhH) with thionyl chloride (SOCl2) in the molar ratio of PhH:SOCl2:AlCl3 = 2:1:1 was studied under different conditions. At about 70°C, adding granular AlCl3 (equimolar to SOCl2) piecewise to a 2:1 mixture of PhH and SOCl2 led to formation of a highly pure (99.9%) sole product of diphenyl sulfoxide (Ph2SO) in an isolated yield of 85% after aqueous workup. It represents a simple and very efficient synthesis of Ph2SO, an important fundamental organic reagent. At about 0°C, adding PhH (twice the molar quantity of SOCl2) to a 1:1 mixture of SOCl2 and AlCl3 gave both diphenyl sulfide Ph2S (yield: 37%) and Ph2SO (yield: 27%) after aqueous workup, together with a small amount of S-phenyl benzenesulfonothioate PhSO2SPh (yield: about 5%) and a trace amount of diphenyl sulfone (Ph2SO2). A possible mechanism has been proposed to account for formation of all these products, especially for the formation of Ph2S [S (II)] from SOCl2 [S (IV)], which involves a novel reduction of sulfur in the course of the EAS reaction.