Formation of Diphenyl Sulfoxide and Diphenyl Sulfide via the Aluminum Chloride–Facilitated Electrophilic Aromatic Substitution of Benzene with Thionyl Chloride, and a Novel Reduction of Sulfur (IV) to Sulfur (II)

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.

Keywords:

• Diphenyl sulfide
• diphenyl sulfoxide
• EAS
• reduction
• sulfur
• thionyl chloride