Reaction of benzyl chloride with ammonium sulfide under liquid–liquid phase transfer catalysis: Reaction mechanism and kinetics

The reaction between benzyl chloride and aqueous ammonium sulfide was carried out in an organic solvent – toluene, using tetrabutylammonium bromide (TBAB) as phase transfer catalyst (PTC). Two products, namely dibenzyl sulfide (DBS) and benzyl mercaptan (BM), were identified in the reaction mixture. The selectivity of DBS was maximised by changing various parameters such as NH3/H2S mole ratio, stirring speed, catalyst loading, concentration of benzyl chloride, volume of aqueous phase, and temperature. The highest selectivity of DBS obtained was about 90% after 445 min of reaction with excess benzyl chloride at 60 °C. Complete conversion of benzyl chloride could be achieved at the cost of very low selectivity of DBS and very high selectivity of BM. The apparent activation energy for the kinetically controlled reaction was found to be 12.3 kcal/mol. From the detailed study of the effects of various parameters on the reaction, a suitable mechanism was established which could explain the course of the reaction.

2C6H5CH2Cl+(NH4)2S⇌C6H5CH2–S–CH2C6H5+2NH4Cl2C6H5CH2Cl+(NH4)2S⇌C6H5CH2–S–CH2C6H5+2NH4ClC6H5CH2Cl+NH4HS⇌C6H5CH2–SH+NH4ClC6H5CH2Cl+NH4HS⇌C6H5CH2–SH+NH4ClC6H5CH2Cl+C6H5CH2–SH⇌C6H5CH2–S–CH2C6H5+HClC6H5CH2Cl+C6H5CH2–SH⇌C6H5CH2–S–CH2C6H5+HCl