Sulfur dichloride

SCl₂ is produced by the chlorination of either elemental sulfur or disulfur dichloride.[1] The process occurs in a series of steps, some of which are:

S₈ + 4 Cl₂ → 4 S₂Cl₂; ΔH = −58.2 kJ/mol

S₂Cl₂ + Cl₂ ↔ 2 SCl₂; ΔH = −40.6 kJ/mol

The addition of Cl₂ to S₂Cl₂ has been proposed to proceed via a mixed valence intermediate Cl₃S−SCl. SCl₂ undergoes even further chlorination to give SCl₄, but this species is unstable at near room temperature. It is likely that several S_nCl₂ exist where n > 2.

Disulfur dichloride, S₂Cl₂, is the most common impurity in SCl₂. Separation of SCl₂ from S₂Cl₂ is possible via distillation with PCl₃ to form an azeotrope of 99% purity, however sulfur dichloride loses chlorine slowly at room temperature and reverts to disulfur dichloride. Pure samples may be stored in sealed glass ampules which develop a slight positive pressure of chlorine, halting the decomposition.

SCl₂ is used in organic synthesis. It adds to alkenes to give chloride-substituted thioethers. Illustrative is its addition to 1,5-cyclooctadiene to give a bicyclic thioether[2] A well tested method for the production of the mustard gas bis(2-chloroethyl)sulfide, is the addition of ethylene to sulfur dichloride:[3]

SCl₂ + 2 C₂H₄ → (ClC₂H₄)₂S

SCl₂ is also a precursor to several inorganic sulfur compounds. Treatment with fluoride salts gives SF₄ via the decomposition of the intermediate sulfur difluoride. With H₂S, SCl₂ reacts to give "lower" sulfanes such as S₃H₂.

Reaction with ammonia affords sulfur nitrides related to S₄N₄. Treatment of SCl₂ with primary amines gives sulfur diimides. One example is di-t-butylsulfurdiimide.[4]

SCl₂ hydrolyzes with release of HCl. Old samples contain Cl₂.