Disulfur dichloride

Disulfur dichloride is the chemical compound with the formula S2Cl2. Some alternative names for this compound are sulfur monochloride (the name implied by its empirical formula, SCl), disulphur dichloride (British English Spelling) and sulphur monochloride (British English Spelling). S2Cl2 has the structure implied by the formula Cl-S-S-Cl, wherein the angle between the Cla-S-S and S-S-Clb planes is 90°. This structure is referred to as gauche, and is akin to that for H2O2. A different isomer of S2Cl2 is S=SCl2; this isomer forms transiently when S2Cl2 is exposed to UV-radiation.

Synthesis and basic properties
Disulfur dichloride is an orange/yellow liquid that smokes in air due to reaction with water:
 * 2 S2Cl2 +  2 H2O  →  SO2  +  4 HCl  +  3/8 S8

It is synthesized by partial chlorination of elemental sulfur:
 * S8 +  4 Cl2  →  4 S2Cl2   ΔH = −58.2 kJ/mol

Excess chlorine gives sulfur dichloride:
 * S2Cl2 + Cl2  →  2 SCl2   ΔH = −40.6 kJ/mol

Both reactions are reversible, and upon standing, SCl2 releases chlorine to revert to the disulfur dichloride. It has the ability to dissolve sulfur, which reflects in part the formation of polysulfanes:
 * S2Cl2 +  n S  →  S2+nCl2

S2Cl2 also arises from the chlorination of CS2 as in the synthesis of thiophosgene.

Applications in synthesis
S2Cl2 has been used to introduce C-S bonds. In the presence of AlCl3, S2Cl2 reacts with benzene to give diphenyl sulfide:
 * S2Cl2 +  C6H6  →  (C6H5)2S  +  2 HCl  +  1/8 S8

Anilines react with S2Cl2 in the presence of NaOH via the so-called Herz reaction to give ortho-aminothiophenolates. These species are precursors to thioindigo dyes. It is also used to prepare the sulfur mustard "gas" by reaction with ethylene:
 * S2Cl2 +  2 C2H4  →  (ClC2H4)2S  +  1/8 S8