Disulfur dichloride. Disulfur dichloride (or disulphur dichloride by the British English spelling) is the inorganic compound of sulfur and chlorine with the formula . It is an amber oily liquid.

Sometimes, this compound is incorrectly named sulfur monochloride (or sulphur monochloride by the British English spelling), the name implied by its empirical formula SCl.

 has the structure implied by the formula , wherein the dihedral angle between the  and  planes is 85.2°. This structure is referred to as gauche, and is akin to that for . A rare isomer of  is  (thiothionyl chloride); this isomer forms transiently when  is exposed to UV-radiation (see thiosulfoxides).

Synthesis, basic properties, reactions
Disulfur dichloride is a yellow liquid that fumes in moist air due to reaction with water:

It is produced by partial chlorination of elemental sulfur. The reaction proceeds at usable rates at room temperature. In the laboratory, chlorine gas is led into a flask containing elemental sulfur. As disulfur dichloride is formed, the contents become a golden yellow liquid:
, ΔH = −58.2 kJ/mol
Excess chlorine produces sulfur dichloride, which causes the liquid to become less yellow and more orange-red:
, ΔH = −40.6 kJ/mol
The reaction is reversible, and upon standing,  releases chlorine to revert to the disulfur dichloride. Disulfur dichloride has the ability to dissolve large quantities of sulfur, which reflects in part the formation of polysulfanes:

Disulfur dichloride can be purified by distillation from excess elemental sulfur.

 also arises from the chlorination of  as in the synthesis of thiophosgene or carbon tetrachloride.

Reactions
 hydrolyzes to sulfur dioxide and elemental sulfur. When treated with hydrogen sulfide, polysulfanes are formed as indicated in the following idealized formula:

It reacts with ammonia to give tetrasulfur tetranitride as well as heptasulfur imide () and related S−N rings  (n = 2, 3).

With primary and secondary alkoxide equivalents, it forms disulfoxylate esters:
2 ROH + S2Cl2 + 2 NEt3 → (R–O–S)2 + 2 [HNEt3]Cl
In principle the subsequent addition of base should give sulfoxylate esters, but typically induces disproportionation to aldehydes and alcohols instead.

Applications
 has been used to introduce C−S bonds. In the presence of aluminium chloride (),  reacts with benzene to give diphenyl sulfide:

Anilines (1) react with  in the presence of NaOH to give 1,2,3-benzodithiazolium chloride (2) (Herz reaction) which can be transformed into ortho-aminothiophenolates (3), these species are precursors to thioindigo dyes. 

It is also used to prepare mustard gas via ethylene at 60 °C (the Levinstein process):

If the reaction is performed at a temperature under 30 °C, the sulfur stays in "pseudo-solution" and avoids the problems associated with the sulfur that is formed during the reaction.

Other uses of  include the manufacture of sulfur dyes, insecticides, and synthetic rubbers. It is also used in cold vulcanization of rubbers, as a polymerization catalyst for vegetable oils and for hardening soft woods.

Safety and regulation
 can be used to produce bis(2-chloroethyl)sulfide , known as the mustard gas:

Consequently, it is listed in Schedule 3 of the Chemical Weapons Convention. Facilities that produce and/or process and/or consume scheduled chemicals may be subject to control, reporting mechanisms and inspection by the Organisation for the Prohibition of Chemical Weapons.

References

Disulfides
Sulfur chlorides