# Disulfur dichloride

Disulfur dichloride (or disulphur dichloride by the British English spelling) is the inorganic compound of sulfur and chlorine with the formula S$_2$Cl$_2$. 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.
S$_2$Cl$_2$ has the structure implied by the formula Cl−S−S−Cl, wherein the dihedral angle between the Cl$^a$−S−S and S−S−Cl$^b$ planes is 85.2°. This structure is referred to as gauche, and is akin to that for H$_2$O$_2$. A rare isomer of S$_2$Cl$_2$ is S=SCl$_2$ (thiothionyl chloride); this isomer forms transiently when S$_2$Cl$_2$ 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:
    16 S$_2$Cl$_2$ + 16 H$_2$O → 8 SO$_2$ + 32 HCl + 3 S$_8$
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:
    S$_8$ + 4 Cl$_2$ → 4 S$_2$Cl$_2$, ΔH = −58.2 kJ/mol
Excess chlorine produces sulfur dichloride, which causes the liquid to become less yellow and more orange-red:
    S$_2$Cl$_2$ + Cl$_2$ ⇌ 2 SCl$_2$, ΔH = −40.6 kJ/mol
The reaction is reversible, and upon standing, SCl$_2$ 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:
    8 S$_2$Cl$_2$ + n S$_8$ → 8 S$_{n+2}$Cl$_2$
Disulfur dichloride can be purified by distillation from excess elemental sulfur.
S$_2$Cl$_2$ also arises from the chlorination of CS$_2$ as in the synthesis of thiophosgene or carbon tetrachloride.

### Reactions

S$_2$Cl$_2$ hydrolyzes to sulfur dioxide and elemental sulfur. When treated with hydrogen sulfide, polysulfanes are formed as indicated in the following idealized formula:
    2 H$_2$S + S$_2$Cl$_2$ → H$_2$S$_4$ + 2 HCl
It reacts with ammonia to give tetrasulfur tetranitride as well as heptasulfur imide (S$_7$NH) and related S−N rings S$_{8−n}$(NH)$_n$ (n = 2, 3).
    16 NH$_3$ + 6 S$_2$Cl$_2$ → S$_4$N$_4$ + S$_8$ + 12 NH$_4$Cl
With primary and secondary alkoxide equivalents, it forms disulfoxylate esters:
    2 ROH + S$_2$Cl$_2$ + 2 NEt$_3$ → (R–O–S)$_2$ + 2 [HNEt$_3$]Cl
In principle the subsequent addition of base should give sulfoxylate esters, but typically induces disproportionation to aldehydes and alcohols instead.

## Applications

S$_2$Cl$_2$ has been used to introduce C−S bonds. In the presence of aluminium chloride (AlCl$_3$), S$_2$Cl$_2$ reacts with benzene to give diphenyl sulfide:
    8 S$_2$Cl$_2$ + 16 C$_6$H$_6$ → 8 (C$_6$H$_5$)$_2$S + 16 HCl + S$_8$
Anilines (1) react with S$_2$Cl$_2$ 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):
    8 S$_2$Cl$_2$ + 16 H$_2$C=CH$_2$ → 8 (ClCH$_2$CH$_2$)$_2$S + S$_8$
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 S$_2$Cl$_2$ 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

S$_2$Cl$_2$ can be used to produce bis(2-chloroethyl)sulfide S(CH$_2$CH$_2$Cl)$_2$, known as the mustard gas:
    S$_2$Cl$_2$ + 2 H$_2$C=CH$_2$ → S(CH$_2$CH$_2$Cl)$_2$ + "S"
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.