Disulfur Dichloride

Disulfur dichloride is the inorganic compound of sulfur and chlorine 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 Cl^a−S−S and S−S−Cl^b planes is 90°. This structure is referred to as ''gauche'', and is akin to that for H₂O₂. A rare isomer of S₂Cl₂ is S=SCl₂; this isomer forms transiently when S₂Cl₂ is exposed to UV-radiation (see thiosulfoxides).

Synthesis, basic properties, reactions

Pure disulfur dichloride is a yellow liquid that "smokes" in moist air due to reaction with water. An idealized (but complicated) equation is:
:16 S₂Cl₂ + 16 H₂O → 8 SO₂ + 32 HCl + 3 S₈

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:F. Fehér "Dichlorodisulfane" in ''Handbook of Preparative Inorganic Chemistry'', 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 371.
:S₈ + 4 Cl₂ → 4 S₂Cl₂, ''ΔH'' = −58.2 kJ/mol
Excess chlorine produces sulfur dichloride, which causes the liquid to become less yellow and more orange-red:
:S₂Cl₂ + Cl₂ ↔ 2 SCl₂, ''ΔH'' = −40.6 kJ/mol
The reaction is reversible, and upon standing, SCl₂ 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:
:S₂Cl₂ + n S → S_2+nCl₂
Disulfur dichloride can be purified by distillation from excess elemental sulfur.

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

Reactions

S₂Cl₂ hydrolyzes to sulfur dioxide and elemental sulfur. When treated with hydrogen sulfide, polysulfanes are formed as indicated in the following idealized formula:
:2 H₂S + S₂Cl₂ → H₂S₄ + 2 HCl
It reacts with ammonia to give heptasulfur imide (S₇NH) and related S−N rings S_8−x(NH)_x (x = 2, 3).

Applications

S₂Cl₂ has been used to introduce C−S bonds. In the presence of aluminium chloride (AlCl₃), S₂Cl₂ reacts with benzene to give diphenyl sulfide:
:S₂Cl₂ + 2 C₆H₆ → (C₆H₅)₂S + 2 HCl + 1/8 S₈
Anilines (1) react with S₂Cl₂ in the presence of NaOH to give 1,2,3-benzodithiazolium salts (2) (Herz reaction) which can be transformed into ''ortho''-aminothiophenolates (3), these species are precursors to thioindigo dyes.
:
It is also used to prepare sulfur mustard, commonly known as "mustard gas", by reaction with ethylene at 60 °C (the Levinstein process):
:S₂Cl₂ + 2 C₂H₄ → (ClC₂H₄)₂S + 1/8 S₈
Other uses of S₂Cl₂ include the manufacture of sulfur dyes, insecticides, and synthetic rubbers. It is also used in cold vulcanization of rubbers, as polymerization catalyst for vegetable oils and for hardening soft woods.

Safety and regulation

This substance is listed in Schedule 3 Part B – Precursor Chemicals of the Chemical Weapons Convention (CWC). Facilities that produce and/or process and/or consume Scheduled chemicals may be subject to control, reporting mechanisms and inspection by the OPCW (Organisation for the Prohibition of Chemical Weapons).

References

{{Chlorides

Disulfides
Sulfur chlorides