Disulfur dichloride

Disulfur dichloride is the chemical compound with the formula S₂Cl₂. An alternative name for this compound is sulfur monochloride, the name implied by its empirical formula, SCl. 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 different isomer of S₂Cl₂ is S=SCl₂; this isomer forms transiently when S₂Cl₂ is exposed to UV-radiation.

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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 → S_2+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

References

1. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. W. W. Hartman, L. A. Smith, and J. B. Dickey "Diphenylsulfide" Organic Syntheses, Coll. Vol. 2, p.242; Vol. 14, p.36.
3. R. J. Cremlyn “An Introduction to Organosulfur Chemistry” John Wiley and Sons: Chichester (1996). ISBN 0-471-95512-4
4. Garcia-Valverde M., Torroba T. (2006). "Heterocyclic chemistry of sulfur chlorides - Fast ways to complex heterocycles". European J. Org. Chem. 4: 849-861.