Polysulfane

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A polysulfane is a compound of formula H₂S_x, where x > 1, although sometimes disulfane, H₂S₂ is excluded.^[1] Polysulfanes consist of un-branched chains of sulfur atoms terminated with hydrogen. Compounds containing 2-8 concatenated sulfur atoms have been isolated, longer chain compounds have been detected, but only in solution. H₂S₂ is colourless, higher members are yellow with the colour increasing with the sulfur content. Even a trace of alkali will cause decomposition and containers need to treated with acid to remove any traces of alkali.
In the chemical literature the term polysulfanes is sometimes used for compounds containing −(S_n)−, eg organic polysulfanes R−(S_n)−R.

[edit] Chemistry and properties

Polysulfanes are thermodynamically unstable with respect to decomposition (disproportionation) to H₂S and sulfur:

H₂S_n → H₂S + (n−1)/8 S₈

However the production of polysulfide ions from S²⁻ and S is thermodynamically favourable:

S₂− + S → S_n²⁻ ; ΔH= −ve

Attempting to make a polysulfane by acidifying an alkali metal polysulfide salt (e.g. sodium disulfide) simply produces hydrogen sulfide, H₂S, and sulfur. Polysulfanes can be made from polysulfides by pouring polysulfide into cooled concentrated hydrochloric acid to produce a mixture of metastable polysulfanes as a yellow oil, from which individual compounds may be separated by fractional distillation. Other more selective syntheses are:

Na₂S_n + 2HCl → 2NaCl + H₂S_n (n=4-6)

S_nCl₂ + 2H₂S_m → 2HCl + H2S_n+2m

The reaction of polysulfanes with sulfur dichloride or disulfur dichloride produces long chain dichloropolysulfanes:

2SCl₂ + H₂S_n → 2HCl + S_2+nCl₂

2S₂Cl₂ + H₂S_n → 2HCl +S_4+nCl₂

The reaction with a sulfite salt (a base) quantitatively decomposes the polysulfane to produce thiosulfate and hydrogen sulfide:

(n−1)SO₃²⁻ + H₂S_n → (n−1)S₂O₃²⁻ + H₂S

[edit] References

Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0-123-52651-5
Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 

1. ^ International Union of Pure and Applied Chemistry. "polysulfanes". Compendium of Chemical Terminology Internet edition.