Polysulfane

A polysulfane is a chemical compound of formula H2S_{n}, where n > 1 (although disulfane (H2S2) is sometimes excluded). Compounds containing 2 – 8 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. In the chemical literature the term polysulfanes is sometimes used for compounds containing \s(S)_{n}\s, e.g. organic polysulfanes R^{1}\s(S)_{n}\sR^{2}.

Structures
Polysulfanes consist of unbranched chains of sulfur atoms terminated with hydrogen atoms. The branched isomer of tetrasulfane H2S4, in which the fourth sulfur is bonded to the central sulfur, would be described as trithiosulfurous acid, S\dS(\sSH)2. Computations suggests that it is less stable than the linear isomer HS\sS\sS\sSH. The S-S-S angles approach 90° in trisulfane H2S3 and higher polysulfanes.

Reactions and properties
Polysulfanes can easily be oxidised, and are thermodynamically unstable with respect to decomposition (disproportionation) readily to H2S and sulfur:
 * 8 H2S_{n} → 8 H2S + (n \s 1) S8 (in this chemical reaction, S8 is cyclo-octasulfur, one of the allotropes of sulfur)

This decomposition reaction is catalyzed by alkali. To suppress this behavior, containers for polysulfanes are often pretreated with acid to remove traces of alkali.

In contrast to the thermodynamic instability of polysulfates, polysulfide anions form spontaneously by treatment of S(2-) with elemental sulfur:
 * S(2-) + (n \s 1) S → [S_{n}](2-)

Beyond H2S and H2S2, many higher polysulfanes H2S_{n} (n = 3 – 8) are known. They have unbranched sulfur chains. Starting with disulfane H2S2, all known polysulfanes are liquids at room temperature. The density, boiling point and viscosity correlate with chain length. Physical properties of polysulfanes are given in the table below.

They also react with sulfite and cyanide producing thiosulfate and thiocyanate respectively.

Polysulfanes can be made from polysulfides by pouring a solution of a polysulfide salt into cooled concentrated hydrochloric acid. A mixture of metastable polysulfanes separates as a yellow oil, from which individual compounds may be separated by fractional distillation. Other more selective syntheses are:


 * Na2S_{n} + 2 HCl → 2 NaCl + H2S_{n} (n = 4, 5, 6)
 * S_{n}Cl2 + 2 H2S_{m} → 2 HCl + H2S_{n+2m}

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


 * 2 SCl2 + H2S_{n} → 2 HCl + S_{2+n}Cl2
 * 2 S2Cl2 + H2S_{n} → 2 HCl + S_{4+n}Cl2

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


 * (n \s 1) SO3(2-) + H2S_{n} → (n \s 1) S2O3(2-) + H2S