Polysulfane

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 − 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 S²⁻ with elemental sulfur:

S²⁻ + (n − 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_nCl₂ + 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+nCl₂

2 S₂Cl₂ + H₂S_n → 2 HCl + S_4+nCl₂

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

(n − 1) SO2−3 + H₂S_n → (n − 1) S₂O2−3 + H₂S

Polysulfanes are produced in biology via oxidation of H₂S. As catenated sulfur species with a terminal hydropersulfide (-SSH) group, polysulfanes in aqueous solution at physiological pH self-react, resulting in a complex equilibrium of polysulfane species. Polysulfanes are truncated in biological milieu by H₂S/HS^– and thiols, forming hydropersulfides/hydropolysulfides and polysulfides. Some representative reactions are below:

HSS_nSH ⇌ HSS_nS^– + HSS_nSH ⇌ HSS_nSS_nSH + HS^–

HSSSSH + HS^– ⇌ HSSH + HSSS^–

HSS_nSH + RSH ⇌ H₂S + H₂S₂ + ... + H₂SS_n + RSSH + RSSSH + ... + RSS_nSH