Chemistry:Polysulfane

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Short description: Class of chemical compounds

A polysulfane is a chemical compound of formula H
2
S
n
, where n > 1 (although disulfane (H
2
S
2
) is sometimes excluded).[1][2][3] Polysulfanes consist of unbranched chains of sulfur atoms terminated with hydrogen atoms. Compounds containing 2 – 8 concatenated sulfur atoms have been isolated, longer chain compounds have been detected, but only in solution. H
2
S
2
is colourless, higher members are yellow with the colour increasing with the sulfur content. Even a trace of alkali will cause chemical decomposition, and containers need to be treated with acid to remove any traces of alkali. In the chemical literature the term polysulfanes is sometimes used for compounds containing –(S)
n
, e.g. organic polysulfanes R1
–(S)
n
–R2
.

Since sulfur is a chalcogen, polysulfanes can be classed as hydrogen chalcogenides.

Chemistry and properties

Polysulfanes are thermodynamically unstable with respect to decomposition (disproportionation) to H
2
S
and sulfur:

8 H
2
S
n
→ 8 H
2
S + (n – 1) S
8

where S
8
is cyclo-octasulfur or cyclooctasulfane, one of the allotropes of sulfur. However, the production of polysulfide ions from S2− and S is thermodynamically favourable:

S2− + (n – 1) S → [S
n
]2−
; ΔH = −ve

Attempting to make a polysulfane by acidifying an alkali metal polysulfide salt (e.g. sodium disulfide) simply produces hydrogen sulfide, H
2
S
, 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:

Na
2
S
n
+ 2 HCl → 2 NaCl + H
2
S
n
(n = 4, 5, 6)
S
n
Cl
2
+ 2 H
2
S
m
→ 2 HCl + H
2
S
n+2m

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

2 SCl
2
+ H
2
S
n
→ 2 HCl + S
2+n
Cl
2
2 S
2
Cl
2
+ H
2
S
n
→ 2 HCl +S
4+n
Cl
2

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

(n – 1) SO2−
3
+ H
2
S
n
→ (n – 1) S
2
O2−
3
+ H
2
S

References

  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "polysulfanes". doi:10.1351/goldbook.P04753
  2. Wiberg, E.; Holleman, A. F.; Wiberg, Nils (2001) (in en). Inorganic Chemistry. San Diego : Academic Press; New York : De Gruyter. ISBN 978-0-12-352651-9. OCLC 48056955. https://books.google.com/books?id=Mtth5g59dEIC. 
  3. Greenwood, N. N.; Earnshaw, A. (2012-12-02) (in en). Chemistry of the Elements. Boston, Massachusetts, MA: Elsevier. ISBN 978-0-08-050109-3. OCLC 48138330. https://books.google.com/books?id=EvTI-ouH3SsC.