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] Compounds containing 2 – 8 sulfur atoms have been isolated, longer chain compounds have been detected, but only in solution.[4] H
2
S
2
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)
n
, e.g. organic polysulfanes R1
–(S)
n
–R2
.

Structures

Polysulfanes consist of unbranched chains of sulfur atoms terminated with hydrogen atoms. The branched isomer of tetrasulfane H
2
S
4
, in which the fourth sulfur is bonded to the central sulfur, would be described as trithiosulfurous acid, S=S(–SH)
2
. Computations suggests that it is less stable than the linear isomer HS–S–S–SH.[5] The S-S-S angles approach 90° in trisulfane H
2
S
3
and higher polysulfanes.[4]

Reactions and properties

Polysulfanes can easily be oxidised, and are thermodynamically unstable with respect to decomposition (disproportionation) readily to H
2
S
and sulfur:

8 H
2
S
n
→ 8 H
2
S + (n – 1) S
8
(in this chemical reaction, S
8
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.[6]

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 H
2
S
and H
2
S
2
, many higher polysulfanes H
2
S
n
(n = 3 – 8) are known.[7] They have unbranched sulfur chains. Starting with disulfane H
2
S
2
, 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.[6]

Chemical formula Name Density at 20 °C (g/cm3) Vapour pressure Extrapolated boiling point
H
2
S
Sulfane 1.363 g/dm3 (gas) 1740 (kPa, 21 °C) (gas) −60 °C (−76 °F)
H
2
S
2
Disulfane 1.334 87.7 mmHg (11,690 Pa) 70 °C (158 °F)
H
2
S
3
Trisulfane 1.491 1.4 mmHg (190 Pa) 170 °C (338 °F)
H
2
S
4
Tetrasulfane 1.582 0.035 mmHg (4.7 Pa) 240 °C (464 °F)
H
2
S
5
Pentasulfane 1.644 0.0012 mmHg (0.16 Pa) 285 °C (545 °F)
H
2
S
6
Hexasulfane 1.688 ? ?
H
2
S
7
Heptasulfane 1.721 ? ?
H
2
S
8
Octasulfane 1.747 ? ?

They also react with sulfite and cyanide producing thiosulfate and thiocyanate respectively.[6]

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:

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. 
  4. 4.0 4.1 R. Steudel "Inorganic Polysulfanes H2Sn with n > 1" in Elemental Sulfur and Sulfur-Rich Compounds II (Topics in Current Chemistry) 2003, Volume 231, pp 99–125. doi:10.1007/b13182
  5. Laitinen, Risto S.; Pakkanen, Tapani A.; Steudel, Ralf (1987). "Ab initio study of hypervalent sulfur hydrides as model intermediates in the interconversion reactions of compounds containing sulfur–sulfur bonds". J. Am. Chem. Soc. 109 (3): 710–714. doi:10.1021/ja00237a012. 
  6. 6.0 6.1 6.2 Greenwood and Earnshaw, p. 683
  7. R. Steudel "Inorganic Polysulfanes H2S2 with n > 1" in Elemental Sulfur and Sulfur-Rich Compounds II (Topics in Current Chemistry) 2003, Volume 231, pp 99-125. doi:10.1007/b13182