Polysulfane: Difference between revisions
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==Chemistry and properties== |
==Chemistry and properties== |
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Polysulfanes can easily be [[oxidised]], and are thermodynamically unstable with respect to decomposition ([[disproportionation]]) readily to {{chem2|H2S}} and sulfur, a reaction for which [[alkali]] acts as a [[catalyst]]:<ref name=Greenwood683>Greenwood and Earnshaw, p. 683</ref> |
Polysulfanes can easily be [[oxidised]], and are thermodynamically unstable with respect to decomposition ([[disproportionation]]) readily to {{chem2|H2S}} and sulfur, a reaction for which [[alkali]] acts as a [[catalyst]]:<ref name=Greenwood683>Greenwood and Earnshaw, p. 683</ref> |
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:{{chem2|8 H2S_{''n''} → 8 H2S + (''n'' \s 1) S8}} |
:{{chem2|8 H2S_{''n''} → 8 H2S + (''n'' \s 1) S8}} |
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Revision as of 08:07, 28 July 2023
A polysulfane is a chemical compound of formula H2Sn, where n > 1 (although disulfane (H2S2) 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.[4] H2S2 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 can easily be oxidised, and are thermodynamically unstable with respect to decomposition (disproportionation) readily to H2S and sulfur, a reaction for which alkali acts as a catalyst:[5]
- 8 H2Sn → 8 H2S + (n − 1) S8
where S8 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 → [Sn]2−; ΔH = −ve[clarification needed]
Beyond H2S and H2S2, many higher polysulfanes H2Sn (n = 3 – 8) are known as stable compounds.[6] They have unbranched sulfur chains, because sulfur has tendency for catenation. Starting with disulfane H2S2, all known polysulfanes are liquids at room temperature. H2S2 is colourless while the other polysulfanes are yellow, and their colour becomes richer as n increases, as do the density, boiling point and viscosity. Physical properties of polysulfanes are given in the table below.[5]
| Compound | Density at 20 °C (g·cm−3) | Vapour pressure (mmHg) | Extrapolated boiling point (°C) |
|---|---|---|---|
| H2S | 1.363 (g·dm−3) | 1740 (kPa, 21 °C) | −60 |
| H2S2 | 1.334 | 87.7 | 70 |
| H2S3 | 1.491 | 1.4 | 170 |
| H2S4 | 1.582 | 0.035 | 240 |
| H2S5 | 1.644 | 0.0012 | 285 |
| H2S6 | 1.688 | ? | ? |
| H2S7 | 1.721 | ? | ? |
| H2S8 | 1.747 | ? | ? |
They also react with sulfite and cyanide producing thiosulfate and thiocyanate respectively.[5]
An structural isomer of trisulfane H2S3, in which the two hydrogen atoms are attached to the central sulfur of the three-sulfur chain (H2S2+(−S−)2), rather than one on each end (HS−S−SH), has been computationally studied.[7] The sulfur analog of thiosulfurous acid (HS−S(=O)−OH or HS−S+(−O−)−OH), is a branched isomer of tetrasulfane H2S4, in which the fourth sulfur is bonded to the central sulfur of a linear dihydrogen trisulfide structure (trithiosulfurous acid, S=S(−SH)2 or −S−S+(−SH)2), and it has been computationally studied as well.[8] The sulfur analog of thiosulfuric acid (HS−S(=O)2−OH), in which two sulfur atoms branch off of the central sulfur atom of a linear dihydrogen trisulfide structure (tetrathiosulfuric acid, (S=)2S(−SH)2 or (−S−)2S2+(−SH)2) has also been computationally studied.[9]
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:
- Na2Sn + 2 HCl → 2 NaCl + H2Sn (n = 4, 5, 6)
- SnCl2 + 2 H2Sm → 2 HCl + H2Sn+2m
The reaction of polysulfanes with sulfur dichloride or disulfur dichloride produces long-chain dichloropolysulfanes:
- 2 SCl2 + H2Sn → 2 HCl + S2+nCl2
- 2 S2Cl2 + H2Sn → 2 HCl + S4+nCl2
The reaction with a sulfite salt (a base) quantitatively decomposes the polysulfane to produce thiosulfate and hydrogen sulfide:
- (n − 1) SO2−3 + H2Sn → (n − 1) S2O2−3 + H2S
References
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "polysulfanes". doi:10.1351/goldbook.P04753
- ^ Wiberg, E.; Holleman, A. F.; Wiberg, Nils (2001). Inorganic Chemistry. San Diego : Academic Press; New York : De Gruyter. ISBN 978-0-12-352651-9. OCLC 48056955.
{{cite book}}: CS1 maint: location missing publisher (link) - ^ Greenwood, N. N.; Earnshaw, A. (2012-12-02). Chemistry of the Elements. Boston, Massachusetts, MA: Elsevier. ISBN 978-0-08-050109-3. OCLC 48138330.
- ^ 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
- ^ a b c Greenwood and Earnshaw, p. 683
- ^ 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
- ^ Dobado, J. A.; Martínez-García, Henar; Molina, José; Sundberg, Markku R. (1999). "Chemical Bonding in Hypervalent Molecules Revised. 2. Application of the Atoms in Molecules Theory to Y2XZ and Y2XZ2 (Y = H, F, CH3; X = O, S, Se; Z = O, S) Compounds". J. Am. Chem. Soc. 121 (13): 3156–3164. doi:10.1021/ja9828206.
- ^ 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.
- ^ Nishimoto, Akiko; Zhang, Daisy Y. (2003). "Hypervalency in sulfur? Ab initio and DFT studies of the structures of thiosulfate and related sulfur oxyanions". Sulfur Letters. 26 (5/6): 171–180. doi:10.1080/02786110310001622767. S2CID 95470892.