Octasulfur

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Octasulfur
Stereo, skeletal formula of octathiocane Spacefill model of octathiocane
Ball and stick model of octathiocane
Sample of pulverised octasulfur
Identifiers
CAS number 10544-50-0 YesY
PubChem 66348
ChemSpider 59726 YesY
MeSH Cyclooctasulfur
ChEBI CHEBI:29385
ChEMBL CHEMBL1235452 YesY
Gmelin Reference 2973
Jmol-3D images Image 1
Properties
Molecular formula S8
Molar mass 256.52 g mol−1
Appearance Vivid, yellow, translucent crystals
Density 2.07 g cm-3
Melting point 119 °C; 246 °F; 392 K
Boiling point 159 °C; 318 °F; 432 K (decomposes)
log P 6.117
Thermochemistry
Std molar
entropy
So298
32 J·mol−1·K−1[2]
Std enthalpy of
formation
ΔfHo298
0 kJ·mol−1[2]
Related compounds
Related compounds Hexathiane
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Octasulfur (systematically named octathiocane, and cyclo-octasulfur) is an inorganic chemical with the chemical formula S
8
. It is a yellow solid, and is odourless and tasteless.

It is the standard allotrope of sulfur. It is also the final member of the thiocane heterocylic series, where every carbon is substituted with a sulfur atom.

Nomenclature[edit]

The name octasulfur, is the most commonly used, the preferred IUPAC name is cyclo-octasulfur.

Chemical properties[edit]

Octasulfur exists as three distinct polymorphs, rhombohedral, and two monoclinic forms, of these only two are stable at standard conditions. The rhombohedral crystal form is the accepted standard. The remaining polymorph is only stable between 96 °C and 115 °C at 100 kPa, above 115 °C octasulfur starts to slowly disproportionate. However, if heated fast enough, with minimal degradation, octasulfur will melt at 119 °C, before being completely degraded above 159 °C.

Octasulfur forms several sulfur allotropes:

α-Sulfur
β-Sulfur
γ-Sulfur
λ-Sulfur

λ-Sulfur is the liquid form of octasulfur, from which γ-sulfur can be crystallised by quenching. If λ-sulfur is crystallised slowly, it will revert to β-sulfur. Since it must have been heated over 115 °C, neither crystallised β-sulfur, or γ-sulfur will be pure. The only known method of obtaining pure γ-sulfur, is by crystallising from solution.

Octasulfur easily forms large sized crystals, these crystals are typically vivid yellow in colour, and are somewhat translucent. As is typical of other crystalline compounds, pulverised sulfur is completely different in appearance - it is a paler colour, and opaque as is shown in the image.

Production[edit]

Very pure octasulfur can be produced through photolysis of 4,4'-dipyridyl disulfide, using aqueous pyridine as a catalyst. The ratio of octasulfur polymorphs can be altered by altering the pyridine-water ratio. This process can also produce the gamma form of octasulfur.

See also[edit]

References[edit]

  1. ^ "cyclooctasulfur (CHEBI:29385)". Chemical Entities of Biological Interest. UK: European Bioinformatics Institute. Main. 
  2. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X.