||This article needs attention from an expert in Chemistry. (August 2011)|
|Jmol-3D images||Image 1|
|Molar mass||256.52 g mol−1|
|Appearance||Vivid, yellow, translucent crystals|
|Density||2.07 g cm-3|
119 °C, 392 K, 246 °F
159 °C, 432 K, 318 °F (decomposes)
|Std enthalpy of
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)|
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.
The name octasulfur, is the most commonly used, the preferred IUPAC name is cyclo-octasulfur.
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:
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.
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.