Octasulfur

Nomenclature

The name octasulfur is the most commonly used for this chemical. It is systematically named cyclo-octasulfur (which is the preferred IUPAC name) and cyclooctasulfane. It is also the final member of the thiocane heterocylic series, where every carbon atom is substituted with a sulfur atom, thus this sulfur allotrope is systematically named octathiocane as well.

Structure

Main article: Allotropes of sulfur

The chemical consists of rings of 8 sulfur atoms. It adopts a crown conformation with D4d point group symmetry. The S–S bond lengths are equal, at about 2.05 Å. Octasulfur crystallizes in three distinct polymorphs: rhombohedral, and two monoclinic forms, of which only two are stable at standard conditions. The rhombohedral crystal form is the accepted standard state. The remaining polymorph is only stable between 96 and 115 °C at 100 kPa. Octasulfur forms several allotropes: α-sulfur, β-sulfur, γ-sulfur, and 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 crystals, which are typically yellow and are somewhat translucent.

Production and reactions

Main article: Sulfur

It is the main (99%) component of elemental sulfur, which is recovered from volcanic sources and is a major product of the Claus process, associated with petroleum refineries. In space, sulfur can be formed by energetic processing of solid hydrogen sulfide, which explains the recent detection of octasulfur in the return samples from the carbonaceous asteroid Ryugu.

References

References

1. {{RedBook2005
2. "cyclooctasulfur (CHEBI:29385)". European Bioinformatics Institute.
3. Zumdahl, Steven S.. (2009). "Chemical Principles 6th Ed.". Houghton Mifflin Company.
4. Steudel, R., "Homocyclic Sulfur Molecules", Topics Curr. Chem. 1982, 102, 149.
5. (1 July 2025). "Missing interstellar sulfur in inventories of polysulfanes and molecular octasulfur crowns". Nature Communications.
6. (10 July 2024). "Primordial aqueous alteration recorded in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu". Nature Communications.