Plastic Sulfur Sulfur Rings Hexasulfur Heptasulfur, Nonasulfur


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Sulfur
Plastic Sulfur
If molten liquid sulfur is quenched from 350 ◦C rapidly to 20 ◦C by spraying into cold water, the resulting solid sulfur is known as “plastic sulfur”. Fibers obtained this way have helical S∞ chains with ten sulfur atoms for every helix repeat length. In addition, this type of sulfur is a composite material, having cyclic S8 molecules packed into cavities between the fibers.
When liquid sulfur is discharged into cold seawater in an undersea volcanic eruption, probably the form of sulfur generated is plastic sulfur.
Sulfur Rings
Most of the allotropes of sulfur consist of molecular rings, Sn, where n = 6-20. The most stable of these rings is S8, a molecule having S–S interatomic distances of 2.05 ˚A and belonging to the D4d point group. Commercially available sulfur consists mostly of S8 molecules, but it typically has a bright yellow color (characteristic of elemental sulfur) that comes from a small amount of S7 impurity. Pure S8 is pale yellow-green.
Another impurity that may be present in volcanic sulfur is S7Se, where one of the sulfur atoms in the eight-membered ring is replaced with selenium.
Hexasulfur
Molecular hexasulfur has been synthesized in various ways. One way is via the thermal decomposition of S2I2, itself being generated by reaction of 2 KI with S2Cl2, analogous to the organic Finkelstein reaction. Using this method, it is possible to obtain S6 in ca. 36% yield after separation from other S2n molecules by fractional precipitation.
Of course, it would be desirable to have a reaction for selective generation of S6, and the organometallic reagent known as titanocene pentasulfide has been developed for this purpose. It has the formula Cp2TiS5, and a molecular structure featuring a six-membered TiS5 ring that effectively acts as a source of S52 – because of the polarity (sulfur negative, titanium positive) of the Ti–S bonds. Accordingly, S6 has been synthesized in 87% yield by the reaction of Cp2TiS5 with SCl2, giving titanocene dichloride (Cp2TiCl2) as the organometallic co-product of the reaction. Additionally generated in the reaction are small amounts of S12. The sulfur dichloride reagent, SCl2, does not have a long shelf life; it must be generated, distilled, and used while still fresh.
Heptasulfur, Nonasulfur, and Decasulfur
The seven-membered ring compound S7 can also be generated in a selective reaction employing titanocene dichloride together with S2Cl2. Pure S7 is not thermally stable and must be stored below −50 ◦C.
The molecular S9 ring has been prepared similarly, by reaction of Cp2TiS5 with S4Cl2 In the case of S10, the synthesis method is different. Here SO2Cl2 is employed effectively as a source of chlorine and SO2 gas as a leaving group. The balanced reaction uses 2 Cp2TiS5 + 2 SO2Cl2 to provide S10, 2 SO2 ↑ , and 2 equiv of titanocene dichloride.

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Principles of Inorganic Chemistry I

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Sulfur
Sulfur as a Crystalline Mixture of Rings
When equal amounts of S6 and S10 are recrystallized together from CS2, the crystals so obtained consist of equal amounts of S6 and S10 molecules. These crystals have a melting point of 92 ◦C. It has been said that this is the only solid allotrope of an element containing molecules of different sizes, but this use of the term “allotrope” is probably not appropriate here given our accepted definition.
Twelve- and Twenty-Membered Rings
The S12 molecule provides a nice example of D3d symmetry, while S20 molecules take on a D4 structure.
Vulcanization
An important application of elemental sulfur is its combination with natural and synthetic rubber in the process known as vulcanization. The most important synthetic polymer processed this way is polystyrene/polybutadiene, which when vulcanized is used to make automobile tires and other products. The vulcanization process installs –SSnS– cross-links between the polymer chains, making the rubber hard and not sticky.
The S2 Molecule
Just like when comparing nitrogen and phosphorus, in the case of oxygen and sulfur the light element is a gas while the heavier element adopts a variety of solid allotropes. What are the properties of S2? One way this has been approached is by the synthesis of a bicyclic disulfide molecule, C5H6S2, that could release S2 in a thermal retro Diels-Alder reaction. The S2 so-generated in the presence of 2,3-dimethylbutadiene led to detectable formation of a new cyclic disulfide product, interpreted as evidence for the intermediacy of S2 as a reactive transient.
Where is S2 found in nature? The very close approach to earth of comet IRAS-Araki-Alcock 1983 VII led to the observation of UV emission spectra showing the presence of S2 close to the cometary nucleus!

5.03

Principles of Inorganic Chemistry I

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Plastic Sulfur Sulfur Rings Hexasulfur Heptasulfur, Nonasulfur