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16.4C: Sulfur - Allotropes

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    34267
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    Allotropes of sulfur

    No other element forms more solid allotropes than sulfur. At present, about 30 well characterized sulfur allotropes are known of which the most common form found in nature is the greenish-yellow orthorhombic α-sulfur, containing puckered rings of S8.

    α-sulfur

    When pure it has a greenish-yellow colour (traces of cyclo-S7 in commercially available samples make it appear yellower). It is practically insoluble in water and is a good electrical insulator with poor thermal conductivity. It is quite soluble in carbon disulfide: 35.5 g/100 g solvent at 25 °C. It has a rhombohedral crystal structure. This is the predominant form found in "flowers of sulfur", "roll sulfur" and "milk of sulfur". It contains S8 puckered rings, alternatively called a crown shape. The S-S bond lengths are all 206 pm and the S-S-S angles are 108° with a dihedral angle of 98°. At 95.3 °C, α-sulfur converts to β-sulfur.

    β-sulfur

    This is a yellow solid with a monoclinic crystal form and is less dense than α-sulfur. Like the α- form it contains puckered S8 rings and only differs from it in the way the rings are packed in the crystal. It is unusual because it is only stable above 95.3 °C, below this it converts to α-sulfur. It can be prepared by crystallising at 100 °C and cooling rapidly to slow down formation of α-sulfur. It has a melting point of about 120 °C and decomposes at around this temperature.

    γ-sulfur

    This form, first prepared by F.W Muthmann in 1890, is sometimes called "nacreous sulfur" or "mother of pearl sulfur" because of its appearance. It crystallises in pale yellow monoclinic needles. It contains puckered S8 rings like α-sulfur and β-sulfur and only differs from them in the way that these rings are packed. It is the densest form of the three. It can be prepared by slowly cooling molten sulfur that has been heated above 150 °C or by chilling solutions of sulfur in carbon disulfide, ethyl alcohol or hydrocarbons. It is found in nature as the mineral rosickyite.

    Some allotropes of Sulfur
     
    S6 - cyclohexasulfur
     

    α-S8
     

    S12 - cyclododecasulfur

    S6 - cyclo-hexasulfur
    This was first prepared by M.R. Engel in 1891 who reacted HCl with thiosulfate, HS2O3-. Cyclo-S6 is orange-red and forms rhombohedral crystals. It is called ρ-sulfur, ε-sulfur, Engel's sulfur and Aten's sulfur. Another method of preparation involves reacting a polysulfane with sulfur monochloride: H2S4 + S2Cl2 → cyclo-S6 + 2 HCl (dilute solution in diethyl ether)

    The sulfur ring in cyclo-S6 has a "chair" conformation, reminiscent of the chair form of cyclohexane. All of the sulfur atoms are equivalent.

    Cyclo-dodecasulfur
    Thermodynamically, S12 is the second most stable sulfur ring after S8. Therefore, S12 is formed in many chemical reactions in which elemental sulfur is a product. In addition, S12 is a component of liquid sulfur at all temperatures. The same holds for S18 and S20 which are often formed together with S12. Its structure can be visualised as having sulfur atoms in three parallel planes, 3 in the top, 6 in the middle and three in the bottom.

    Liquid sulfur after equilibration contains sulfur homocycles of all sizes and some of these can be isolated by quenching, extraction, fractional precipitation and crystallization depending on their differing solubilities.

    Cyclo-S12 can be prepared by heating elemental sulfur to about 200 °C for 5-10 min and then allowing the mixture to cool to 140-160 °C within about 15 min. Once the melt has become less viscous, it is poured in as thin a stream as possible into liquid nitrogen in order to quench the equilibrium. Recrystallization of the yellow powder from CS2 allows the isolation of an adduct which slowly loses the solvent to give the cyclo-dodecasulfur.

    Note that both B and S form stable E12 species but the structures (and coordination numbers) are quite different.


    16.4C: Sulfur - Allotropes is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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