Allotropes of Sulphur

The property of chemical elements to exist in numerous different forms in the same physical state is known as allotropism or allotropy. These forms are also called allotropes of elements. Sulphur is a non-metal having atomic mass 40 and atomic number 16, belonging to the chalcogen family of group 16 and period 3 of the modern long form periodic table. There are several types of sulphur allotropes. All allotropes are divided into two groups–crystalline and amorphous forms. Crystalline forms are further divided into orthorhombic and monoclinic forms. Amorphous forms are further divided into three forms–plastic, colloidal, and milk of sulphur.

From the allotropes of sulphur, the yellow rhombic and monoclinic forms are most important. These forms are interchangeable to each other under special conditions. In the last century, these allotropes were given traditional names such as mother of pearl sulphur, Muthmann’s sulphur I, Engel’s sulphur, etc. From the five forms of allotropes, two are metallic. The number of sulphur allotropes reflects the relative stability of the strong S – S bond of bond dissociation energy 265 KJ/mol.

Rhombic Sulphur 

Commonly named – sulphur, this allotrope is yellow and has a melting point of 385.8K and specific gravity 2.06. Its crystals are formed by evaporating the solution of roll sulphur in CS₂. As this is not a polar solution, it is insoluble in water but dissolves slightly in benzene alcohol and rapidly dissolves in CS₂. It is commonly known as ‘’flower of sulphur” or “roll of sulphur”. It is the most thermodynamic stable form of sulphur at ordinary temperatures. It has a crown-like S8 puckered ring structure.

Monoclinic Sulphur 

Commonly known as – sulphur, monoclinic sulphur has various common names such as prismatic sulphur and Muthmann’s sulphur II. It is also a yellow solid containing S8 puckered rings. It is only stable above 95.3°C. Its melting point is 393K and its specific gravity is 1.98. It is also soluble in CS₂. This sulphur allotrope is prepared by melting the rhombic sulphate in a container and cooling it until the crust has formed. Two holes are made in the crust and the remaining liquid is drained. When the crust is removed, crystals of yellow needles of – sulphur form. It is stable above 369K and converts to sulphur beneath it. In contrast, – sulphur is stable below 369K and converts to – sulphur above it. At 369K, both forms are stable. The temperature of 369K is known as the alternating temperature of sulphur allotropes.

Plastic Sulphur

Plastic sulphur is an amorphous type of sulphur. Its heat remains amorphous indefinitely, however, at room temperature, it shines in a few hours, continuing the crystallisation (like selenium) from the outside to the inside. 

Milk of Sulphur

Milk of sulphur is an amorphous white solid commonly called precipitated sulphur containing sulphate of calcium. Generally, it is made up of the precipitate obtained by the use of sulphuric acid instead of hydrochloric acid. When it precipitates from pure sulphur, it is called milk of sulphur. It is used in the treatment of various skin disorders when we heat it, and changes to the conventional yellow colour of sulphur. It is dissolved in nonpolar solvents like ether, alcohol, etc. 

Colloidal Sulphur

Colloidal sulphur is an amorphous type of sulphur obtained through special treatment. It is prepared when small particles of sulphur hang and easily dissolve in water to form a colloidal mixture. The best method of cathodic pulverisation, commonly used in the production of colloidal solutions of metallic substances, has also been successfully expanded. The use of colloidal sulphur accelerates the conversion of maleic acid to fumaric acid.

Disulphur

Disulphur is predominantly a form of sulphur vapour above 720°C and with low pressure (1 mmHg) at 530°C. It has an S−S bond length of 188.7 pm. It is blue-coloured due to burning it in the emission of light by the S2 molecules and producing a flame. Its formula is S2. The molecule gets trapped in the compound [S2I4][EF6]2 (E = As, Sb) for crystallographic measurements and is produced by treating elemental sulphur with excess iodine in liquid sulphur dioxide. The [S2I4]2+ cation has an “open-book” structure, in that each [I2]+ cation donates the unpaired electron in the π* molecular orbital to a vacant orbital of the S2 molecule. It is paramagnetic in nature.

Hexasulfur

This allotrope was first prepared by M.R. Engel in 1891 by treating thiosulphate with HCl. It is an orange-red allotrope and forms a rhombohedral crystal form. It is commonly called ρ-sulphur, ε-sulphur, Engel’s sulphur, and Aten’s sulphur. Its alternative method of preparation involves the reaction of a polysulfone with sulphur monochloride. It is a closed ring structure. The sulphur ring in cyclo-S6has a “chair” conformation, similar to the chair form of cyclohexane. All sulphur atoms are equivalent with bond lengths at 205.7pm and bond angles of 102.2 degrees.

Conclusion 

Each allotrope of sulphur has different physical properties and is interconvertible. Rhombic sulphur is converted to monoclinic sulphur when heated above 369K. Monoclinic sulphur converts to rhombic sulphur below 369k. Many other modifications of sulphur containing 6–20 sulphur atoms have been synthesised in the last two decades.