Allotropes of sulfur
Encyclopedia
There are a large number of allotropes
of sulfur
. In this respect, sulfur is second only to carbon
.
The most common form found in nature is yellow orthorhombic α-sulfur, which contains puckered rings of . Chemistry students may have seen "plastic sulfur"; this is not an allotrope but a mixture of long chain polymeric sulfur forms, two of which have been identified as allotropes. In addition to these there are other solid forms that contain sulfur rings of 6, 7, 9–15, 18 and 20 atoms. There are also gases, S2, S3; some species only detected in the vapour phase, S4 and S5 and perhaps five or more high-pressure forms, two of which are metallic.
The range of molecular allotropes possessed by sulfur can in part be ascribed to the wide range of bond lengths (180–260 pm) and bond angles (90–120°) exhibited by the S–S bond and its strength (the unrestrained S–S single bond has a high bond energy of 265 kJ mol−1).
Early workers identified some forms that have later proved to be allotropes, i.e. pure forms, whilst others have proved to be mixtures. Some forms have been named for their appearance, e.g. "mother of pearl sulfur", or alternatively named for a chemist who was pre-eminent in identifying them, e.g. "Muthmann's sulfur I" or "Engel's sulfur". A commonly used naming system uses Greek suffixes (α, β, etc.); however, this system predates the discovery of the new forms that have been synthesised rather than prepared from elemental sulfur.
, S2 is the predominant species in sulfur vapour above 720 °C. At low pressure (1 mmHg) at 530 °C, it comprises 99% of vapour species. It is a triplet diradical
like dioxygen with an S-S bond length of 188.7 pm. The blue colour of burning sulfur is due to emission of light by the S2 molecule produced in the flame.
, O3.
This was first prepared by M.R. Engel in 1891 by reacting concentrated HCl with thiosulfate
, HS2O3−. Cyclo-S6 is orange-red and forms a rhombohedral crystal. 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:
The sulfur ring in cyclo-S6 has a "chair" conformation
, reminiscent of the chair form of cyclohexane
. All of the sulfur atoms are essentially equivalent.
: 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.
, ethyl alcohol or hydrocarbons. It is found in nature as the mineral rosickyite.
These allotropes have been synthesised by various methods for example, reacting titanocene pentasulfide and a dichlorosulfane of suitable sulfur chain length, Sn−5Cl2:
2TiS5 + Sn−5Cl2 → cyclo-Sn
or alternatively reacting a dichlorosulfane, Sn−mCl2 and a polysulfane, H2Sm:
S12, S18 and S20 can also be prepared from S8. With the exception of cyclo-S12, the rings contain S-S bond lengths and S-S-S bond angle that differ one from another.
Cyclo-S12 is the second most stable cyclo- allotrope after cyclo-S8. 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.
Two forms (α-, β-) of cyclo-S9 are known, one of which has been characterized.
Two forms of cyclo-S18 are known where the conformation of the ring is different. To differentiate these structures, rather than using the normal crystallographic convention of α-, β-, etc., which in other cyclo-Sn compounds refer to different packings of essentially the same conformer
, these two conformers
have been termed endo- and exo-.
of rubber.
Allotropy
Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, known as allotropes of these elements...
of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
. In this respect, sulfur is second only to carbon
Allotropes of carbon
This is a list of the allotropes of carbon.-Diamond:Diamond is one of the most well known allotropes of carbon. The hardness and high dispersion of light of diamond make it useful for both industrial applications and jewellery. Diamond is the hardest known natural mineral. This makes it an...
.
The most common form found in nature is yellow orthorhombic α-sulfur, which contains puckered rings of . Chemistry students may have seen "plastic sulfur"; this is not an allotrope but a mixture of long chain polymeric sulfur forms, two of which have been identified as allotropes. In addition to these there are other solid forms that contain sulfur rings of 6, 7, 9–15, 18 and 20 atoms. There are also gases, S2, S3; some species only detected in the vapour phase, S4 and S5 and perhaps five or more high-pressure forms, two of which are metallic.
The range of molecular allotropes possessed by sulfur can in part be ascribed to the wide range of bond lengths (180–260 pm) and bond angles (90–120°) exhibited by the S–S bond and its strength (the unrestrained S–S single bond has a high bond energy of 265 kJ mol−1).
Early workers identified some forms that have later proved to be allotropes, i.e. pure forms, whilst others have proved to be mixtures. Some forms have been named for their appearance, e.g. "mother of pearl sulfur", or alternatively named for a chemist who was pre-eminent in identifying them, e.g. "Muthmann's sulfur I" or "Engel's sulfur". A commonly used naming system uses Greek suffixes (α, β, etc.); however, this system predates the discovery of the new forms that have been synthesised rather than prepared from elemental sulfur.
List of allotropes and forms
Allotropes are in bold.| Formula/name | Common name | Other names | Notes |
|---|---|---|---|
| S2 | disulfur Disulfur Disulfur is the diatomic molecule with the formula S2. It is analogous to the dioxygen molecule but rarely occurs at room temperature. This violet gas is commonly generated by heating sulfur above 720 °C and comprises 99% of vapor species at low pressure at 530 °C. S2 is one of minor components... |
A diatomic gas with a triplet ground state like dioxygen. | |
| S3 | trisulfur | A cherry red triatomic gas with a bent ozone Ozone Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope... -like structure. |
|
| S4 | tetrasulfur | Structure not determined but calculations indicate it to be cyclo-S4. | |
| cyclo-S5 | cyclo-pentasulfur | Not yet isolated only detected in sulfur vapour. | |
| cyclo-S6 | ρ-sulfur | cyclo-hexasulfur, "ε-sulfur", "Engel's" sulfur, "Aten's sulfur" | The ring adopts a chair form in the solid. |
| cyclo-S6/cyclo-S10 adduct | A mixed crystal with alternating layers of cyclo-S6 and cyclo-S10. | ||
| cyclo-S7 | α-, β-, γ-, δ- cycloheptasulfur | Four forms known, two(γ-, δ- ) characterized. | |
| cyclo-S8 | α-sulfur | "orthorhombic sulfur" "rhombic sulfur", "flowers of sulfur", "roll sulfur" "milk of sulfur", "Muthmann's sulfur I" | Yellow solid consisting of S8 puckered rings. The thermodynamically stable form at ordinary temperatures. |
| cyclo-S8 | β-sulfur | "monoclinic sulfur" "prismatic sulfur" "Muthmann's sulfur II" | Yellow crystalline solid, consisting of S8 puckered rings. Only stable above 95.6 °C, it reverts to α-sulfur at room temperature. |
| cyclo-S8 | γ-sulfur | "nacreous sulfur" "mother of pearl sulfur" "Gernez’s sulfur" or "Muthmann's sulfur III". | Light yellow solid, crystal monoclinic, consisting of S8 puckered rings. Found in nature as the rare mineral rosickyite. |
| cyclo-Sn n = 9–15, 18, 20 |
cyclo-(nona; deca; undeca; dodeca; trideca; tetradeca; pentadeca; octadeca; eicosa)sulfur | Pure forms all allotropes, cyclo-S9 has four forms, cyclo-S18 has two forms. Generally synthesised rather than obtained by treatment of another form of elemental sulfur. | |
| catena-Sx | fibrous (ψ) sulfur | Well characterized, contains parallel helical sulfur chains and is difficult to obtain pure. | |
| catena-Sx | lamina sulfur | Not well characterized, contains helical chains partially crossed. | |
| amorphous sulfur | Quenched molten sulfur plastic sulfur at first crystallises to amorphous or glassy sulfur. Consists of a mixture of catena sulfur and cyclo sulfur. | ||
| insoluble sulfur | Quenched liquid sulfur with soluble species extracted with CS2. Sometimes called polymeric sulfur, μ-S or ω-S. | ||
| φ-sulfur | A mixture of allotropic ψ-sulfur and cyclo forms mainly γ-cyclo-S8. | ||
| ω-sulfur | insoluble sulfur | A mixture of chains with a minimum of soluble species. | |
| λ-sulfur | Light yellow mobile liquid formed when β-sulfur first melts at 119.6 °C. Consists of S8 rings. | ||
| μ-sulfur | The dark-coloured viscous liquid formed when π-sulfur is heated and the solid when cooled. Contains a mixture of polymeric chains. | ||
| π-sulfur | Dark-coloured liquid that develops as λ-sulfur is left molten. Contains mixture of Sn rings. | ||
| High-pressure forms of α-sulfur | S-II, S-III, S-IV, S-V and others | Four high-pressure phases (at ambient temperature) including two that are metallic and become superconducting Superconductivity Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum... at low temperature and some additional phases photo-induced below 20–30 GPa. |
|
S2, disulfur
DisulfurDisulfur
Disulfur is the diatomic molecule with the formula S2. It is analogous to the dioxygen molecule but rarely occurs at room temperature. This violet gas is commonly generated by heating sulfur above 720 °C and comprises 99% of vapor species at low pressure at 530 °C. S2 is one of minor components...
, S2 is the predominant species in sulfur vapour above 720 °C. At low pressure (1 mmHg) at 530 °C, it comprises 99% of vapour species. It is a triplet diradical
Diradical
A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals . They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one bond less than the number...
like dioxygen with an S-S bond length of 188.7 pm. The blue colour of burning sulfur is due to emission of light by the S2 molecule produced in the flame.
S3, trisulfur
S3 is found in sulfur vapour, comprising 10% of vapour species at 440 °C and 10 mmHg. It is cherry red in colour, with a bent structure, similar to ozoneOzone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
, O3.
S4, tetrasulfur
This has been detected in the vapour phase but has not been fully characterized. Various forms, (e.g. chains, branched chains and rings) have been proposed. The latest view, based on theoretical calculations is that it has a ring structure.cyclo-S5, pentasulfur
This has not been isolated, but has been detected in the vapour phase.S6, cyclo-hexasulfur
Thiosulfate
Thiosulfate is an oxyanion of sulfur. The prefix thio indicates that thiosulfate ion is a sulfate ion with one oxygen replaced by a sulfur. Thiosulfate occurs naturally and is produced by certain biochemical processes...
, HS2O3−. Cyclo-S6 is orange-red and forms a rhombohedral crystal. 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 etherDiethyl etherDiethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
)
The sulfur ring in cyclo-S6 has a "chair" conformation
Conformational isomerism
In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds...
, reminiscent of the chair form of cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...
. All of the sulfur atoms are essentially equivalent.
cyclo-S6.cyclo-S10 adduct
This is produced from a solution of cyclo-S6 and cyclo-S10 in CS2. It has a density midway between cyclo-S6 and cyclo-S10. The crystal consists of alternate layers of cyclo-S6 and cyclo-S10. For all the elements this may be the only allotrope which contains molecules of different sizes.S7,cyclo-heptasulfur
It is a bright yellow solid. Four (α-, β-, γ-, δ-) forms of cyclo-heptasulfur are known. Two forms (γ-, δ-)have been characterized. The cyclo-S7 ring has an unusual range of bond lengths of 199.3–218.1 pm. It is said to be the least stable of all of the sulfur allotropes.Cyclo-S8
α-sulfur
α-sulfur is the form most commonly found in nature. 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 disulfideCarbon disulfide
Carbon disulfide is a colorless volatile liquid with the formula CS2. The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non-polar solvent...
: 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 variously quoted as 119.6 °C and 119.8 °C but as it decomposes to other forms at around this temperature the observed melting point can vary from this. The 119 °C melting point has been termed the "ideal melting point" and the typical lower value (114.5°) when decomposition occurs, the "natural melting point".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 disulfideCarbon disulfide
Carbon disulfide is a colorless volatile liquid with the formula CS2. The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non-polar solvent...
, ethyl alcohol or hydrocarbons. It is found in nature as the mineral rosickyite.
cyclo-Sn, (n = 9–15, 18, 20)
2TiS5 + Sn−5Cl2 → cyclo-Sn
or alternatively reacting a dichlorosulfane, Sn−mCl2 and a polysulfane, H2Sm:
- Sn−mCl2 + H2Sm → cyclo-Sn
S12, S18 and S20 can also be prepared from S8. With the exception of cyclo-S12, the rings contain S-S bond lengths and S-S-S bond angle that differ one from another.
Cyclo-S12 is the second most stable cyclo- allotrope after cyclo-S8. 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.
Two forms (α-, β-) of cyclo-S9 are known, one of which has been characterized.
Two forms of cyclo-S18 are known where the conformation of the ring is different. To differentiate these structures, rather than using the normal crystallographic convention of α-, β-, etc., which in other cyclo-Sn compounds refer to different packings of essentially the same conformer
Conformational isomerism
In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds...
, these two conformers
Conformational isomerism
In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds...
have been termed endo- and exo-.
Solid catena sulfur allotropes
The production of pure forms of catena-sulfur has proved to be extremely difficult. Complicating factors include the purity of the starting material and the thermal history of the sample.ψ-sulfur
This form, also called fibrous sulfur or ω1-sulfur, has been well characterized. It has a density of 2.01 g·cm−3 (α-sulfur 2.069 g·cm−3) and decomposes around its melting point of 104 °C. It consists of parallel helical sulfur chains. These chains have both left and right-handed "twists" and a radius of 95 pm. The S-S bond length is 206.6 pm, the S-S-S bond angle is 106° and the dihedral angle is 85.3°, (comparable figures for α-sulfur are 203.7 pm, 107.8° and 98.3°).lamina sulfur
This has not been well characterized but is believed to consist of criss-crossed helices. It is also called χ-sulfur or ω2-sulfur.Catena sulfur forms
The naming of the different forms is very confusing and care has to be taken to determine what is being described as the same names are used interchangeably.amorphous sulfur
This is the quenched product of sulfur melts above 160 °C (at this point the properties of the liquid melt change remakably, e.g. large increase in viscosity ). Its form changes from an initial plastic form gradually to a glassy form, hence its other names of plastic, glassy or vitreous sulfur. It is also called χ-sulfur. It contains a complex mixture of catena-sulfur forms mixed with cyclo-forms.insoluble sulfur
This is obtained by washing quenched liquid sulfur with CS2. It is sometimes called polymeric sulfur, μ-S or ω-S.ω-sulfur
This is a commercially available product prepared from amorphous sulfur that has NOT been stretched prior to extraction of soluble forms with CS2. It sometimes called "white sulfur of Das" or supersublimated sulfur. It is a mixture of ψ-sulfur and lamina sulfur. The composition depends on the exact method of production and the samples history. One well known commercial form is "Crystex". ω-sulfur is used in the vulcanizationVulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
of rubber.