Organosulfur compounds

Organosulfur compounds are organic compounds that contain sulfur (sulphur). They are often associated with foul odours, but ironically many of the sweetest compounds known are organosulfur derivatives. Nature abounds with organosulfur compounds—sulfur is essential for life. Two of the 20 common amino acids are organosulfur compounds. Fossil fuels, coal, petroleum, and natural gas, which are derived from ancient organisms, necessarily contain organosulfur compounds, the removal of which is a major focus on oil refineries.

Sulfur shares the chalcogen group with oxygen, and it is expected that organosulfur compounds have similarities with carbon-oxygen compounds, which is true to some extent.

A classical chemical test for the detection of sulfur compounds is the Carius halogen method.

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Classes of organosulfur compounds

Organosulfur compounds can be classified according to the sulfur-containing functional groups, which are listing in decreasing order of their occurrence.

Thioethers, thioesters, thioacetals

Thioethers are characterized by C-S-C bonds. The C-S bond is both longer, because S is larger, and weaker than C-C bonds. Selected bond lengths in sulfur compounds are 183 pm for the S-C single bond in methanethiol and 173 pm in thiophene. The C-S bond dissociation energy for thiomethane is 89 kcal/mol (370 kJ/mol) compared to methane's 100 kcal/mol (420 kJ/mol) and when hydrogen is replaced by a methyl group the energy decreases to 73 kcal/mol (305 kJ/mol).^[1]

The single carbon to oxygen bond is shorter than that of the C-C bond. The bond dissociation energies for dimethyl sulfide and dimethyl ether are respectively 73 and 77 kcal/mol (305 and 322 kJ/mol.

Thioethers are typically prepared by alkylation of thiols. They can also be prepared via the Pummerer rearrangement. In one named reaction called the Ferrario reaction phenyl ether is converted to phenoxthin by action of elemental sulfur and aluminium chloride ^[2]

Thioacetals, which are useful in umpolung of carbonyl groups, are a special class of thioethers as well as thioesters with general structure R-CO-S-R.

Thiophenes represent a special class of thioethers that are aromatic. The resonance stabilization of thiophene is 29 kcal/mol (121 kJ/mol) compared to 20 kcal/mol (84 kJ/mol) for the oxygen analogue furan. The reason for this difference is the higher electronegativity for oxygen drawing away electrons to itself at the expense of the aromatic ring current. Yet as an aromatic substituent the thio group is less effective as an activating group than the alkoxy group.

Thiols, disulfides

Thiol group contain the functionality R-SH. Thiols are structurally similar to the alcohol group, but these functionalities are very different in their chemical properties. Thiols are correspondingly more nucleophilic, more acidic, and more readily oxidized. This acidity can differ by 5 pKa units ^[3].

The difference in electronegativity between sulfur (2.58) and hydrogen (2.20) is small and therefore hydrogen bonding in thiols is not prominent. Aliphatic thiols form monolayers on gold, which are topical in nanotechnology.

Certain aromatic thiols can be accessed through a Herz reaction.

Disulfides R-S-S-R with a covalent sulfur to sulfur bond are important for crosslinking: in biochemistry for the folding and stability of some proteins and in polymer chemistry for the crosslinking of rubber.

Double bonds between C and S

Double bonds of carbon and sulfur are relatively uncommon, because such species often tend to oligomerize or polymerize. Exception to this rule include carbon disulfide, carbonyl sulfide, and thiophosgene. Resonance-stabilized C=S bonds are more common, as found in thioamides (see below) and related species.

Thioketones have the general structure RC(=S)R'. These species are quite rare, in contrast to their oxygen analogues. Thioaldehydes are rarer still, reflecting their lack of steric protection.

Thioamides, with the formula R₁C(=S)N(R₂)R₃ are more common. They are typically prepared by the reaction of amides with Lawesson's reagent.

Double bonds of carbon and sulfur exist as Sulfonium ylides for instance in the Johnson-Corey-Chaykovsky reaction.

Sulfonic acids, esters, amides

Sulfonic acids have functionality RS(=O)₂OH. They are strong acids that are typically soluble in organic solvents. Sulfonic acids like Trifluoromethanesulfonic acid is a frequently used reagent in organic chemistry. Sulfa drugs are sulfonamides derived from aromatic sulfonation.

Sulfuranes and persulfuranes

Sulfuranes are relatively specialized functional group that are tetravalent, hypervalent sulfur compounds, with the formula SR₄ ^[4] and likewise persulfuranes are hexavalent SR₆. All-carbon persulfuranes have been known for the heavier representatives of the chalcogen group, for instance the compound hexamethylpertellurane (Te(Me)₆) was discovered in 1990 ^[5] by reaction of tetramethyltellurium with xenon difluoride to Te(Me)₂)F₂ followed by reaction with diethyl zinc. The sulfur analogue hexamethylpersulfurane SMe₆ has been predicted to be stable ^[6] but has not been synthesized yet.

The first ever all-carbon persulfurane actually synthesized in a laboratory has two methyl and two biphenyl ligands ^[7]:

It is prepared from the corresponding sulfurane 1 with xenon difluoride / boron trifluoride in acetonitrile to the sulfuranyl dication 2 followed by reaction with butyllithium in tetrahydrofuran to (a stable) persulfurane 3 as the cis isomer. X-ray diffraction shows C-S bond lengths ranging between 189 and 193 pm (longer than the standard bond length) with the central sulfur atom in a distorted octahedral molecular geometry.

In silico experiments suggest that these bonds are very polar with the negative charges residing on carbon.

Naturally occurring organosulfur compounds

Not all organosulfur compounds are foul-smelling pollutants. Compounds like allicin and ajoene are responsible for the odor of garlic, and lenthionine contributes to the flavor of shiitake mushrooms. Many of these natural products also have important medicinal properties such as preventing platelet aggregation or fighting cancer.

Organosulfur compounds in pollution

Most organic sulfur compounds in the environment are naturally occurring, as a consequence of the fact that sulfur is essential for life and two amino acids contain this element.

Some organosulfur compounds in the environment, are generated as minor by-products of industrial processes such as the manufacture of plastics and tires.

Selected smell-producing processes are organosulfur compounds produced by the coking of coal designed to drive out sulfurus compounds and other volatile impurities in order to produce 'clean carbon' (coke), which is primarily used for steel production.

Organosulfur compounds in fossil fuels

Odours occur as well in chemical processing of coal or crude oil into precursor chemicals (feedstocks) for downstream industrial uses (e.g. plastics or pharmaceutical production) and the ubiquitous needs of petroleum distillation for (gasolines, diesel, and other grades of fuel oils production.

Organosulfur compounds might be understood as smelly contaminants that need to be removed from natural gas before commercial uses, from exhaust stacks and exhaust vents before discharge. In this latter context, organosulfur compounds may be said to account for the pollutants in sulfurous acid rain, or equivalently, said to be pollutants within most common fossil fuels, especially coal.

See also

References

1. ^ Handbook of Chemistry and Physics, 81st Edition CRC Press ISBN 0-8493-0481-4
2. ^ Organic Syntheses, Coll. Vol. 2, p.485 (1943); Vol. 18, p.64 (1938). Article link
3. ^ Organosulfur chemistry. reviews of current research JANSSEN, M.J. Interscience, New York,(1967)
4. ^ For an example bis[2,2,2-trifluoro-1-phenyl-1-(trifluoromethyl) ethoxy] diphenyl sulfurane Organic Syntheses, Coll. Vol. 6, p.163 (1988); Vol. 57, p.22 (1977) Link.
5. ^ Synthesis and characterization of hexamethyltellurium(VI) Latif Ahmed, John A. Morrison J. Am. Chem. Soc.; 1990; 112(20); 7411-7413. Abstract
6. ^ The S6 Point Group Conformers of the Hexamethylchalcogens: Me6S, Me6Se, Me6Te Fowler, J. E.; Schaefer, H. F., III; Raymond, K. N. Inorg. Chem.; (Article); 1996; 35(2); 279-281. doi:10.1021/ic940240d 10.1021/ic940240d
7. ^ Isolation and Molecular Structure of the Organo-persulfuranes [12-S-6(C6)] Sato, S.; Matsunaga, K.; Horn, E.; Furukawa, N.; Nabeshima, T. J. Am. Chem. Soc.; (Communication); 2006; 128(21); 6778-6779. doi:10.1021/ja060497y