Organosulfur compounds
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2008-03-26T00:45:54Z
Roboto de Ajvol
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'''Organosulfur compounds''' are [[organic compound]]s 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 of [[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]].
==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 [[chemical bond|bond]]s. The C-S bond is both longer, because S is larger, and weaker than C-C bonds. Selected [[bond length]]s in sulfur compounds are 183 [[picometer|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).<ref>''Handbook of Chemistry and Physics'', 81st Edition CRC Press ISBN 0-8493-0481-4</ref>
The single [[carbon-oxygen bond|carbon to oxygen bond]] is shorter than that of the C-C bond. The [[bond dissociation energy|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]] <ref>[[Organic Syntheses]], Coll. Vol. 2, p.485 (1943); Vol. 18, p.64 (1938). [http://orgsynth.org/orgsyn/pdfs/CV2P0485.pdf Article link]</ref>
:[[Image:FerrarioReaction.png|400px|Ferrario reaction]]
[[Thioacetal]]s, which are useful in [[umpolung]] of carbonyl groups, are a special class of thioethers as well as [[thioester]]s 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 [[nucleophile|nucleophilic]], more acidic, and more readily oxidized. This acidity can differ by 5 [[pKa]] units <ref>''Organosulfur chemistry. reviews of current research'' JANSSEN, M.J. Interscience, New York,('''1967''')</ref>.
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 [[monolayer]]s on [[gold]], which are topical in [[nanotechnology]].
Certain aromatic thiols can be accessed through a [[Herz reaction]].
[[Disulfide bond|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 bond]]s 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.
[[Thioketone]]s 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.
[[Thioamide]]s, with the formula R<sub>1</sub>C(=S)N(R<sub>2</sub>)R<sub>3</sub> 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 ylide]]s for instance in the [[Johnson-Corey-Chaykovsky reaction]].
==Sulfonic acids, esters, amides ==
[[Sulfonic acid]]s have functionality RS(=O)<sub>2</sub>OH. 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 drug]]s are [[sulfonamide]]s derived from [[aromatic sulfonation]].
===Sulfuranes and persulfuranes===
Sulfuranes are relatively specialized functional group that are [[tetravalent]], [[hypervalent]] sulfur compounds, with the formula SR<sub>4</sub> <ref>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) [http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv6p0163 Link].</ref> and likewise '''persulfuranes''' are [[hexavalent]] SR<sub>6</sub>. All-carbon persulfuranes have been known for the heavier representatives of the [[chalcogen]] group, for instance the compound hexamethylpertellurane (Te(Me)<sub>6</sub>) was discovered in 1990 <ref>''Synthesis and characterization of hexamethyltellurium(VI)'' Latif Ahmed, John A. Morrison [[J. Am. Chem. Soc.]]; '''1990'''; 112(20); 7411-7413. [http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1990/112/i20/f-pdf/f_ja00176a061.pdf Abstract]</ref> by reaction of tetramethyltellurium with [[xenon difluoride]] to Te(Me)<sub>2</sub>)F<sub>2</sub> followed by reaction with [[diethyl zinc]]. The sulfur analogue ''hexamethylpersulfurane'' SMe<sub>6</sub> has been predicted to be stable <ref>''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}} </ref> but has not been synthesized yet.
The first ever all-carbon persulfurane actually synthesized in a laboratory has two [[methyl]] and two [[biphenyl]] [[ligand]]s <ref>''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}}</ref>:
[[Image:AllCarbonPersulfurane.png|500px|center|All-Carbon persulfurane]]
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 length]]s 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 (fuel)|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 ([[gasoline]]s, [[diesel oil|diesel]], and other grades of [[fuel oil]]s 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 ==
{{ChemicalBondsToCarbon}}
== External links ==
* Organosulfur chemistry at http://users.ox.ac.uk [http://users.ox.ac.uk/~mwalter/web_05/resources/sulph_chem/organosulphur_chemistry.shtml Link]
== References ==
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[[Category:Organosulfur compounds|*]]
[[Category:Soil chemistry]]
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