Organosulfur compounds
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Organosulfur compounds are organic compounds containing sulfur. They are one subset of many compounds, usually of distasteful odor, created naturally during the decay of growing organisms containing carbohydrate chains and compounds of sulfur (British spelling: sulphur). Both carbohydrates and sulfur are chemically active, and such compounds form readily during decay processes of living matter as in natural gas. Organosulfur chemistry is the science exploring the properties and reactivity of organosulfur compounds.
Sulfur shares the chalcogen group with oxygen and it is expected that organosulfur compounds have similarities with compounds of carbon and oxygen.
A classical chemical test for the detection of sulfur compounds is the Carius halogen method.
Thioethers and thiophenes
The carbon to sulfur bond in thioethers is both longer and weaker than that of the carbon to carbon bond. Selected bond lengths in sulfur compounds are 183 pm for the S-C single bond in methanethiol and 173 pm in thiophene. The bond dissociation energy for thiomethane is 89 Kcal/mol compared to methane's 100 Kcal/mol and when hydrogen is replaced by a methyl group the energy decreases to 73 Kcal/mol Handbook of Chemistry and Physics, 81st Edition CRC Press ISBN 0849304814.The single carbon to oxygen bond is shorter than that of the C-C bond. The bond dissociation energies for dimethylsulfide and dimethyl ether are respectively 73 and 77 Kcal/mol. Thioethers can be prepared via the Pummerer rearrangement.
The resonance stabilization of thiophene is 29 Kcal/mol compared to 20 kcal/mole for the oxygen pendant 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.
Double bonds of carbon and sulfur exist as Sulfonium ylides for instance in the Johnson-Corey-Chaykovsky reaction.
Thiols
The thiol group (R-SH) is similar to the alcohol group but sulfur in a thiol is more nucleophilic and a thiol is also more acidic. This acidity can differ by 5 pKa units Organosulfur chemistry. reviews of current research JANSSEN, M.J. Interscience, New York,(1967).The diffence in electronegativity between sulfur (2.58) and hydrogen (2.20) is small and therefore hydrogen bonding in thiols is not prominent. Aliphatic thiols are very effective in forming gold monolayers in nanotechnology. Certain aromatic thiols can be accessed through a Herz reaction.
The related thioacetals represent umpolung of carbonyl groups.
Sulfoxides, sulfones & thioketones
A sulfoxide has the general structure RS=OR and can be obtained by oxidation of thioethers. Sulfoxides can be oxidized even further to Sulfones (R(O=S=O)R).A thioketone has the general structure RC=SR and is the sulfur carbonyl equivalent. An important route to thioketones is reaction of ketones with Lawesson's reagent.
Sulfonic acids, esters, amides
Sulfonic acids with general structure R(S=0)OH are strong acids and acids with solubility in organic solvents. Sulfonic acids like Trifluoromethanesulfonic acid is a frequently used reagent in organic chemistry. Sulfonamides or sulfa drugs are important drugs derived from aromatic sulfonation.Sulfuranes and persulfuranes
Sulfuranes are tetravalent and hypervalent sulfur compounds SR4 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]. 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 Synthesis and characterization of hexamethyltellurium(VI) Latif Ahmed, John A. Morrison J. Am. Chem. Soc.; 1990; 112(20); 7411-7413. [Abstract] 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 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] but has not been synthesized yet.The first ever all-carbon persulfurane actually synthesized in a laboratory has two methyl and two biphenyl ligands 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] :
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.
Organosulfur pollution
Some organosulfur compounds in the environment, particularly the air, are created as rare by-products of chemical plant production processes, where sulfur is used as one reagent in a process such as making plastics or tires. But most sulfurous pollutants occur from use of fossil fuels as part of a process feedstock, or when used as fuel as in power generation.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.
Odors 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.
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.
See also
- Compounds of carbon with period 3 elements: organosilicon compounds, organophosphorus compounds, organosulfur compounds,
- Compounds of carbon with Group 16 elements: organosulfur compounds, organoselenium compounds,organotellurium compounds
External links
- Organosulfur chemistry at http://users.ox.ac.uk [Link]
References
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