Thiophene

Chembox new
Name = Thiophene
ImageFile = Thiophene chemical structure.png ImageSize = 200px
ImageName = Thiophene
IUPACName = Thiophene
OtherNames = Thiofuran
Thiacyclopentadiene
Section1 = Chembox Identifiers
SMILES = C1=CC=CS1
PubChem = 8030
CASNo = 110-02-1
RTECS = XM7350000
Section2 = Chembox Properties
Formula = C₄H₄S
MolarMass = 84.14 g/mol
Appearance = colorless liquid
Density = 1.051 g/ml, liquid
MeltingPt = −38 °C
BoilingPt = 84 °C
Viscosity = 8.712 cP at 0.2 °C 6.432 cP at 22.4 °C
RefractIndex = 1.5287
Section3 = Chembox Structure
Dipole =
Section7 = Chembox Hazards
ExternalMSDS = [http://physchem.ox.ac.uk/MSDS/TH/thiophene.html External MSDS]
EUClass = not listed
NFPA-H = 2
NFPA-F = 3
NFPA-R =
FlashPt = −1 °C
Section8 = Chembox Related
Function = thioethers
OtherFunctn = Tetrahydrothiophene
Diethyl sulfide
OtherCpds = Furan
Pyrrole

Thiophene is the heterocyclic compound with the formula C₄H₄S. Consisting of a flat five-membered ring, it is aromatic as indicated by its extensive substitution reactions. Related to thiophene are benzothiophene and dibenzothiophene, containing the thiophene ring fused with one and two benzene rings, respectively. Compounds analogous to thiophene include furan (C₄H₄O) and pyrrole (C₄H₄NH).

Isolation, occurrence

Thiophene was discovered as a contaminant in benzene. [cite journal
author= Viktor Meyer
title = Ueber den Begleiter des Benzols im Steinkohlenteer
journal = Berichte der Deutschen chemischen Gesellschaft
year = 1883
volume = 16
issue =
pages = 1465–1478
url = http://gallica.bnf.fr/ark:/12148/bpt6k90696b/f1470.chemindefer
doi = 10.1002/cber.188301601324] It was observed that isatin forms a blue dye if it is mixed with sulfuric acid and crude benzene. The formation of the blue indophenin was long believed to be a reaction with benzene. Victor Meyer was able to isolate the substance responsible for this reaction from benzene. This new heterocyclic compound was thiophene. [cite journal
author= Ward C. Sumpter
title = The Chemistry of Isatin
journal = Chemical Reviews
year = 1944
volume = 34,
issue = 3
pages = 393–434
doi = 10.1021/cr60109a003]

Thiophene and its derivatives occur in petroleum, sometimes in concentrations up to 1-3%. The thiophenic content of oil and coal is removed via the hydrodesulfurization (HDS) process. In HDS, the liquid or gaseous feed is passed over a form of molybdenum disulfide catalyst under a pressure of H₂. Thiophenes undergo hydrogenolysis to form hydrocarbons and hydrogen sulfide. Thus, thiophene itself is converted to butane and H₂S. More prevalent and more problematic in petroleum are benzothiophene and dibenzothiophene.

ynthesis and production

Reflecting their high stabilities, thiophenes arise from many reactions involving sulfur sources and hydrocarbons, especially unsaturated ones, e.g. acetylenes and elemental sulfur, which was the first synthesis of thiophene by Viktor Meyer in the year of its discovery. Thiophenes are classically prepared by the reaction of 1,4-diketones, diesters, or dicarboxylates with sulfiding reagents such as P₄S₁₀. Specialized thiophenes can be synthesized similarly using or Lawesson's reagent as the sulfiding agent, via the Gewald reaction, which involves the condensation of two esters in the presence of elemental sulfur. Another method is the Volhard-Erdmann cyclization.

Thiophene is produced on a scale of ca. 2M kg per year worldwide. Production involves the vapor phase reaction of a sulfur source, typically carbon disulfide, and butanol. These reagents are contacted with an oxide catalyst at 500-550 °C.Jonathan Swanston “Thiophene” in Ullmann’s Encyclopedia of Industrial Chemistry Wiley-VCH, Weinheim, 2006. DOI|10.1002/14356007.a26 793.pub2.]

Properties

At room temperature, thiophene is a colorless liquid with a mildly pleasant odor reminiscent of benzene, with which thiophene shares some similarities. The high reactivity of thiophene toward sulfonation is the basis for the separation of thiophene from benzene, which are difficult to separate by distillation due to their similar boiling points (4 °C difference at ambient pressure). Like benzene, thiophene forms an azeotrope with water.

The molecule is flat; the bond angle at the sulphur is around 93 degrees, the C-C-S angle is around 109, and the other two carbons have a bond angle around 114 degrees. The C-C bonds to the carbons adjacent to the sulphur are about 1.34A, the C-S bond length is around 1.70A, and the other C-C bond is about 1.41A (figures from the Cambridge Structural Database).

Reactivity

Thiophene is considered aromatic, although theoretical calculations suggest that the degree of aromaticity is less than that of benzene. The "electron pairs" on sulfur are significantly delocalized in the pi electron system. As a consequence of its aromaticity, thiophene does not exhibit the properties seen for conventional thioethers. For example the sulfur atom resists alkylation and oxidation.

Toward electrophiles

Although the sulfur atom is relatively unreactive, the flanking carbon centers, the 2- and 5-positions, are highly susceptible to attack by electrophiles. Halogens give initially 2-halo derivatives followed by 2,5-dihalothiophenes; perhalogenation is easily accomplished to give C₄X₄S (X = Cl, Br, I). [OrgSynth | author = Henry Y. Lew and C. R. Noller| title = 2-Iodolthiophene| collvol = 4 | collvolpages = 545 | year = 1963 | prep = CV4P0545] Thiophene brominates 10⁷ times faster than does benzene.

Chloromethylation and chloroethylation occur readily at the 2,5-positions. Reduction of the chloromethyl product gives 2-methylthiophene. Hydrolysis followed by dehydration of the chloroethyl species gives 2-vinylthiophene. [OrgSynth | author = W. S. Emerson and T. M. Patrick, Jr.| title = 2-Vinylthiophene| collvol = 4 | collvolpages = 980 | year = 1963 | prep = CV4P0980] [OrgSynth | author = K. B. Wiberg and H. F. McShane| title = 2-Chloromethylthiophene| collvol = 3 | collvolpages = 1 | year = 1955 | prep = CV3P0197]

Desulfurization by Raney Nickel

Desulfurization of thiophene with Raney nickel affords butane. When coupled with the easy 2,5-difunctionalization of thiophene, desulfurization provides a route to 1,4-disubstituted butanes.

Lithiation

Not only is thiophene reactive toward electrophiles, it is also readily lithiated with butyl lithium to give 2-lithiothiophene, which is a precursor to a variety of derivatives, including dithienyl. [OrgSynth | author = E. Jones and I. M. Moodie | title = 2-Thiophenethiol| collvol = 6 | collvolpages = 979| year = 1988 | prep = CV6P0979]

Coordination chemistry

Thiophene exhibits little thioether-like character, but it does serve as a pi-ligand forming piano-stool complexes such as Cr(η⁵-C₄H₄S)(CO)₃. [Rauchfuss, T. B., "The Coordination Chemistry of Thiophenes", Progress in Inorganic Chemistry 1991, volume 39, pp. 259-311. ISBN 978-0-471-54489-0]

Uses

Thiophenes are important heterocyclic compounds that are widely used as building blocks in many agrochemicals and pharmaceuticals. The benzene ring of a biologically active compound may often be replaced by a thiophene without loss of activity.cite book
author = Daniel Lednicer
title = The Organic Chemistry of Drug Synthesis
publisher = Wiley Interscience
volume = 6
year = 1999
location = New York
pages = 187
id = ISBN 0-471-24510-0 ] This is seen in examples such as the NSAID lornoxicam, the thiophene analog of piroxicam.

Polythiophene

The polymer formed by linking thiophene through its 2,5 positions is called polythiophene. Polythiophene itself has poor processing properties. More useful are polymers derived from thiophenes substituted at the 3- and 3- and 4- positions. Polythiophenes become electrically conductive upon partial oxidation, i.e. they become "organic metals." [cite journal | author = J. Roncali | title = Conjugated poly(thiophenes): synthesis, functionalization, and applications | year = 1992 | journal = Chem. Rev. | volume = 92 | issue = 4 | pages = 711–738 | doi = 10.1021/cr00012a009]

References

External links

* [http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc11/icsc1190.htm International Chemical Safety Card 1190]