Everything about Thiophene totally explained
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Thiophene is the
heterocyclic compound with the formula C
4H
4S. Consisting of a five-membered ring, it's
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 (C
4H
4O) and
pyrrole (C
4H
4NH).
Isolation, occurrence
Thiophene was discovered as a contaminant in benzene. It was observed that
isatin forms a blue dye if it's 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 resposible for this reaction from benzene. This new heterocyclic compound was thiophene.
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 H
2. Thiophenes undergo hydrogenolysis to form
hydrocarbons and
hydrogen sulfide. Thus, thiophene itself is converted to butane and H
2S. More prevalent and more problematic in petroleum are
benzothiophene and dibenzothiophene.
Synthesis and production
Reflecting their high stabilities, thiophenes arise from many reactions involving sulfur sources and hydrocarbons, especially unsaturated ones, for example 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-di
ketones, diesters, or dicarboxylates with sulfiding reagents such as P
4S
10. 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.
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.
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 doesn't 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 C
4X
4S (X = Cl, Br, I). Thiophene brominates 10
7 times faster than does benzene.
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's also readily lithiated with
butyl lithium to give 2-lithiothiophene, which is a precursor to a variety of derivatives, including dithienyl.
Coordination chemistry
Thiophene exhibits little thioether-like character, but it does serve as a pi-ligand forming
piano-stool complexes such as Cr(η
5-C
4H
4S)(CO)
3.
Uses
Thiophenes are important heterocyclic compounds that are widly used as building blocks in many agrichemicals and pharmaceuticals. 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, for example they become "organic metals."
Further Information
Get more info on 'Thiophene'.
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