Thiophene

Thiophene is an aromatic heterocyclic compound consisting of four carbon atoms and one sulfur atom in a five-membered ring. Compounds analogous to thiophene include furan and pyrrole where the S atom is replaced by O and NH, respectively. Thiophene was discovered by Viktor Meyer in 1883 as a contaminant in benzene. Related to thiophene are benzothiophene and dibenzothiophene, containing the thiophene ring fused with one and two benzene rings, respectively.

History
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 resposible for this reaction from benzene. This new heterocyclic compound was thiophene.

Properties
At room temperature, thiophene is a colorless liquid with a mildly pleasant odor reminiscent of benzene, with which thiophene shares some similarities.

Thiophene is considered aromatic, although theoretical calculations suggest that the degree of aromaticity is less than that of benzene. The participation of the lone electron pairs on sulfur in the delocalized pi electron system is significant. As a consequence of its aromaticity, thiophene does not exhibit the properties seen for conventional thioethers. For example the sulfur atom is not alkylated by methyl iodide. Although the sulfur atom is unreactive, the flanking CH centers are susceptible to attack by electrophiles. The high reactivity of thiophene toward sulfonation is the basis for the separation of thiophene from benzene, as thiophene and benzene are difficult to separate by distillation due to the mere 4 °C difference in their boiling points at ambient pressure. Treatment of thiophene-benzene mixtures with sulfuric acid results in preferential sulfonation of the thiophene to give water-soluble thiophene sulfonic acid.

Uses
Thiophenes are important heterocyclic compounds and are recurring building blocks in organic chemistry with applications in pharmaceuticals. The benzene ring of a biologically active compound may often be replaced by a thiophene without loss of activity. This is seen in examples such as the NSAID lornoxicam, the thiophene analog of piroxicam.

Thiophenes are used as synthetic intermediates, taking advantage of the susceptibility of the carbon atoms adjacent to S toward electrophilic reactions. Desulfurization of the resulting ring using Raney nickel affords 1,4-disubstituted butanes. The polymer formed by linking thiophene through its 1,5 positions is called polythiophene. Polythiophenes become electrically conductive upon partial oxidation, i.e. they become "organic metals".

Thiophene is used as a denaturant for ethanol with which it forms an azeotrope.

Synthesis and occurrence
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 thiphene by Viktor Meyer in the year of its discovery. Thiophenes are classically prepared by the reaction of 1,4-diketones with sulfiding reagents such as Lawesson's reagent or P4S10. Specialized thiophenes can be synthesized via the Gewald reaction, which involves the condensation of two esters in the presence of elemental sulfur.

Thiophene and its derivatives occur in petroleum, sometimes in concentrations up to 1-3%. The thiophenic content of liquids from oil and coal is removed via the hydrodesulfurization (HDS) process. In HDS, the liquid or gaseous feed is passed over a special form of molybdenum disulfide 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.