Thiophene is a heterocyclic compound with the formula C4H4S. Consisting of a planar five-membered ring, it is aromatic as indicated by its extensive substitution reactions. It is a colorless liquid with a benzene-like odor. In most of its reactions, it resembles benzene. Compounds analogous to thiophene include furan (C4H4O), selenophene (C4H4Se) and pyrrole (C4H4NH), which each vary by the heteroatom in the ring.
Isolation and occurrence Thiophene was discovered by Viktor Meyer in 1882 as a contaminant in benzene. It was observed that isatin (an indole) forms a blue dye if it is mixed with sulfuric acid and crude benzene. The formation of the blue indophenin had long been believed to be a reaction of benzene itself. Viktor Meyer was able to isolate thiophene as the actual substance responsible for this reaction. Thiophene and especially 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.
On Mars Thiophene derivatives have been detected at nanomole levels in 3.5 billions year old Martian soil sediments (Murray Formation, Pahrump Hills) by the rover Curiosity at Gale crater (Mars) between 2012 and 2017.
Synthesis and production Reflecting their high stabilities, thiophenes arise from many reactions involving sulfur sources and hydrocarbons, especially unsaturated ones. The first synthesis of thiophene by Meyer, reported the same year that he made his discovery, involves acetylene and elemental sulfur. Thiophenes are classically prepared by the reaction of 1,4-diketones, diesters, or dicarboxylates with sulfidizing reagents such as P4S10 such as in the Paal-Knorr thiophene synthesis. Specialized thiophenes can be synthesized similarly using Lawesson's reagent as the sulfidizing agent, or 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 modest scale of around 2,000 metric tons per year worldwide. Production involves the vapor phase reaction of a sulfur source, typically carbon disulfide, and a C-4 source, typically butanol. These reagents are contacted with an oxide catalyst at 500–550 °C.
Properties and structure Thiophene is a colorless liquid at room temperature. 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 ethanol. The molecule is flat; the bond angle at the sulfur is around 93°, the C–C–S angle is around 109°, and the other two carbons have a bond angle around 114°. The C–C bonds to the carbons adjacent to the sulfur are about 1.34 Å, the C–S bond length is around 1.70 Å, and the other C–C bond is about 1.41 Å.
Reactivity Thiophene is considered to be 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 sulfides. For example, the sulfur atom resists alkylation and oxidation.
Oxidation Oxidation can occur both at sulfur, giving a thiophene S-oxide, as well as at the 2,3-double bond, giving the thiophene 2,3-epoxide, followed by subsequent NIH shift rearrangement. Oxidation with trifluoroperacetic acid demonstrates both reaction pathways. The major pathway forms the S-oxide as an intermediate, which undergoes subsequent Diels-Alder-type dimerisation and further oxidation, forming a mixture of sulfoxide and sulfone products with a combined yield of 83% (based on NMR evidence):
In the minor reaction pathway, a Prilezhaev epoxidation results in the formation of thiophene-2,3-epoxide that rapidly rearranges to the isomer thiophene-2-one. Trapping experiments demonstrate that this pathway is not a side reaction from the S-oxide intermediate, while isotopic labeling with deuterium confirm that a 1,2-hydride shift occurs and thus that a cationic intermediate is involved. If the reaction mixture is not anhydrous, this minor reaction pathway is suppressed as water acts as a competing base. Oxidation may be relevant to the metabolic activation of various thiophene-containing drugs, such as tienilic acid and the investigational anticancer drug OSI-930.
Alkylation 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). Thiophene brominates 107 times faster than does benzene. Acetylation occurs readily to give 2-acetylthiophene, precursor to thiophene-2-carboxylic acid and thiophene-2-acetic acid. 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.
Desulfurization 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.
Polymerization The polymer formed by linking thiophene through its 2,5 positions is called polythiophene. Polymerization is conducted by oxidation using electrochemical methods (electropolymerization) or electron-transfer reagents. An idealized equation is shown:
n C4H4S → (C4H2S)n + 2n H+ + 2n e− Polythiophene itself has poor processing properties and so is little studied. More useful are polymers derived from thiophenes substituted at the 3- and 3- and 4- positions, such as EDOT (ethylenedioxythiophene). Polythiophenes become electrically conductive upon partial oxidation, i.e. they obtain some of the characteristics typically observed in metals.
Coordination chemistry Thiophene exhibits little sulfide-like character, but it does serve as a pi-ligand forming piano stool complexes such as Cr(η5-C4H4S)(CO)3.
Thiophene derivatives
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