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. 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.
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.
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==