A thioether (similar to sulfide) is a functional group in organic chemistry that has the structure R¹-S-R² as shown on right. Like many other sulfur-containing compounds, volatile thioethers characteristically have foul odors.^[1]

A thioether is similar to an ether except that it contains a sulfur atom in place of the oxygen. Because oxygen and sulfur belong to the chalcogens group in the periodic table, the chemical properties of ethers and thioethers are somewhat similar. This functional group is important in biology, notably in the amino acid methionine and the cofactor biotin.

Contents 1 Preparation 2 Reactions 2.1 Thiophenes 3 References

[edit] Preparation

Thioethers are typically prepared by the alkylation of thiols:

R-Br + HS-R' → R-S-R' + HBr

Such reactions are usually conducted in the presence of base, which converts the thiol into the more nucleophilic thiolate. Analogously, the reaction of disulfides with organolithium reagents produces thioethers:

R₃CLi + R¹S-SR² → R₃CSR¹ + R²SLi

Analogous reactions are known starting with Grignard reagents.

Alternatively, thioethers are often synthesized by the addition of a thiol to an alkene:

R-CH=CH₂ + HS-R' → R-CH₂-CH₂-S-R'

This reaction is often catalysed by free radicals Thioethers can also be prepared by many other methods, such as the Pummerer rearrangement. Trialkysulfonium salts react with nucleophiles with a dialkyl sulfide as a leaving group:

Nu^- + R₃S⁺ → Nu-R + R-S-R

This reaction is exploited in biological systems as a means of transferring an alkyl group. For example, S-adenosylmethionine acts as a methylating agent in biological S_N2 reactions.

[edit] Reactions

While ethers are generally non-oxidizeable, thioethers (R-S-R) can be easily oxidized to the sulfoxides (R-S(=O)-R), which can themselves be further oxidized to sulfones (R-S(=O)₂-R). Hydrogen peroxide is a typical oxidant. For example, dimethyl sulfide can be oxidized as follows:

S(CH₃)₂ + H₂O₂ → OS(CH₃)₂ + H₂O

OS(CH₃)₂ + H₂O₂ → O₂S(CH₃)₂ + H₂O

Ethers can be alkylated at oxygen only with difficulty, but thioethers are readily alkylated to give stable sulfonium salts, such as trimethylsulfonium iodide:

S(CH₃)₂ + CH₃I → [S(CH₃)₃]⁺I^-

In analogy to their easy alkylation, thioethers bind to metals to form coordination complexes. They are classified as soft ligands, but their affinity for metals is lower than typical phosphines. Chelating thioethers are known, such as 1,4,7-Trithiacyclononane.

[edit] Thiophenes

The heterocyclic compound thiophene is formally a thioether. Because of the aromatic character of this heterocycle, the nonbonding electrons on sulfur, normally responsible for the nucleophilicity so characteristic of thioethers, are delocalized into the π-system. Consequently thiophene exhibits few properties expected for a thioether - thiophene is non-nucleophilic at sulfur and, in fact, is sweet-smelling. Upon hydrogenation, thiophene gives tetrahydrothiophene, C₄H₈S, which indeed does behave as a typical thioether.

[edit] References

v • d • e Functional groups Chemical class: Alcohol • Aldehyde • Alkane • Alkene • Alkyne • Amide • Amine • Azo compound • Benzene derivative • Carboxylic acid • Cyanate • Disulfide • Ester • Ether • Haloalkane • Hydrazone • Imine • Isocyanide • Isocyanate • Ketone • Oxime • Nitrile • Nitro compound • Nitroso compound • Peroxide • Phosphoric acid • Pyridine derivative • Sulfone • Sulfonic acid • Sulfoxide • Thioester • Thioether • Thiol