Thioether
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[[Image:thioether structure.png|right|200px]]
A '''thioether''' (similar to ''[[sulfide]]'') is a functional group in [[organic chemistry]] that has the structure R<sup>1</sup>-S-R<sup>2</sup> as shown on right. Like many other sulfur-containing compounds, [[Volatile organic compound|volatile]] thioethers characteristically have foul odors.<ref>R. J. Cremlyn “An Introduction to Organosulfur Chemistry” John Wiley and Sons: Chichester (1996). ISBN 0-471-95512-4.</ref>
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 [[chalcogen]]s group in the [[periodic table]], the chemical properties of ethers and thioethers share some commonalities. This functional group is important in biology, most notably in the [[amino acid]] [[methionine]] and the [[cofactor]] [[biotin]].
==Preparation==
* Thioethers are typically prepared by the [[alkylation]] of [[thiol]]s:
:R-Br + HS-R' → R-S-R' + HBr
Such reactions are accelerated in the presence of base, which converts the thiol into the more nucleophilic thiolate.
* An alternative method of synthesis includes the addition of a thiol to an [[alkene]], typically catalysed by [[free radical]]s:
:R-CH=CH<sub>2</sub> + HS-R' → R-CH<sub>2</sub>-CH<sub>2</sub>-S-R'
* Thioethers can also be prepared via the [[Pummerer rearrangement]].
==Reactions==
*While ethers are generally stable, thioethers (R-S-R) are easily oxidized to the [[sulfoxide]]s (R-S(=O)-R), which can themselves be further oxidized to [[sulfone]]s (R-S(=O)<sub>2</sub>-R). For example, [[dimethyl sulfide]] can be oxidized as follows:
:S(CH<sub>3</sub>)<sub>2</sub> + [[O]] → [[DMSO|OS(CH<sub>3</sub>)<sub>2</sub>]]
:OS(CH<sub>3</sub>)<sub>2</sub> + [[O]] → [[Methylsulfonylmethane|O<sub>2</sub>S(CH<sub>3</sub>)<sub>2</sub>]]
Typical oxidants include [[peroxide]]s.
*The sulfur-sulfur bond in [[disulfide]]s is susceptible to cleavage by nucleophiles, and reaction with a carbon nucleophile produces a thioether:
: R<sub>3</sub>C<sup>-</sup> + R<sup>1</sup>S-SR<sup>2</sup> → R<sub>3</sub>CSR<sup>1</sup> + R<sup>2</sup>S<sup>-</sup>
*Trialkysulfonium salts react with nucleophiles with a dialkyl sulfide as a leaving group:
:Nu<sup>-</sup> + R<sub>3</sub>S<sup>+</sup> → Nu-R + R-S-R
This reaction is exploited in biological systems as a means of transferring an [[alkyl|alkyl group]]. For example, [[S-Adenosyl methionine|S-adenosylmethionine]] acts as a [[methylation|methylating agent]] in biological [[SN2 reaction|S<sub>N</sub>2 reactions]].
*Ethers can be [[alkylate]]d at oxygen only with difficulty to give highly reactive trialkyloxonium salts. In contrast, thioethers are readily alkylated to give stable [[sulfonium]] salts, such as trimethylsulfonium:
:S(CH<sub>3</sub>)<sub>2</sub> + CH<sub>3</sub>I → [S(CH<sub>3</sub>)<sub>3</sub>]<sup>+</sup>I<sup>-</sup>
===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 [[delocalize]]d 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]], C<sub>4</sub>H<sub>8</sub>S, which indeed does behave as a typical thioether.
==References==
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