In organic chemistry, a sulfide (British English sulphide) or thioether is an organosulfur functional group with the connectivity as shown on right. Like many other sulfur-containing compounds, volatile sulfides have foul odors.
Nomenclature
Sulfides are sometimes called thioethers, especially in the old literature. The two organic substituents are indicated by the prefixes. (CH
3)
2S is called
dimethylsulfide. Some sulfides are named by modifying the common name for the corresponding ether. For example, C
6H
5SCH
3 is methyl phenyl sulfide, but is more commonly called
thioanisole, since its structure is related to that for
anisole, C
6H
5OCH
3.
The modern systematic nomenclature in chemistry for the trival name thioether is sulfane.
Structure and properties
Sulfide is an angular functional group, the C–S–C angle approaching 90° The C–S bonds are about 180
picometer.
For the prototype, dimethylsulfide, the C-S-C angles is 99°, which is smaller than the C-O-C angle in
ether (~110°). The C-S distance in dimethylsulfide is 1.81 Å.
Sulfides are characterized by their strong odors, which are similar to thiol odor. This odor limits the applications of volatile sulfides. In terms of their physical properties they resemble ethers, but are less volatile, higher melting, and less hydrophilic. These properties follow from the polarizability of the divalent sulfur center, which is greater than that for oxygen in ethers.
Thiophenes
are a special class of sulfide-containing heterocyclic compounds. The nonbonding electrons on sulfur are
into the
aromatic π-system. As a consequence, thiophene exhibits few properties expected for a sulfide – thiophene is non-nucleophilic at sulfur and, in fact, is sweet-smelling. Upon
hydrogenation, thiophene gives tetrahydrothiophene, C
4H
8S, which indeed does behave as a typical sulfide.
Occurrence and applications
Sulfides are important in biology, notably in the
amino acid methionine and the cofactor
biotin.
Petroleum contains many organosulfur compounds, including sulfides. Polyphenylene sulfide is a useful high temperature plastic.
Coenzyme M, , is the precursor to
methane (i.e. natural gas) via the process of
methanogenesis.
Preparation
Sulfides are typically prepared by
alkylation of
. Alkylating agents include not only alkyl halides, but also
,
, and
.
Such reactions are usually conducted in the presence of a base, which converts the thiol into the more nucleophilic thiolate.
Analogously, the reaction of
with organolithium reagents produces thioethers:
Analogous reactions are known starting with
.
Alternatively, sulfides can be synthesized by the addition of a thiol to an alkene in the thiol-ene reaction:
This reaction is often catalysed by
produced from a
photoinitiator.
Sulfides 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:
This reaction is exploited in biological systems as a means of transferring an
alkyl. For example,
S-adenosylmethionine acts as a
methylation in biological S
N2 reactions.
An unusual but well tested method for the synthesis of thioethers involves addition of alkenes, especially ethylene across the S-Cl bond of sulfur dichloride. This method has been used in the production of bis(2-chloroethyl)sulfide, a mustard gas:
Reactions
The
Lewis basic lone pairs on sulfur dominate the sulfides' reactivity. Sulfides readily alkylate to stable
sulfonium salts, such as trimethylsulfonium iodide:
Sulfides also oxidize easily to
(), which can themselves be further oxidized to
(). Hydrogen peroxide is a typical oxidant—for example, with
dimethyl sulfide ():
In analogy to their easy alkylation, sulfides bind to metals to form thioether complexes. Consequently, Lewis acids do not decompose thioethers as they do ethers. Sulfides are HSAB ligands, but their affinity for metals is lower than typical . Chelating thioethers are known, such as 1,4,7-trithiacyclononane.
Sulfides undergo hydrogenolysis in the presence of certain metals:
Raney nickel is useful for stoichiometric reactions in organic synthesis whereas molybdenum-based catalysts are used to "sweeten" petroleum fractions, in the process called hydrodesulfurization. Similarly dissolving metal reductions can induce
dealkylation or
dearylation.
Unlike ethers, thioethers are stable in the presence of . The adjacent to the sulfur atom are labile, and can be deprotonated with strong bases.
Common sources
