General structure of a sulfide with the blue marked functional group.
In organic chemistry, a sulfide (British Englishsulphide) or thioether is an organosulfurfunctional group with the connectivity R−S−R' as shown on right. Like many other sulfur-containing compounds, volatile sulfides have foul odors.[1] A sulfide is similar to an ether except that it contains a sulfur atom in place of the oxygen. The grouping of oxygen and sulfur in the periodic table suggests that the chemical properties of ethers and sulfides are somewhat similar, though the extent to which this is true in practice varies depending on the application.
Nomenclature
Sulfides are sometimes called thioethers, especially in the old literature. The two organic substituents are indicated by the prefixes. (CH3)2S is called dimethylsulfide. Some sulfides are named by modifying the common name for the corresponding ether. For example, C6H5SCH3 is methyl phenyl sulfide, but is more commonly called thioanisole, since its structure is related to that for anisole, C6H5OCH3.
The modern systematic nomenclature in chemistry for the trival name thioether is sulfane.[2]
Structure and properties
Sulfide is an angular functional group, the C–S–C angle approaching 90° The C–S bonds are about 180 pm.
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.[3]
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
Thiophenes are a special class of sulfide-containing heterocyclic compounds. Because of their aromatic character, they are non-nucleophilic. The nonbonding electrons on sulfur are delocalized into the 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, C4H8S, which indeed does behave as a typical sulfide.
Such reactions are usually conducted in the presence of a base, which converts the thiol into the more nucleophilic thiolate.[5] Analogously, the reaction of disulfides with organolithium reagents produces thioethers:
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:
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.[10] Sulfides are soft ligands, but their affinity for metals is lower than typical phosphines. Chelating thioethers are known, such as 1,4,7-trithiacyclononane.
Sulfides undergo hydrogenolysis in the presence of certain metals:
R-S-R' + 2 H2 → RH + R'H + H2S
Raney nickel is useful for stoichiometric reactions in organic synthesis[11] whereas molybdenum-based catalysts are used to "sweeten" petroleum fractions, in the process called hydrodesulfurization.
Unlike ethers, thioethers are stable in the presence of Grignard reagents.[12] The protons adjacent to the sulfur atom are labile, and can be deprotonated with strong bases.[13]
^D. Landini; F. Rolla (1978). "Sulfide Synthesis In Preparation Of Dialkyl And Alkyl Aryl Sulfides: Neopentyl Phenyl Sulfide". Org. Synth. 58: 143. doi:10.15227/orgsyn.058.0143.
^Hoyle, Charles E.; Bowman, Christopher N. (2010-02-22). "Thiol-Ene Click Chemistry". Angewandte Chemie International Edition. 49 (9): 1540–1573. doi:10.1002/anie.200903924. PMID20166107.
Brendsma, L.; Arens, J. F. (1967). "The chemistry of thioethers; differences and analogies with ethers". In Patai, Saul (ed.). The Chemistry of the Ether Linkage. The Chemistry of Functional Groups. London: Interscience / William Clowes and Sons. pp. 555–559. LCCN66-30401.