Organic sulfide

In organic chemistry, a sulfide (British English sulphide) or thioether is an organosulfur functional 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.

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]

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 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.

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, CH
₃SCH
₂CH
₂SO⁻
₃, is the precursor to methane (i.e. natural gas) via the process of methanogenesis.

Sulfides are typically prepared by alkylation of thiols. Alkylating agents include not only alkyl halides, but also epoxides, aziridines, and Michael acceptors.^[4]

RBr + HSR' → RSR' + HBr

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:

R³CLi + R¹S-SR² → R³CSR¹ + R²SLi

Analogous reactions are known starting with Grignard reagents.

Alternatively, sulfides can be synthesized by the addition of a thiol to an alkene in the thiol-ene reaction:

R-CH=CH₂ + H-SR' → R-CH₂-CH₂-S-R'

This reaction is often catalysed by free radicals produced from a photoinitiator.^[6]

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:

Nu⁻ + R³S⁺ → Nu-R + R²SR¹

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.

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:^[7]

SCl₂ + 2 C₂H₄ → (ClC₂H₄)₂S

The Lewis basic lone pairs on sulfur dominate the sulfides' reactivity. Sulfides readily alkylate to stable sulfonium salts, such as trimethylsulfonium iodide:^[8]

S(CH₃)₂ + CH₃I → [S(CH₃)₃]⁺I⁻

Sulfides also oxidize easily to 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, with dimethyl sulfide (S(CH₃)₂):^[9]

S(CH₃)₂ + H₂O₂ → OS(CH₃)₂ + H₂O

OS(CH₃)₂ + H₂O₂ → O₂S(CH₃)₂ + H₂O

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.^{[citation needed]} Similarly dissolving metal reductions can induce dealkylation or dearylation.^[12]

Unlike ethers, thioethers are stable in the presence of Grignard reagents.^[13] The protons adjacent to the sulfur atom are labile, and can be deprotonated with strong bases.^[14]

• 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. LCCN 66-30401.