11.15 : Preparation and Reactions of Sulfides

Recall that in sulfides or thioethers, the central sulfur atom is bonded to two hydrocarbon groups on either side. Symmetrical sulfides consist of identical groups, whereas asymmetrical sulfides have different groups.

Typically,  when 2 moles of alkyl halide react with 1 mole of sodium sulfide in an S_N2 manner, a symmetrical sulfide is produced. 

The same reaction conditions can be applied to 1,4-dichlorobutane and 1,5-dichloropentane to synthesize five and six-membered cyclic sulfides, respectively.

On the other hand,  when a thiol reacts with an alkyl halide in the presence of a base, an asymmetrical sulfide is produced. 

For instance, the reaction of benzenethiol with methyl iodide and sodium hydroxide yields methyl phenyl sulfide.

Mechanistically, the reaction begins as the base deprotonates the thiol to produce a thiolate ion.

Subsequently, the thiolate ion acts as a nucleophile and attacks the alpha carbon of the alkyl halide, displacing the halide ion and forming the sulfide in an S_N2 reaction.

As the reaction involves the alkylation of the thiolate ion, it is referred to as the sulfur analog of Williamson ether synthesis and works well with methyl and primary alkyl halides.

Unlike ethers that are non-oxidizable at the oxygen, sulfides can readily oxidize at sulfur to produce sulfoxides, which further oxidize to sulfones.

Typically, one equivalent of hydrogen peroxide at room temperature oxidizes methyl phenyl sulfide to methyl phenyl sulfoxide, which upon further treatment with a peroxy acid, oxidizes to methyl phenyl sulfone.

However, methyl phenyl sulfide can be directly converted to methyl phenyl sulfone in the presence of 2 equivalents of hydrogen peroxide.

Common examples of sulfoxides and sulfones include dimethyl sulfoxide and tetramethylene sulfone, which are excellent aprotic solvents.