Sulfides are the sulfur analog of ethers, just as thiols are the sulfur analog of alcohol. Like ethers, sulfides also consist of two hydrocarbon groups bonded to the central sulfur atom. Depending upon the type of groups present, sulfides can be symmetrical or asymmetrical. Symmetrical sulfides can be prepared via an SN2 reaction between 2 equivalents of an alkyl halide and one equivalent of sodium sulfide. Asymmetrical sulfides can be synthesized by treating thiols with an alkyl halide and a base. The reaction follows an SN2 pathway and proceeds via the thiolate ion intermediate. This reaction is the sulfur analog of Williamson ether synthesis and prefers methyl, primary, and secondary alkyl halides but not tertiary alkyl halides. Sulfides can readily oxidize to sulfoxide and sulfones. Treatment of sulfide with one equivalent of hydrogen peroxide at room temperature yields sulfoxide, which upon further oxidation with a peroxy acid yields a sulfone. However, 2 equivalents of hydrogen peroxide oxidize sulfide directly to sulfone. Dimethyl sulfoxide (DMSO) and tetramethylene sulfone are common examples of sulfoxides and sulfones, respectively. They both are excellent dipolar aprotic solvents.