A sulfonium ion, also known as sulphonium ion and sulfanium ion, is a positively charged sulfur ion carrying three alkyl groups as substituents (S+R3). Ionic compounds consisting of a positively charged sulfonium cation and a negatively charged anion are called sulfonium salts.
In the above reaction, the alkylation of a sulfide, the sulfur atom has two lone electron pairs. Using those lone pairs, the sulfide acts as a nucleophile, and attacks the carbon of iodomethane. At the same time, as part of a concerted nucleophilic substitution mechanism (SN2), the iodide leaving group comes off; this leaves a positively charged trimethylsulfonium ion. This cation is attracted to the negatively charged iodide, so the two form an ionic bond, giving trimethylsulfonium iodide.
- IUPAC definition (short pdf)
sulfonium in German: Sulfonium
sulfonium in Japanese: スルホニウム
sulfonium in Portuguese: Sulfônio