A sulfonium ion, also known as sulphonium ion or sulfanium ion, is a positively charged ion (a "cation") featuring three organic substituents attached to sulfur. They have the formula [SR3]+. Together with their negatively charged counterpart, the anion, the compounds are called sulfonium salts.
- CH3–S–CH3 + CH3–I → (CH3)3S+ + I−
Before the above reaction, the sulfur atom has two lone electron pairs. One of these lone pairs links to the methyl group. At the same time, as part of a concerted nucleophilic substitution mechanism (SN2), the iodide leaving group departs. this leaves a positively charged trimethylsulfonium ion, whose charge is balanced by the iodide. The rate of reaction is even faster with stronger methylating agents, such as methyl trifluoromethanesulfonate.
The compounds are pyramidal at sulfur. Thus Me3S+ is structurally similar to the isoelectronic compound trimethylphosphine. Sulfonium compounds wherein the three substituents differ are chiral and optically stable.
Applications and occurrence 
The sulfonium (more specifically methioninium) species S-adenosylmethionine occurs widely in nature, where it is used as a source of the adenosoyl radical. This radical participates in the biosynthesis of various compounds.
Organic synthesis 
Some azo dyes are modified with sulfonium groups to give them a positive charge. The compound triphenylsulfonium triflate is a photoacid, a compound that under light converts to an acid.
- March, J. “Advanced Organic Chemistry” 5th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.
- Layer, G.; Heinz, D. W.; Jahn, D.; Schubert, W.-D. "Structure and function of radical SAM enzymes" Current Opinion in Chemical Biology 2004, volume 8, 468-476. doi:10.1016/j.cbpa.2004.08.001
- Mitchell J. Bogdanowicz, Barry M. Trost (1988), "Cyclopropylphenylsulfonium Tetrafluoroborate", Org. Synth.; Coll. Vol. 6: 364
See also 
- IUPAC definition (short pdf)