chemistry

Sulfonium

Sulfonium is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Sulfonium rather than just read about it. In short: In organic chemistry, 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. These organosulfur compounds have the formula [SR3]+.

Key takeaways

Sulfonium belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sulfonium to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sulfonium from memory before moving on to harder problems.

Reference excerpt

In organic chemistry, 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. These organosulfur compounds have the formula [SR3]+. Together with a negatively charged counterion, they give sulfonium salts. They are typically colorless solids that are soluble in polar organic solvent.

Synthesis

Sulfonium compounds are usually synthesized by the reaction of thioethers with alkyl halides. For example, the reaction of dimethyl sulfide with iodomethane yields trimethylsulfonium iodide:

CH3SCH3 + CH3I → (CH3)3S+I− The reaction proceeds by a nucleophilic substitution mechanism (SN2), where iodide is the leaving group. For weakly electrophilic alkyl halides, the reactions can be accelerated by the addition of silver tetrafluoroborate. In that vein, the rate (and irreversibility) of methylation improves with more electrophilic methylating agents such as methyl trifluoromethanesulfonate. These S-alkylations can be reversible, especially when the leaving group is iodide. For example, alkylation of dimethylsulfide with allyl iodide gives trimethylsulfonium iodide. In a related process, secondary alkyl halides react with dimethylsulfide to give mixed thioether, eliminating methyl halide. Below are listed some other synthetic methods, among many:

sulfonium salts can be made by treating secondary and tertiary alcohols with sulfuric acid in the presence of thioethers. addition of methyl iodide to dimethyldisulfide in the presence of a mercuric halide catalyst. addition of sulfenyl chlorides to alkenes, giving episulfonium salts. alkylation of thioethers with electrophilic alkenes in the presence of a proton donor.

Inversion Sulfonium ions with three different substituents are chiral owing to their pyramidal structure. Unlike the isoelectronic oxonium ions (R3O+), chiral sulfonium ions are resolvable into optically stable enantiomers. [Me(Et)SCH2CO2H]+ is the first chiral sulfonium cation to be resolved into enantiomers. The barrier to inversion ranges from 100 to 130 kJ/mol.

Applications and occurrence

Biochemistry The sulfonium (more specifically methioninium) species S-adenosylmethionine occurs widely in nature, where it is used as a source of the adenosoyl or methyl radicals. These radicals participate in the biosynthesis of many compounds.

Other naturally occurring sulfonium species are S-methylmethionine (methioninium) and the related dimethylsulfoniopropionate (DMSP).

Organic synthesis Sulfonium salts are precursor to sulfur ylides, which are useful in carbon–carbon bond-forming reactions. In a typical application, a R2S+CH2R′ center is deprotonated to give the ylide R2S+CHR−. These ylides add to ketones and aldehydes to give epoxides (Johnson–Corey–Chaykovsky reaction).

Tris(dimethylamino)sulfonium difluorotrimethylsilicate [((CH3)2N)3S]+[F2Si(CH3)3]− is a popular fluoridation agent. 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. Organic sulfides react with liquid bromine to give bromosulfonium bromides, i.e.:

RSR′ + [RR′S+Br]Br−

Illustrations

Worked examples

Example 1 — a first encounter with Sulfonium

Start with the simplest possible case. Write down what Sulfonium claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sulfonium before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sulfonium ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sulfonium

In research: Sulfonium appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sulfonium in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sulfonium is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cations, Sulfonium compounds, Sulfur ions, so understanding it makes those chapters shorter.
In everyday life: Look for Sulfonium outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Sulfonium” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Sulfonium in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Sulfonium means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Sulfonium out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Sulfonium in simple terms?

Why does Sulfonium matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sulfonium?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sulfonium.