Sulfonic acid

A compound with the general structure R–SO₃H, in which a sulfur atom bonds to an organic group, two oxygen atoms, and a hydroxyl group. Sulfonic acids are strong acids, and their conjugate bases are sulfonates.

Sulfonyl group: The –SO₂– functional group, with sulfur bonded to two oxygen atoms and two substituents. Its two sulfur–oxygen bonds form the core of the sulfonic acid functional group.
Proton transfer: The movement of a proton from one chemical species to another. Dissociation transfers the sulfonic acid proton to a solvent or base.
Ion-exchange resin: An insoluble polymer bearing charged groups that exchange ions with a surrounding solution. Resins with fixed sulfonate groups exchange cations and can function as strong-acid exchangers.
Carboxylic acid: An organic compound containing a carboxyl group, generally written R–CO₂H. Both contain an acidic hydroxyl group, but sulfonic acids are generally much stronger.
Sulfonate: The conjugate base or anion derived from a sulfonic acid, with structure R–SO₃⁻. Loss of the acidic proton converts a sulfonic acid into this ion.
Sulfonation: The introduction of a sulfonic acid group into an organic molecule. This reaction creates carbon–sulfur bonds that install the defining group.
Polystyrene sulfonate: A polymer bearing sulfonate groups on a polystyrene backbone. Its fixed acidic groups make it useful in ion exchange and water treatment.
Sulfinic acid: An organosulfur acid with general structure R–S(O)OH. It has one fewer oxygen atom bonded to sulfur than a sulfonic acid.
Conjugate acid–base pair: Two chemical species related by the gain or loss of a proton. The acid’s strength is expressed through its equilibrium with its sulfonate conjugate base.
Electrophilic aromatic substitution: A reaction in which an electrophile replaces a hydrogen atom on an aromatic ring. Aromatic sulfonation commonly forms aryl sulfonic acids by this mechanism.

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