Illustrated Glossary of Organic Chemistry

Sulfonic acid: A functional group characterized by a sulfur atom doubly bonded to two oxygen atoms, a hydroxyl group, and a carbon atom of any hybridization. Also the parent member of this class of compounds, HS(=O)2OH.

General sulfonic acid
molecular structure.
Sulfonic acid
Benzenesulfonic acid
Trifluoromethanesulfonic acid
(triflic acid; a superacid).