Sulfonic acid

Methanesulfonic acid (CH₃SO₂OH) is the simplest sulfonic acid.

A sulfonic acid is a chemical compound related to sulfuric acid, where one of the pairs of oxygen and hydrogen atoms (called a hydroxy group, −OH) in sulfuric acid is replaced with a carbon atom. Because of the chemical bond between sulfur and carbon, sulfonic acids are a type of organosulfur compound. Sulfonic acids are ingredients in detergents (alkylbenzene sulfonate), medications (sulfa drugs), and some dyes.

The parent compound, which has hydrogen instead of carbon, would also be called "sulfonic acid" (HSO₂OH). but research has shown that it is a tautomer of sulfurous acid (H₂SO₃), meaning that the two chemicals change into each other so easily that they can never be separated. Pure sulfurous acid has never been produced, as it disproportionates to sulfur dioxide and water.