Sulfonic acid is an unstable acid with the formula H-S(=O)2-OH. Sulfonic acid is the less stable tautomer of sulfurous acid HO-S(=O)-OH into which sulfonic acid converts rapidly. Derived compounds which replace the sulfur-bonded hydrogen with organic groups are stable. These may then form salts or esters, called sulfonates.
Sulfonic acids are a class of organic acids with the general formula R-S(=O)2-OH, where R is usually a hydrocarbon side chain. Sulfonic acids are typically much stronger acids than their carboxylic equivalents, and have the unique tendency to bind to proteins and carbohydrates tightly; most "washable" dyes are sulfonic acids (or have the functional sulfonyl group in them) for this reason. They are also used as catalysts and intermediates for a number of different products. Sulfonic acids and their salts (sulfonates) are used extensively in such diverse products like detergents, antibacterial drugs sulfa drugs, anion exchange resins (water purification) and dyes. The simplest example is methanesulfonic acid, CH3SO2OH, which is a reagent regularly used in organic chemistry. p-Toluenesulfonic acid is also an important reagent.