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Chemical structure of sulfonic acid.

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.

Ball-and-stick model of methanesulfonic acid

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.

Sulfonic acid chlorides are a class of organic compounds with the general formula R-SO₂-Cl. These compounds react readily with nucleophiles (like alcohols, amines etc.) If the nucleophile is an alcohol the product is a sulfonic ester, if it is an amine the product is a sulfonamide. Important sulfonic acid chlorides are tosyl chloride, brosyl chloride, nosyl chloride and mesyl chloride. One synthetic procedure to synthesize sulfonic acid chlorides is the Reed reaction.

Sulfonic esters are a class of organic compounds with the general formula R-SO₂-OR. Sulfonic esters are considered good leaving groups in nucleophilic aliphatic substitution.

• Sulfuric acid H₂SO₄
• Sulfonium S⁺, S⁺R₃