Sulfonic acid, sulfonic also spelled sulphonic , any of a class of organic acidscontaining sulfur and having the general formula RSO3H, in which R is an organic combining group. The sulfonic acids are among the most important of theorganosulfur compounds; the free acids are widely used as catalysts in organic syntheses, while the salts and other derivatives form the basis of the manufacture of detergents, water-soluble dyes and catalysts, sulfonamide pharmaceuticals, and ion-exchange resins. Aromatic sulfonic acids are particularly useful as intermediates or starting materials in synthesis—for example, in the preparation of phenols. Sulfonic acid groups can greatly enhance the water solubility of compounds, as seen with the sulfonic acid derivative of triphenyl phosphine (TPPTS), P(C6H4-m-SO3Na)3. Metal complexes of this compound are used as homogeneous catalysts for the syntheses of organic compounds in two-phase systems (e.g., in a mixture of water and an organic solvent) in industry and in the laboratory.
Several sulfonic acids occur naturally—for example, the essential nutrient taurine (2-aminoethanesulfonic acid; NH2CH2CH2SO3H), the sulfobacins and other sulfonolipids (the biologically active products from bacterial cultures that contain 15- to 17-carbon chains attached to the carbon and nitrogen of 2-aminoethanesulfonic acid), and the echinosulfonic acid C (an α-hydroxysulfonic acid containing two brominated indole rings). The aliphatic sulfonic acids methanesulfonic acid and trifluoromethanesulfonic acid (triflic acid; CF3SO3H) are also commercially important reagents and catalysts. Triflic acid, one of the strongest known organic acids, is used as a polymerization catalyst and in fuel cells, in gasoline production, and in the synthesis of organic and organometallic compounds.