Formula R-SO3H wherein R is a hydrocarbyl group. Most of the sulfonic acids are synthetic products, and only a few sulfonic acids such as β-aminoethanesulfonic acid NH2CH2CH2SO3H exist in nature. The sulfonic acid group is a strongly water-soluble strong acidic group, and the sulfonic acid is a water-soluble strongly acidic compound. The sulfonic acid group in the aromatic sulfonic acid molecule can be substituted by a hydroxyl group or a cyano group, and is an intermediate for preparing a phenol or a nitrile. The sulfonic acid can be prepared by a sulfonation reaction of an aromatic hydrocarbon or an oxidation reaction of a thiol. Methanesulfonic acid can be used as a catalyst for esterification, hydrolysis and alkylation. The sodium salt of a long chain sulfonic acid such as n-C11H23-CH2 is a class of detergents resistant to hard water. Sulfonic acid groups in many dye molecules allow water-soluble compounds to be water soluble. Sodium dodecylbenzene sulfonate can be used as a detergent. Chlorinated sulfone can be obtained from sulfonic acid and is widely used in organic synthesis. The hydrazine H moiety in the sulfonic acid molecule may be substituted by a halogen atom, an amino group or the like. For example, sulfonyl chloride is a commonly used intermediate in organic synthesis and analytical chemistry. Aminosulfonamide is a class of anti-inflammatory drugs. Sodium saccharin is also a sulfonamide.