Sulfonic Acid

sulfonic acid (səlfŏn`ĭk), organic compound containing the functional group RSO₂OH, which consists of a sulfur atom, S, bonded to a carbon atom that may be part of a large aliphatic or aromatic hydrocarbon, R, and also bonded to three oxygen atoms, O, one of which has a hydrogen atom, H, attached to it. The hydrogen atom makes the compound acidic, much as the hydrogen of a carboxylic acid (see carboxyl group) makes it acidic (see acids and bases). However, while carboxylic acids are weak (with dissociation constants of about 10⁻⁵), sulfonic acids are considered strong acids (with dissociation constants of about 10⁻²). Because sulfonic acids are so acidic, they generally exist as their salts and thus tend to be quite soluble in water. Sulfonic acid groups are often introduced into organic molecules such as dyes to stabilize them for use in aqueous dye baths. Sulfonic acid groups also improve the washfastness of wool and silk dyes by enabling the dye to bind more tightly to the fabric. The most important use of sulfonic acid salts (sulfonates) is in the detergent industry. Sodium salts of long-chain aliphatic or aromatic sulfonic acids are used as detergents. Unlike ordinary soaps, which contain carboxylic acid salts, soaps containing sulfonates do not form a scum in hard water because the calcium and magnesium ions present in the hard water do not form insoluble precipitates with sulfonates as they do with carboxylates. Some sulfonic acid derivatives, e.g., the sulfa drugs, are important as antibiotics.

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sulfonic acid [¦səl¦fän·ik ′as·əd]

(organic chemistry)

A compound with the radical ‒SO₂OH, derived by the sulfuric acid replacement of a hydrogen atom; for example, conversion of benzene, C₆H₆, to the water-soluble benzenesulfonic acid, C₆H₅SO₃H, by treatment with sulfuric acid; used to make dyes and drugs.

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Sulfonic Acid 

any of several acids of the general formula RSO₃H, where R is an aliphatic or aromatic hydrocarbon radical. Sulfonic acids are usually hygroscopic, crystalline substances that dissolve readily in water. Their acidity is close to that of mineral acids.

Aromatic sulfonic acids are obtained by sulfonation of aromatic hydrocarbons and their derivatives. Aliphatic sulfonic acids are prepared by sulfochlorination of hydrocarbons or by replacement of the halogen with the sulfonic acid group, —SO₃H (see).

Under the action of PC1₅, sulfonic acids are converted to sulfo-nyl chlorides, RSO₂C1. Reduction of sulfonic acids produces sulfinic acids, RSO₂H, and mercaptans, RSH. The SÓ₃H group in the aromatic ring is easily replaced by H, OH, OR, H₂N, R₂N, and other groups under the action of nucleophilic reagents.

Aromatic sulfonic acids are important intermediates in industrial organic synthesis, such as the production of phenols. They are used in the production of azo dyes, sulfa drugs, ion-exchange resins, and additives for electroplating baths. The basic and quaternary ammonium salts of aliphatic sulfonic acids, called sulfonates, are surfactants and are used as components of detergents.

In inorganic chemistry, the term “sulfonic acid” is sometimes used in reference to thio acids.

B. L. DIATKIN