sulfonic acid

sulfonic acid

sulfonic acid, 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.

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

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

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. Sulfonyl chloride is very prone to hydrolysis. An ice/water bath can be used to decrease this effect (Titus 1982 & Lefevre 1996).

Sulfonic esters

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

See also


  • J.A. Titus, R. Haugland, S.O. Sharrow and D.M. Segal , Texas red, a hydrophilic, red-emitting fluorophore for use with fluorescein in dual parameter flow microfluorometric and fluorescence microscopic studies. J. Immunol. Methods 50 (1982), pp. 193–204.
  • C. Lefevre, H.C. Kang, R.P. Haugland, N. Malekzadeh, S. Arttamangkul, and R. P. Haugland, Texas Red-X and Rhodamine Red-X, New Derivatives of Sulforhodamine 101 and Lissamine Rhodamine B with Improved Labeling and Fluorescence Properties, Bioconj Chem 1996, 7(4):482-9

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