Sulfonic acid is an hypothetical acid with formula H-S(=O)₂-OH. This compound is a tautomer of sulfurous acid HO-S(=O)-OH, but less stable, and would likely convert to that very quickly if it were formed. Although this compound is unimportant, there are many derived compounds, with formula R-S(=O)₂-OH for various R. These may then form salts or esters, called sulfonates.

Sulfonic acids

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Sulfonic acids are a class of organic acids with the general formula RSO₃H, 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 acid salts (sulfonates) are important as detergents, and the antibacterial sulfa drugs are also sulfonic acid derivatives. The simplest example is methanesulfonic acid, CH₃SO₂OH, which is a reagent regularly used in organic chemistry. P-toluenesulfonic acid is also an important reagent.

Note that the sulfonic acids and sulfonates are analogous to carboxylic acids and carboxylates; in both cases, -C(=O)- is replaced by -S(=O)2-. Chemical properties are similar as well, although sulfonic acids are often even stronger acids than carboxylic acids, the hydrogen being easier to leave than in most compounds, and they readily form esters.

The sulfonic acid and sulfonate functional groups, -SO₂OH and -SO₂O-, are found in many chemical compounds, e.g. certain detergents and dyes as well as in strongly acidic cation exchange resins.

Sulfonic acid chlorides

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Sulfonic acid chlorides are a class of organic compounds with the general formula R-SO₂-Cl. These compounds react readily with alcohols to sulfonic esters. Important 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

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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.

See also

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

External links

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Sulfonic acids | Functional groups

Sulfonat | Acide sulfonique | スルホン酸 | Sulfonácia