Description

Sulfonic acid is a hypothetical acid with the formula H-S(=O)2-OH. This compound is a less stable tautomer of the sulfurous acid HO-S(=O)-OH, so the sulfonic acid is rapidly converted upon formation.

Derived compounds that replace hydrogen by sulfur bonding with organic groups are stable. These may form salts or esters called sulfonates.
Structure of sulfonic acid

Sulphonic acids are a group of organic acids with the general formula R-S(=O)2-OH, where R is usually a hydrocarbon side chain.

Sulfonic acids are generally much stronger acids than their carboxylic counterparts and have a unique propensity to bind tightly to proteins and carbohydrates.

Most “washable” dyes are sulfonic acids (or have a functional sulfonyl group) 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 sulfa antibacterial 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 sulfonic acids and sulfonates are similar to carboxylic acids and carboxylates. In both cases, -C(=O)- is replaced by -S(=O)2-.

The chemical properties are similar, although sulfonic acids are often even stronger acids than carboxylic acids, the hydrogen is removed more easily from most compounds, and they readily form esters.
Application of sulfonic acid

Sulphonic acid and sulfonate functional groups, -SO2OH and -SO2O- are found in many chemical compounds, e.g. Special detergents and dyes as well as in highly acidic cation exchange resins