In organic chemistry, the desulfonation reaction is the hydrolysis of aryl and naphthyl sulfonic acids:
- RC6H4SO3H + H2O → RC6H5 + H2SO4
It is the reverse of sulfonation. The temperature of desulfonation correlates with the ease of the sulfonation.
In organic chemistry, the desulfonation reaction is the hydrolysis of aryl and naphthyl sulfonic acids:
It is the reverse of sulfonation. The temperature of desulfonation correlates with the ease of the sulfonation.
This reactivity is exploited in the regiospecific preparation of di- and tri-substituted aromatic compounds. The approach exploits the meta-directing effect of the sulfonic acid group. 2-Chlorotoluene for example can be prepared by chlorination of p-toluenesulfonic acid, followed by hydrolysis. The method is also useful for the preparation of 2,6-dinitroaniline and 2-bromophenol via phenol-2,4-disulfonic acid.
In some cases, sulfonation gives a disulfonated product, which can be followed by mono-desulfonation to give a distinct monosulfonated product. 2-Hydroxynaphthalene-4-sulfonic acid is prepared by desulfonation of 2-hydroxynaphthalene-4,8-di-sulfonic acid, which in turn is produced by disulfonation of 2-naphthol.
In contrast to the acid-catalyzed desulfonation is the alkaline fusion of aryl sulfonates, which is used to prepare phenols:
The first stage in alkaline fusion entails the dissolution of the aryl sulfonic acid or arylsulfonate salt in concentrated or even molten alkali metal base, such as a mixture of sodium hydroxide-potassium hydroxide at high temperatures, say 250 °C. One commercial process is the preparation of 1-hydroxynaphthalene-5-sulfonic acid, a precursor to dyes and pigments, by treatment of 1,5-naphthalenedisulfonic acid with hot aqueous sodium hydroxide.