Desulfonation reaction

In organic chemistry, the desulfonation reaction is the hydrolysis of aryl and naphthyl sulfonic acids:^[1]

RC₆H₄SO₃H + H₂O → RC₆H₅ + H₂SO₄

It is the reverse of sulfonation.^[2] 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^[3] and 2-bromophenol via phenol-2,4-disulfonic acid.^[4]

In contrast to the acid-catalyzed desulfonation is the alkaline fusion of aryl sulfonates, which is used to prepare phenols:

ArSO₃Na + 2 NaOH → ArONa + Na₂SO₃ + H₂O (Ar = aryl)

ArONa + H⁺ → ArOH + Na⁺

The first stage in alkaline fusion entails the dissolution of the aryl sulfonic acid or arylsulfonate salt in molten alkali metal base, such as a mixture of sodium hydroxide-potassium hydroxide at high temperatures, say 250 °C.^[5]