Fischer reaction

In organic chemistry, the Fischer reaction is the conversion of an aryl sulfonic acid to an aryl chloride using chlorine:^[1]

ArSO₃H + Cl₂ → ArCl + ClSO₃H

The method was once used extensively in the preparation of chlorinated anthraquinones such as 1-chloroanthraquinone and 2-anthraquinones. 1,5-Dichloroanthraquinone [wd] forms from the corresponding disulfonic acid [wd]. Illustrating some scope for this reaction, 1-chloroanthraquinone-2-carboxylic acid forms from the sulfonic-carboxylic acid.

Chlorine can be generated in situ from sodium chlorate and hydrochloric acid:^[2]

NaClO₃ + 6 HCl → 3 Cl₂ + NaCl + 3 H₂O

The acidity of this mixture means that sodium sulfonate salts can be used in situ, since they are readily protonated.