Aromatic sulfonation by halosulfuric acids and other sulfonating agents

When aromatic compounds are sulfonated by chlorosulfuric acid side reactions such as formation of the sulfonyl chloride or even the sulfone occur to a greater extent than in the procedures described above. For example, o- and p-toluenesulfonyl chloride are produced industrially from toluene and an excess of chlorosulfuric acid. Nevertheless, this acid can be used for introducing the sulfonic acid group if the proportions of the reactants, the temperature, and the solvent are selected correctly. 

It is best to sulfonate by chlorosulfuric acid in an inert solvent such as carbon disulfide, carbon tetrachloride, chloroform, chlorobenzene, dichlorobenzene, or nitrobenzene. 

Using chlorosulfuric acid has the advantage over using sulfuric acid that milder conditions can generally be applied, so that the products are purer and substitution is more confined to one site also substituents that are affected by sulfuric acid, such as the cyano group, remain intact during sulfonation by chlorosulfuric acid. 

Examples of sulfonation by chlorosulfuric acid are the preparation of 2-hydroxy-1 -naphthalenesulfonic acid,124h 6-chrysenesulfonic acid,180 4-hydroxy-2-methylbenzenesulfonic acid.181 Aromatic amines are sulfonated with particular ease by this reagent. 

4-Amino-3-(trifluoromethyl)benzenesulfonic acid 182 m-(Trifluoromethyl)aniline (161 g) is dissolved in carefully dried 0-dichlorobenzene (11), freshly distilled chlorosulfuric acid (120 g) is added with stirring, and the mixture is heated slowly to 180°, whereupon there is lively 

---