Aromatic sulfonation by sulfuric acid or oleum

Sulfonation if of great practical importance as one of the fundamental operations of aromatic chemistry, and there are many methods of carrying it out. The term sulfonation denotes introduction of an S03H group into a molecule, in the simplest case by means of sulfuric acid 104 

Price and S. Oae, Sulfur Bonding, Ronald Press Co., New York, 1962. 

37 (1941) E. S. Gould, Mechanismus und Struktur in der organischen Chemie, Verlag Chemie, Weinheim/Bergstrafle, 1962, p. 528. 

For the reaction mechanism see reviews by Price and by Gould.104 The reaction converts a compound into a water-soluble derivative which may be the free sulfonic acid or, more often, a salt thereof. Moreover, sulfonic acids are extremely important intermediates for the preparation of phenols, particularly those of the naphthalene series. 

Free sulfonic acids rarely have characteristic melting points. They are all appreciably stronger acids than the corresponding carboxylic acids. 

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