Sulfonation by sulfuric acid and oleum in the presence of catalysts

Metal salts such as sodium and ammonium sulfate exert a dilution effect that effectively weakens the aggressive action of sulfuric acid. Thus, Tietze and Bayer,159 sulfonating pyrene in the presence of sodium sulfate, weakened the oxidizing action of the pure acid on this hydrocarbon in fact, heating pyrene, sulfuric acid, and sodium sulfate in the molar proportions 1 3 3 at 180° for 6 hours gives a 100% yield of 1-pyrenesulfonic acid.160 Adding sodium sulfate is also advantageous in preparation of l-aminoanthraquinone-2-sulfonic acid.161 

The longest known influence of catalysts on sulfonation is that of mercury and mercury salts, which not only accelerates the reaction but also determines the site of substitution owing to intermediate formation of organometallic compounds.156 Thus, whereas anthraquinone-2-sulfonic acid is formed from anthraquinone and oleum,124 1 the 1-sulfonic acid is obtained when mercury salts are present.124 1 

4-Sulfophthalic anhydride is formed on sulfonation of phthalic anhydride by oleum, but presence of mercury(i) sulfate leads to the 3,5-disulfonic acid.162 The catalytic effect of mercury and mercury salts is also applied to the conversion of 4-methylpyridine into 4-methyl-3-pyridinesulfonic acid163 and of 

Sulfonation is also easier in the presence of hydrogen fluoride, boron trifluoride112 (e.g., for benzene, toluene, naphthalene, naphthylamine, naphthol, phenol, carbazole, and biphenyl), or boric acid166 (e.g., for 6-quinizarinsul-fonic acid), but iodine is reported111 not to influence the course of a sulfonation. 

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