Sulfonation with sulfur trioxide and its addition compounds

Direct introduction of the sulfo group by means of sulfur trioxide167,168 has the advantage over sulfonation by sulfuric acid that no water is formed, so that the site of sulfonation is fixed more cleanly.169 The reaction is generally very violent, often even leading to carbonization, so that it is wise to use a low temperature or a solvent. The reaction may be performed by passing gaseous sulfur trioxide into the reactant or in solution. 

Methyl 5-sulfosalicylate (COOCH3 = l),170 for example, is obtained by passing sulfur trioxide into methyl salicylate until the reaction mass is solid, during which process the temperature rises slowly from 25° to 107°. 

Liquid sulfur dioxide is particularly suitable as solvent for sulfonation by sulfur trioxide.171 

The powerful oxidizing action of sulfur trioxide can be overcome by using the sulfur trioxide addition compounds mentioned above (page 612) their activity in sulfonation depends on their stability.167 Disulfur hexaoxide or its adducts to mineral acids (e.g., H2S207) are more powerful sulfonating agents than the stable adducts to dioxan55,101 or tertiary amines very mild conditions can be used with the latter 172 compounds that are especially sensitive to acids can be sulfonated by the pyridine-sulfur trioxide adduct without decomposition. 

Pyridine-sulfur trioxide adduct 173 Pyridine (1 mole) is added, with stirring and good cooling, to sulfur trioxide (1 mole) in three parts of carbon tetrachloride. The adduct produced is filtered off, washed with a little ice-water (to remove small amounts of pyridinium sulfate), and dried. The yield is about 90%. 

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