Aromatic sulfonation

Aromatic sulfonation is an organic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution.

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Sulfur trioxide is the electrophile in this reaction generated from concentrated sulfuric acid (or fuming sulfuric acid) when heated.

In contrast to aromatic nitration and other electrophilic aromatic substitutions this reaction is reversible. Sulfonation takes place in strong acidic conditions and desulfonation is the mode of action in a hot aqueous acid.

It is very useful in protecting a piece of an aromatic compound because of this reversibility. See Aromatic Hydration.

Application

Aromatic sulfonic acids are intermediates in the preparation of dyes and many pharmaceuticals. Sulfonation of aniline produces p-aminobenzenesulfonic acid or sulfanilic acid which is a zwitterion with an unusual high melting point. The amide of this compound and related compounds form a large group of sulfa drugs.

Sulfonation of polystyrene can be used to prepare sodium polystyrene sulfonate.