Mechanism
Sulfur trioxide is the electrophile in this electrophilic aromatic substitution generated from concentrated sulfuric acid (or fuming sulfuric acid) when heated. With benzene as substrate the reaction product is benzenesulfonic acid.
In contrast to aromatic nitration and other electrophilic aromatic substitutions this reaction is reversible. Sulfonation takes place in concentrated acidic conditions and desulfonation is the mode of action in a dilute hot aqueous acid. It is very useful in protecting the aromatic system because of this reversibility.
Read more about this topic: Aromatic Sulfonation
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