sulfonation
/ sul-fuh-NAY-shun /
Sulfonation hangs a sulfonic acid group (-SO3H) onto a benzene ring. The most striking thing about it is something the other EAS reactions cannot do: it is reversible. You can put the -SO3H group on, and later take it right back off, which makes it an unusually handy tool for steering a synthesis rather than just decorating a ring.
The electrophile is sulfur trioxide, SO3 (or its protonated form), supplied by fuming sulfuric acid (sulfuric acid with extra dissolved SO3, also called oleum). The sulfur in SO3 is strongly electron-poor, so the ring attacks it, forms the usual arenium ion, loses a proton, and ends up as benzenesulfonic acid (C6H5-SO3H). Because every step is an equilibrium, the whole reaction runs forward in hot, concentrated, SO3-rich acid — but if you later heat the product with dilute aqueous acid (steam), the equilibrium reverses and the -SO3H comes off as the ring picks up an H again.
That reversibility is the reaction's superpower in synthesis. A chemist can install -SO3H as a temporary, bulky, ortho/para-directing blocking group: it occupies a position (often the para spot), forces the next electrophile to go somewhere else, and is then removed at the end by desulfonation. Sulfonic acid groups are also valuable in their own right — they make compounds water-soluble and acidic, which is why they appear in many detergents, dyes, and sulfa drugs.
Benzene + fuming H2SO4 (SO3) -> benzenesulfonic acid. Heat that product with dilute sulfuric acid and steam, and the -SO3H falls back off, regenerating benzene — the reaction running in reverse.
Unique among EAS reactions, sulfonation can be run forward or backward at will.
Reversibility is the whole point: chemists exploit it to use -SO3H as a removable blocking group that occupies a position temporarily to steer the next reaction, then is stripped off later — something none of the other EAS reactions allow.