29.2 Characteristic organic reactions

Electrophilic substitution replaces an atom or group on an electron-rich aromatic ring with an electrophile while restoring aromaticity. Addition–elimination first adds across a multiple bond or carbonyl system and then eliminates a leaving group to restore a π bond.

The names describe the sequence, not just the reactants. Identify the electron-rich site, the electrophile or nucleophile, and the group that leaves.

Nitration of benzene is electrophilic substitution: NO₂⁺ enters the ring and H⁺ is removed. Acyl substitution at an acid derivative follows addition of a nucleophile to C=O, then elimination of the leaving group.

Electrophilic substitution is not electrophile addition: the ring ends with its aromatic π system restored.