Electrophilic aromatic substitution

Electrophilic aromatic substitution replaces a hydrogen on an aromatic ring with an electrophile. The ring temporarily loses aromaticity during bond formation, then regains it when a proton is removed.

Arenium ion: A resonance-stabilized carbocation formed when an electrophile bonds to an aromatic ring. This intermediate forms after electrophilic attack and before proton loss restores aromaticity.
Benzene: A six-carbon aromatic hydrocarbon with a planar ring and delocalized π electrons. Benzene is the simplest common substrate for illustrating this substitution reaction.
Nitration: A reaction that introduces a nitro group into a molecule, commonly using nitric acid and sulfuric acid. Benzene nitration is a standard example of electrophilic aromatic substitution.
Electrophilic addition: A reaction in which an electrophile and another group add across a multiple bond. Unlike addition to an alkene, substitution ultimately restores the aromatic π system.
Electrophile: An electron-pair acceptor that forms a bond by accepting electron density. The electrophile accepts electron density from the aromatic ring in the bond-forming step.
Resonance: A way to represent electron delocalization using multiple contributing Lewis structures. Resonance describes both aromatic-ring electron distribution and stabilization of the arenium ion.
Sulfonation: A reaction that introduces a sulfonic acid group into an organic molecule. Aromatic sulfonation uses electrophilic substitution and can often be reversed by desulfonation.
Nucleophilic aromatic substitution: A reaction in which a nucleophile replaces a leaving group on an aromatic ring. It replaces the usual electrophilic attack with nucleophilic attack under different electronic conditions.
Aromaticity: A stabilizing property of cyclic, conjugated systems with delocalized π electrons. Temporary loss and subsequent restoration of aromaticity shape the reaction pathway.
Inductive effect: A shift in electron density transmitted through σ bonds because of electronegativity differences. Inductive effects help explain how substituents alter a ring's reactivity.

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