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Why do electron-releasing groups speed up electrophilic substitution on aromatic molecules?

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dissociation of E–N — reaction mechanism Possible energy diagram for the dissociation of a covalent molecule, E–N, into its ions E⁺ and N⁻ (see text).

Electron-releasing groups increase the rate of electrophilic aromatic substitution because they facilitate the reaction by increasing the electron density of the aromatic ring.1

In this reaction mechanism, the process typically involves the formation of a carbonium ion intermediate. Electron-releasing groups, such as the hydroxyl group found in phenol, stabilize this positively charged intermediate through the donation of nonbonding electrons. By lowering the energy of this transition state, these groups make the aromatic ring more reactive toward electrophiles compared to unsubstituted benzene.12

Conversely, electron-withdrawing groups, such as nitro groups, have the opposite effect; they destabilize the intermediate cation, thereby deactivating the ring and retarding the reaction.321