30.1.2—Mechanism: electrophilic substitution in arenes

In electrophilic substitution, an electron-rich arene attacks an electrophile to form a positively charged intermediate. A base then removes H⁺, restoring the delocalised π system.

The mechanism explains both the need for an electrophile and the final substitution product: addition is temporary, while loss of the ring proton regenerates aromatic stabilisation.

During nitration, NO₂⁺ attacks benzene, the sigma complex forms, and removal of H⁺ gives nitrobenzene plus restored aromaticity.

Do not draw a permanent addition product or leave the ring with one fewer π bond; the final product is aromatic again.