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Aromatic nitration replaces an aromatic hydrogen with a nitro group. In a common pathway, aromatic attack on a nitronium ion forms a Wheland intermediate and deprotonation restores aromaticity.

The nitronium ion is $\mathrm{N}{\mathrm{O}}_2^{+}$, a strong electrophile in mixed-acid aromatic nitration. protonation and dehydration of nitric acid can generate it.