Nitration of Benzene: A Step-by-Step Mechanism

1. Formation of the Electrophile: The electrophile in nitration reactions is the nitronium ion (NO2+​). It is generated by reacting concentrated nitric acid (HNO3​) with a strong acid, typically concentrated sulfuric acid (H2SO4). The sulfuric acid protonates the nitric acid, leading to the formation of the nitronium ion and water.

   

2. Electrophilic Attack and Formation of the Sigma Complex: The nitronium ion (NO2+​) then attacks the $\pi$ electrons of the benzene ring, forming a high-energy carbocation intermediate known as the sigma complex or arenium ion. This step disrupts the aromaticity of the benzene ring temporarily but is stabilized by resonance as the positive charge can be delocalized across three carbon atoms of the ring.
   

3. Deprotonation and Restoration of Aromaticity: A base, typically the bisulfate ion (HSO4−​) produced alongside the nitronium ion, removes a proton (H+) from the carbon adjacent to the newly added nitro group. This action restores the aromaticity of the ring by re-establishing the conjugated $\pi$ electron system, resulting in the formation of nitrobenzene.

4. Regeneration of the Acid Catalyst: This step involves the recovery of the sulfuric acid catalyst, allowing it to participate in further reactions. The removal of H+ by HSO4−​ helps in maintaining the acid concentration.

    

In summary, the nitration of benzene involves the formation of a strong electrophile (nitronium ion), which attacks the benzene ring to form a sigma complex. This intermediate is then deprotonated to restore aromaticity, yielding nitrobenzene. This process is widely used in the chemical industry for the production of various nitroaromatic compounds.