7.4.2 Nitration of Benzene

• The electrophilic substitution reaction in arenes consists of three steps:
  • Generation of an electrophile
  • Electrophilic attack
  • Regenerating aromaticity

Mechanism of electrophilic substitution

• The nitration of benzene is one example of an electrophilic substitution reaction
  • A hydrogen atom is replaced by a nitro (-NO₂) group

The overall reaction of nitration of arenes

• In the first step, the electrophile is generated
  • The electrophile NO₂⁺ ion is generated by reacting concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄)

• Once the electrophile has been generated, it will carry out an electrophilic attack on the benzene ring
  • The nitrating mixture of HNO₃ and H₂SO₄ is refluxed with the arene at 25 - 60 ^oC

Nitration of Benzene Mechanism:

Addition reactions of arenes

• The delocalisation of electrons (also called aromatic stabilisation) in arenes is the main reason why arenes predominantly undergo substitution reactions over addition reactions
• In substitution reactions, the aromaticity is restored
• In addition reactions, on the other hand, the aromaticity is not restored and is in some cases completely lost
  • The hydrogenation of arenes is an example of an addition reaction during which the aromatic stabilisation of the arene is completely lost
  • The cyclohexane formed is energetically less stable than the benzene