Nitration of Benzene (AQA A Level Chemistry)

Nitration

• 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