Nitration of Benzene (AQA A Level Chemistry): Revision Note

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

• 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 heated under reflux with the benzene at 25 - 60 ^oC

Nitration of Benzene Mechanism:

Addition reactions of arenes

• The delocalisation of electrons in arenes (aromatic stabilisation) explains why arenes usually undergo substitution rather than addition reactions

• In electrophilic substitution reactions, the aromatic ring is temporarily disrupted, but aromaticity is restored in the final product

  • As a result, the benzene ring retains its stabilised, delocalised π system

• In contrast, addition reactions permanently break the delocalised π system, so aromaticity is lost

• For example, hydrogenation of benzene is an addition reaction that converts benzene into cyclohexane

  • In this process, the delocalised π system is destroyed, and the product does not possess aromatic stabilisation

  • This loss of aromatic stabilisation makes addition reactions less favourable than substitution reactions under normal conditions