7.2.2 Electrophilic Substitution of Arenes

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

Mechanism of electrophilic substitution

• The halogenation and nitration of arenes are both examples of electrophilic substitution reactions
  • A hydrogen atom is replaced by a halogen atom or a nitro (-NO₂) group

The overall reaction of halogenation of arenes

The overall reaction of nitration of arenes

• In the first step, the electrophile is generated
  • For the halogenation reaction, this is achieved by reacting the halogen with a halogen carrier
  • The halogen molecules form a dative bond with the halogen carrier by donating a lone pair of electrons from one of its halogen atoms into an empty 3p orbital of the halogen carrier
  • In the nitration reaction, the electrophile NO₂⁺ ion is generated by reacting it with concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄)

Step 1 of the halogenation reaction of arenes

Step 1 of the nitration reaction of arenes

• 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

• A pair of electrons from the benzene ring is donated to the electrophile to form a covalent bond
  • This disrupts the aromaticity in the ring as there are now only four π electrons and there is a positive charge spread over the five carbon atoms

Step 2 of the halogenation reaction of arenes

Step 2 of the nitration reaction of arenes

• In the final step of the reaction, this aromaticity is restored by heterolytic cleavage of the C-H bond so that bond electrons in this bond go into the benzene π bonding system

Step 3 of the halogenation reaction of arenes

Step 3 of the nitration reaction of arenes

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 by heterolytic cleavage of the C-H bond
• 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