• 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

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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