AQA A Level Chemistry

Revision Notes

7.4.2 Nitration of Benzene

Nitration

  • 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

 

Hydrocarbons - Overall Halogenation, downloadable AS & A Level Chemistry revision notes

The overall reaction of halogenation of arenes

 

Hydrocarbons - Overall Nitration, downloadable AS & A Level Chemistry revision notes

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)

Hydrocarbons - Step 1 Halogenation, downloadable AS & A Level Chemistry revision notes

Step 1 of the halogenation reaction of arenes

 

Hydrocarbons - Step 1 Nitration, downloadable AS & A Level Chemistry revision notes

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

Hydrocarbons - Step 2 Halogenation, downloadable AS & A Level Chemistry revision notes

Step 2 of the halogenation reaction of arenes

 

Hydrocarbons - Step 2 Nitration, downloadable AS & A Level Chemistry revision notes

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

 

Hydrocarbons - Step 3 Halogenation, downloadable AS & A Level Chemistry revision notes

Step 3 of the halogenation reaction of arenes

 

Hydrocarbons - Step 3 Nitration, downloadable AS & A Level Chemistry revision notes

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
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