CIE A Level Chemistry (9701) exams from 2022

Revision Notes

7.2.2 Electrophilic Substitution of Arenes

Arenes: Electrophilic Substitution

  • 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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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