Edexcel A Level Chemistry:复习笔记7.4.3 Electrophilic Substitution

Electrophilic Substitution


Reactions of Benzene

  • The main reactions which benzene will undergo include the replacement of one of the 6 hydrogen atoms from the benzene ring
    • This is different to the reactions of unsaturated alkenes, which involve the double bond breaking and the electrophile atoms 'adding on' to the carbon atoms


  • These reactions where at least one of the H atoms from benzene are replaced, are called electrophilic substitution reactions
    • The hydrogen atom is substituted by the electrophile


  • You must be able to provide the mechanisms for specific examples of the electrophilic substitution of benzene

General Electrophilic Substitution Mechanism:





  • The delocalised π system is extremely stable and is a region of high electron density
  • Electrophilic substitution reactions involve an electrophile, which is either a positive ion or the positive end of a polar molecule
  • There are numerous electrophiles which can react with benzene
    • However, they usually cannot simply be added to the reaction mixture to then react with benzene
    • The electrophile has to be produced in situ, by adding appropriate reagents to the reaction mixture
  • The electrophilic substitution reaction in arenes consists of three steps:
    • Generation of an electrophile
    • Electrophilic attack
    • Regenerating aromaticity


Nitration of benzene mechanism 

  • 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)



Halogenation of benzene mechanism 

  • Benzene will undergo a substitution reaction with a halogen if a metal halide carrier is present
  • This generates the electrophile for the reaction to occur


The different stages in the chlorination of benzene


Friedel-Crafts acylation mechanism 

  • In the Friedel-Crafts acylation reaction, an acyl group is substituted into the benzene ring
    • An acyl group is an alkyl group containing a carbonyl, C=O group


Example of a Friedel-Crafts acylation reaction