Edexcel A Level Chemistry

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7.4.3 Electrophilic Substitution

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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:

7-4-1-general-electrophilic-substitution-mechanism-1-1

General electrophilic substitution mechanism 2, downloadable AS & A Level Chemistry revision notes

  • 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)

7-4-2-nitration-of-benzene-mechanism

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, downloadable IB Chemistry revision notes

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

Hydrocarbons - Friedel-Crafts Acylation (1), downloadable AS & A Level Chemistry revision notesHydrocarbons - Friedel-Crafts Acylation (2), downloadable AS & A Level Chemistry revision notes

Example of a Friedel-Crafts acylation reaction

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Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.