Benzene - Electrophilic Substitution (Edexcel International A Level Chemistry)

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

Diagram to show the chlorination of benzene 

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