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