Electrophilic Substitution in Benzene (HL) (HL IB Chemistry)

Reactions of Benzene

• Benzene undergoes a wide range of reactions including combustion - (complete and incomplete) and nitration
• Nitration involves the substitution of a hydrogen atom from the benzene ring with an electrophilic atom or group of atoms

Nitration of benzene

In nitration reactions, a nitro (-NO₂) group replaces a hydrogen atom on the arene

• Nitration is an electrophilic substitution reaction
  • This is different to the reactions of unsaturated alkenes, which involve the double bond breaking and the electrophile atoms 'adding across' the carbon atoms

Electrophilic substitution reaction

• The electrophilic substitution reaction in arenes consists of three steps:
  1. Generation of an electrophile
  2. Electrophilic attack
  3. Regenerating aromaticity

Generation of an electrophile

• The delocalised π system is extremely stable and is a region of high electron density
• Consequently, the first step of an electrophilic substitution reaction involves the generation of an electrophile
  • An electrophile can be a positive ion or the positive end of a polar molecule
• The electrophile for nitration is the nitronium ion, NO₂⁺ 
  • This is produced in situ, by adding a mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄), at a temperature between 25 and 60 ^oC, to the reaction mixture

Electrophilic attack

• The second step in nitration is when 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

The electrophilic attack of the nitronium ion by benzene

Electrons from the benzene π bonding system attack the electrophile

Regenerating aromaticity

• In the third step of electrophilic substitution, the aromaticity of the benzene ring system is restored
• This happens by heterolytic cleavage of the C-H bond
  • This means that the electrons in this bond go into the benzene π bonding system

Breaking a C-H bond to restore aromaticity

The C-H bond breaks heterolytically to restore the aromaticity of the benzene π bonding system