Electrophilic Substitution in Benzene (HL) (DP IB Chemistry): Revision Note

Electrophilic substitution in benzene

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

• Nitration is an electrophilic substitution reaction, unlike alkenes where electrophiles add across double bonds

Electrophilic substitution reaction

• The electrophilic substitution reaction 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

• In the second step, a pair of π electrons from benzene forms a covalent bond with the electrophile, disrupting aromaticity and creating a positively charged intermediate

The electrophilic attack of the nitronium ion by benzene

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