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
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:
- Generation of an electrophile
- Electrophilic attack
- 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
