Benzene (C6H6) is a highly stable aromatic hydrocarbon due to its conjugated π-electron system.
This stability makes benzene a prime candidate for electrophilic substitution reactions, where an electrophile replaces one of the hydrogen atoms on the benzene ring.
Here are the key electrophilic substitution reactions of benzene: nitration, sulphonation, halogenation, Friedel-Crafts alkylation, and Friedel-Crafts acylation.
Nitration of Benzene
Nitration involves substituting a hydrogen atom on benzene with a nitro group (-NO2).
1) Reaction Equation:
2) Mechanism:
Formation of the Electrophile:
Electrophilic Attack on Benzene:
C6H6 + NO2+ → C6H5NO2 + H+
Deprotonation:
H+ + HSO4− → H2SO4
Sulphonation of Benzene
Sulphonation involves substituting a hydrogen atom on benzene with a sulfonyl group (-SO3H).
1) Reaction Equation:
2) Mechanism:
Formation of the Electrophile:
Electrophilic Attack on Benzene:
Deprotonation:
Halogenation of Benzene
Halogenation involves substituting a hydrogen atom on benzene with a halogen atom (X).
1) Reaction Equation:
2) Mechanism:
Formation of the Electrophile:
Electrophilic Attack on Benzene:
Formation of the Catalyst:
Friedel-Crafts Alkylation
Friedel-Crafts Alkylation involves substituting a hydrogen atom on benzene with an alkyl group (R).
1) Reaction Equation:
2) Mechanism:
Formation of the Electrophile:
Electrophilic Attack on Benzene:
Regeneration of the Catalyst:
Limitations:
Not effective with deactivated aromatic rings.
Over-alkylation can occur due to increased reactivity of the alkylbenzene product.
Ineffective with sterically hindered alkyl halides.
Friedel-Crafts Acylation (Reactions of Benzene)
Friedel-Crafts Acylation involves substituting a hydrogen atom on benzene with an acyl group (RCO-).
1) Reaction Equation:
2) Mechanism:
Formation of the Electrophile:
Electrophilic Attack on Benzene:
Regeneration of the Catalyst:
Limitations:
Ineffective with sterically hindered acyl halides.
Not suitable for highly deactivated aromatic rings.
Not directly used to prepare alkylbenzenes, as the resulting acylbenzene needs to be reduced.