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Benzene: Electrophilic Substitution Reactions

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Benzene: Electrophilic Substitution Reactions - Lesson Summary

The resonance stabilization of benzene is due to the delocalization of these pi electrons. These pi electrons available for electrophiles, thus the substitution reactions in benzene are electrophillic in nature.

Benzene undergoes typical electrophillic substitution reactions due to the retention of resonance stabilization of the ring (aromaticity). Some common electrophillic substitution reactions of benzene are:
  • Halogenation,
  • Nitration,
  • Friedal Craft’s alkylation
  • Friedal Craft’s acylation Reactions
  • Sulphonation

A general electrophillic substitution reaction involves three steps:
  • Generation of the electrophile,
  • Formation of sigma complex intermediate, and
  • Removal of proton from the sigma complex.

Halogenation reaction: The replacement of a hydrogen atom by a halogen atom such as fluorine, chlorine, bromine and iodine in the presence of a Lewis acid such as anhydrous aluminium chloride or ferric chloride or ferric bromide is called halogenation of benzene.

Ex: Chlorination and bromination which occur at a temperature between 310 and 320 K
Benzene is treated with chlorine in the presence of anhydrous aluminium chloride results in the formation of chloro benzene.

Chloronium ion is generated when anhydrous aluminium chloride combines with the attacking reagent chlorine. This Chloronium ion attacks the pi electron cloud of benzene ring and forms the less stable intermediate called sigma complex or arenium ion. This sigma complex is stabilized by the resonance.

Proton is removed from the intermediate sigma complex in the presence of nucleophile AlCl₄⁻ results in the formation of chlorobenzene.



Nitration:
Reactions, in which hydrogen atom in the benzene ring is replaced by the nitro group is known as nitration.

Nitration is carried out by heating benzene at about 330 K with the nitrating mixture consisting of concentrated HNO₃ and concentrated H₂SO₄.

 
Friedal-Crafts alkylation reaction:
Reactions in which the hydrogen atom in Benzene ring is replaced by the alkyl group in the presence of anhydrous aluminium chloride known as Friedal-crafts alkylation reaction.

Ex:  
Benzene reacts with ethyl chloride in the presence of anhydrous aluminium chloride to form ethyl benzene.


 
Friedal-Crafts acylation:  
When benzene is treated with acyl halide or acid anhydride in the presence of anhydrous aluminium chloride, hydrogen atom in a benzene ring is replaced by the acyl group and forms acyl benzene. This reaction is called the Friedal-Crafts acylation.

Ex:   When benzene is treated with acetyl chloride in the presence of anhydrous aluminium chloride it forms Acetophenone and hydrochloric acid.


 
Sulphonation reaction:
When the hydrogen atom in a Benzene ring is replaced by the sulphonic acid (-SO₃H) group, is known as Sulphonation. Sulphonation reaction is carried out by heating Benzene with fuming sulphuric acid or oleum. Sulphonation can also be carried out by treating benzene with chloro sulphonic acid.
 


Benzene reacts with hydrogen in the presence of a catalyst such as nickel or platinum at 473 to 573 K under pressure to form cyclohexane.


Benzene reacts with chlorine or bromine in the presence of ultra violet light to form benzene hexachloride or BHC.

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