Benzene mainly undergoes __________?

Benzene (C₆H₆) is the simplest aromatic hydrocarbon and exhibits very unique chemical properties due to its aromatic stability. Aromaticity arises from its cyclic conjugated π-electron system, where six π-electrons are delocalized over the six carbon atoms, making the structure extremely stable.

When we study the reactions of benzene, one of the most striking features is that it does not behave like alkenes, even though it contains double bonds. Alkenes typically undergo addition reactions, where π-bonds are broken to form new σ-bonds. However, in benzene, such addition reactions would disrupt its aromatic conjugation, leading to a significant loss of stability.

To preserve aromaticity, benzene usually undergoes substitution reactions, especially electrophilic aromatic substitution (EAS). In these reactions, one hydrogen atom on the ring is replaced by another atom or group, while the aromatic ring system remains intact. This ensures that the delocalized π-system is preserved.

Common substitution reactions of benzene include:

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  Nitration: Reaction with a nitrating mixture (HNO₃ + H₂SO₄) to give nitrobenzene.

  
  


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  Sulfonation: Reaction with concentrated H₂SO₄ to give benzene sulfonic acid.

  
  


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  Halogenation: Reaction with Cl₂ or Br₂ in the presence of Lewis acid catalysts (FeCl₃, FeBr₃).

  
  


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  Friedel–Crafts alkylation: Reaction with alkyl halides in the presence of AlCl₃.

  
  


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  Friedel–Crafts acylation: Reaction with acid chlorides to introduce an acyl group.

  
  

While benzene can undergo halogenation, it is just one specific type of substitution reaction under controlled conditions. Addition reactions, though possible under extreme conditions (like hydrogenation to form cyclohexane), are not characteristic of benzene.

Key Point:
Benzene’s preference for substitution reactions is due to the need to retain its aromatic stability. This property distinguishes aromatic compounds from alkenes and is a central concept in organic chemistry.