Benzene reaction mechanism

[Yajurved]

Benzene reaction with no2 or halogens does h+ leaves compound leaving benzene- and mechanim

 

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$\mathbf(1)$ The reaction involves replacing a hydrogen atom on the benzene ring with a nitro group to produce nitrobenzene

Step 1: Generation of the Electrophile (Nitronium Ion)
Nitric acid alone is not strong enough to attack the stable benzene ring. Concentrated sulfuric acid acts as a catalyst by protonating the nitric acid to create a much more powerful electrophile: the nitronium ion

• Reaction:
  $HNO_3 + 2H_2SO_4 \rightarrow NO2^{+}+H_3O^{+} + 2HSO_4^{−}$

Step 2: Formation of the Arenium Ion (Sigma Complex)
The pi-electrons of the benzene ring attack the electrophilic $NO_2^{+}$ ion. This breaks the ring's aromaticity and forms a carbocation intermediate known as the arenium ion or Wheland complex.

Nature: This is the slowest, rate-determining step (RDS) of the reaction.

• Stability: The positive charge in the arenium ion is delocalized over the ortho and para positions through resonance, providing some temporary stability.

Step 3: Deprotonation and Restoration of Aromaticity
A weak base in the mixture (typically $HSO_4^{-}$ or $H_2O$) abstracts the proton from the $sp^{3}$ hybridized carbon that was attacked by the nitro group.

• Result: The two electrons from the C-H bond reform the pi-system restoring the stable aromatic ring.
  
  
• Product: Nitrobenzene is formed as the final product

$\mathbf(2)$ Second reaction involves replacing a hydrogen atom on the benzene ring with halogen to produce $\textbf{aryl halide}$

In this we prepare aryl chlorides and bromides by reacting an aromatic ring with a halogen in the presence of a Lewis acid catalyst (e.g.,
$FeCl_3,AlCl_3,FeBr_3$)

1. Generation of the Electrophile:The Lewis acid reacts with the halogen molecule ($X_2$) to create a strong electrophile, often a halonium ion ($X^+$)
   • Example:
     $Cl_2+FeCl_3 \rightarrow [FeCl_4]^{−} + Cl^{+}$
2. Formation of the Arenium Ion (Sigma Complex): The electron-rich pi cloud of the aromatic ring attacks the electrophile. This disrupts aromaticity and creates a resonance-stabilized carbocation known as an arenium ion. This is the slow, rate-determining step.
   
   
3. Restoration of Aromaticity: A weak base (like the $[FeCl_4]^-$ ion removes a proton from the carbon atom bonded to the halogen. The electrons from the broken C−H bond return to the ring, restoring its stable aromatic system.