Reactions of Benzene

• Reactions of Benzene: 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.

Reactions of Benzene – Nitration

• Nitration involves substituting a hydrogen atom on benzene with a nitro group (-NO2).

  1. Reaction Equation:

     • • ${C6H6 + HNO3 → C6H5NO2 + H2O} \quad \text{(in the presence of H2SO4)}$

  2. Mechanism:

     • Formation of the Electrophile:
       • ${HNO3 + H2SO4 → NO2+ + HSO4- + H2O}$
     • Electrophilic Attack on Benzene:
       • C6H6 + NO2+ → C6H5NO2 + H+
     • Deprotonation:
       • H+ + HSO4− → H2SO4

Reactions of Benzene- Sulphonation

• Sulphonation involves substituting a hydrogen atom on benzene with a sulfonyl group (-SO3H).

  1. Reaction Equation:

     • • ${C6H6 + H2SO4 → C6H5SO3H + H2O}$

  2. Mechanism:

     • Formation of the Electrophile:
       • $\text{H}_2\text{SO}_4 \; \text{↔} \; \text{H}^+ + \text{HSO}_4^- \quad \text{and} \quad \text{HSO}_4^- \; \text{↔} \; \text{H}^+ + \text{SO}_3^{2-}$
     • Electrophilic Attack on Benzene:
       • ${C6H6 + SO3^{2-} → C6H5SO3H + H+}$
     • Deprotonation:
       • ${H+ + HSO4- → H2SO4}$

Reactions of Benzene- Halogenation of Benzene

• Halogenation involves substituting a hydrogen atom on benzene with a halogen atom (X).

  1. Reaction Equation:

     • • ${C6H6 + X2 → C6H5X + HX} \quad \text{(in the presence of FeX3 or AlX3)}$

  2. Mechanism:

     • Formation of the Electrophile:
       • ${X2 + FeX3 → X+ + FeX4-}$
     • Electrophilic Attack on Benzene:
       • ${C6H6 + X+ → C6H5X + H+}$
     • Formation of the Catalyst:
       • ${H+ + FeX4- → HX + FeX3}$

Reactions of Benzene- Friedel-Crafts Alkylation

• Friedel-Crafts Alkylation involves substituting a hydrogen atom on benzene with an alkyl group (R).

  1. Reaction Equation:

     • • ${C6H6 + RCO-X -> C6H5COR + HX} \quad \text{(in the presence of AlCl3 or FeCl3)}$

  2. Mechanism:

     • Formation of the Electrophile:
       • ${RCO-X + AlCl3 -> RCO+ + AlCl4-} \quad \text{RCO-X$^+$}$
     • Electrophilic Attack on Benzene:
       • ${C6H6 + RCO+ -> C6H5COR + H+}$
     • Regeneration of the Catalyst:
       • ${H+ + AlCl4- -> HX + AlCl3}$

• 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.

Reactions of Benzene- Friedel-Crafts Acylation

• Friedel-Crafts Acylation involves substituting a hydrogen atom on benzene with an acyl group (RCO-).

  1. Reaction Equation:

     • • ${C6H6 + R-X -> C6H5R + HX} \quad \text{(in the presence of AlCl3 or FeCl3)}$

  2. Mechanism:

     • Formation of the Electrophile:
       • ${R-X + AlCl3 -> R+ + AlCl4-}$
     • Electrophilic Attack on Benzene:
       • ${C6H6 + R+ -> C6H5R + H+}$
     • Regeneration of the Catalyst:
       • ${H+ + AlCl4- -> HX + AlCl3}

• 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.

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