Preparation of Haloarenes

Full Notes

Haloarenes From Hydrocarbons (by Electrophilic Substitution)

Haloarenes are formed by the direct halogenation of aromatic hydrocarbons, such as benzene.

An arene (e.g. toluene) is reacted with Cl₂ or Br₂ in the presence of a Lewis acid catalyst like Fe or FeCl₃, in the dark.

• The reaction gives ortho and para isomers.
• These isomers can be separated easily due to their different melting points.
• Iodination is reversible and requires an oxidising agent (like HNO₃, HIO₄) to oxidize the HI formed during the process.
• Fluorination is not feasible this way due to the high reactivity of fluorine.

Example: C₆H₆ + Cl₂ (FeCl₃ catalyst) → C₆H₅Cl + HCl

This is a typical electrophilic aromatic substitution reaction.

Haloarenes from Amines (by Sandmeyer’s Reaction)

Aromatic amines (e.g. aniline) can be converted into haloarenes via the diazonium salt route, specifically using the Sandmeyer reaction.

• Step 1: Diazotisation
  Aniline is treated with NaNO₂ + HCl at 273–278 K to form benzene diazonium chloride.
  Reaction: C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O
• Step 2: Substitution using Cu(I) halide
  The diazonium salt is then treated with CuCl or CuBr to introduce the halogen atom.
  • C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂
  • C₆H₅N₂⁺Cl⁻ + CuBr → C₆H₅Br + N₂
  This process is known as the Sandmeyer reaction.
• For iodination: C₆H₅N₂⁺Cl⁻ + KI → C₆H₅I + KCl + N₂ (This reaction does not require Cu⁺ salts.)

Summary

• Haloarenes can be prepared by halogenation of benzene with Lewis acid catalysts.
• Diazotisation of anilines followed by Sandmeyer substitution gives chloro and bromo derivatives.
• Iodination proceeds via diazonium salts with KI without Cu⁺.
• Fluorination and iodination require modified conditions due to reactivity constraints.