Question

Discuss some important methods of preparation of haloarenes.

Answer 1

Haloarenes (aryl halides) can be prepared by the following important methods:

1. From Diazonium Salts (Sandmeyer Reaction): This involves the conversion of aromatic amines (anilines) to haloarenes. The aromatic amine is first converted into a diazonium salt by treating with nitrous acid (generated in situ from NaNO₂ and HCl) at low temperature. The diazonium salt is then treated with cuprous halides (CuCl, CuBr) or KI to replace the diazonium group with Cl, Br, or I respectively.
   Example:
   \[\mathrm{C_6H_5NH_2} \xrightarrow[]{NaNO_2, HCl, 0-5^\circ C} \mathrm{C_6H_5N_2^+Cl^-} \xrightarrow[]{CuCl} \mathrm{C_6H_5Cl}\]
2. By Gattermann’s Reaction: Similar to Sandmeyer but uses copper powder and halogen acid (e.g., HCl) directly with the diazonium salt to introduce the halogen into the aromatic ring. It’s useful mostly for chlorination or bromination.
   Example:
   
3. By Electrophilic Substitution Reaction (Direct Halogenation): Direct halogenation of benzene or substituted benzene using halogens (Cl₂ or Br₂) in the presence of a Lewis acid catalyst (like FeCl₃ or FeBr₃). This method yields chlorobenzene or bromobenzene.
   Example:
   \[\mathrm{C_6H_6} + \mathrm{Cl_2} \xrightarrow[]{FeCl_3} \mathrm{C_6H_5Cl} + HCl\]