Question

Describe a method for the preparation of haloarenes from diazonium salts.

Answer 1

The preparation of haloarenes from diazonium salts, including the relevant equations:

1. Preparation of Diazonium Salt: An aromatic amine (aniline) is treated with nitrous acid (generated in situ from NaNO2 and HCl) at 0–5 °C to form the diazonium salt.
   \[\mathrm{C_6H_5NH_2} + \mathrm{HNO_2} + \mathrm{HCl} \rightarrow \mathrm{C_6H_5N_2^+Cl^-} + 2\mathrm{H_2O}\]
2. Sandmeyer Reaction (For chloro- and bromoarenes): Diazonium salt reacts with cuprous chloride (CuCl) or cuprous bromide (CuBr) to give haloarenes.
   \[\mathrm{C_6H_5N_2^+Cl^-} + \mathrm{CuCl} \rightarrow \mathrm{C_6H_5Cl} + \mathrm{N_2} + \mathrm{CuCl_2} ] [ \mathrm{C_6H_5N_2^+Cl^-} + \mathrm{CuBr} \rightarrow \mathrm{C_6H_5Br} + \mathrm{N_2} + \mathrm{CuClBr}\]
3. Gattermann Reaction (For chlorobenzene): Diazoniun salt is treated with Cu powder in HCl to yield chlorobenzene.
   \[ \mathrm{C_6H_5N_2^+Cl^-} + \mathrm{Cu} \rightarrow \mathrm{C_6H_5Cl} + \mathrm{N_2} + \mathrm{CuCl}\]
4. Iodination: For iodoarenes, diazonium salt is treated with potassium iodide (KI).
   \[\mathrm{C_6H_5N_2^+Cl^-} + \mathrm{KI} \rightarrow \mathrm{C_6H_5I} + \mathrm{N_2} + \mathrm{KCl}\]

This method efficiently replaces the diazo group with halogens to form haloarenes through substitution reactions involving diazonium salts.