Preparation of Haloarenes

Chloroarenes and bromoarenes are prepared by reaction of halogens (Chlorine and bromine) with benzene derivatives in the presence of lewis acids e.g., Fe, FeCl₃, AlCl₃ etc.

If halogen is used in large quantity, ortho and para isomers are formed because halogen is an o,p-directing group.

If we use I₂ instead of Cl₂ or Br₂, the reaction becomes reversible because HI produced during the reaction is a good reducing agent and converts aryl iodide back to the aromatic hydrocarbon.

To shift the equilibrium in forward, some oxidizing agents like HNO₃, HIO₃, HIO₄ or HgO are used. The oxidising agents either oxidise HI to I₂ or convert it into HgI₂.

2HI + 2HNO₃ → 2NO₂ + 2H₂O + I₂

5HI + HIO₃ → 3I₂ + H₂O

2HI + HgO → HgI₂ + H₂O

Preparation of haloarenes from diazonium salt

Diazonium salts (R-N₂X⁻) can alternatively be used to prepare haloarenes.

Benzenediazonium chloride

Diazotization reaction

Diazotization reaction is used to prepare diazonium salt. In diazotization reaction, aniline is dissolved in dilute HX with an aqueous solution of sodium nitrite at 0 to 5°C.

Sandmeyer's reaction

Aryl chlorides and aryl bromides are prepared by Sandmeyer's reaction. In Sandmeyer's reaction, diazonium salt reacts with copper halides dissolved in HX to form haloarenes.

Preparation of iodoarenes

Iodoarenes are prepared by warming diazonium salt in aquous KI. Copper halides are not needed in this reaction.

Gattermann's reaction

Haloarenes (Chloro and bromoarenes) can alternatively be prepared by gattermann's reaction. Gattermann's reaction is a modification of Sandmeyer's reaction.

In Gattermann's reaction, instead of cuprous halide (CuCl or CuBr), copper powder is used.