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.
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.
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.
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.