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.
Note : Ortho and para isomers can be easily separated due to large difference in their melting points.
Note : Fluoro compounds are not prepared by this method because the reaction is highly exothermic and causes explosion.
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.
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.