Aryl diazonium salts are prepared by reacting an aromatic primary amine with nitrous acid at a temperature between 0-5 oC.
Diazonium salts are very useful intermediates in the synthesis of a number of aromatic compounds. Some of them are as follows.
1. Replacement of Hydrogen:
Synthesis of 1,3,5,-tribromobenzene
2. Replacement by halogen atom:
   Synthesis of chlorobenzene
3. Replacement by Cyano group:
   Synthesis of p-toluinitrile
4. Replacement by hydroxy group:
   Synthesis of o-cresol
