Diazonium salt formation and coupling reaction: When a reaction mixture of phenyl amine and nitrous acid is kept below 10^(@)C, a diazonium salt is formed. This reaction is called diazotization reaction. The diazonium ion, -N_(2)^(+), is rather unstable and decomposes readily to nitrogen. However, delocalization of the diazonium from pi-bond electron over a benzene ring phenyl diazonium sufficiently for it to form at low temperature. The phenyl diazonium ion behaves as an electrophile, and will attack another arene molecule such as phenol. Electrophilic subsitiution takes at the 4 position, producing 4-hydroxy phenyl azobenzene. The reaction is known as coupling reaction. The compound formed is an energetically stable, yellow azo dye (the azo group is -N=N-) The stability is due to extensive delocalisation of electrons via the nitrogen- nitrogen double bonds. The product which is a red azodye obtained on reacting benzene diazonium chloride with one of the following compounds :