Summary

• Typical reagents : sodium nitrite and aq. HCl or H₂SO₄ (this mixture yields nitrous acid, HNO₂)
• The most useful reactions are probably those of primary aryl amines, Ar-NH₂, which give aryl diazonium salts, Ar-N₂⁺ which can then be used to prepare substituted benzenes (see below)
• The actual nitrosation reagent is the nitrosyl cation, NO⁺  which is formed in situ:

• The nature of the product depends on the nature of the initial amine
• Primary alkyl or aryl amines yield diazonium salts (hence the diazotisation reaction)

• Alkyl diazonium salts are very unstable and yield carbocation-derived products by loss of the very good leaving group, N₂:

 
• Secondary alkyl or aryl amines yield N-nitrosoamines:

• Tertiary alkyl amines do not react in a useful fashion.
• Tertiary aryl amines undergo nitrosation of the ring (an electrophilic aromatic substitution reaction)

Synthetically Useful Transformations Involving Aryl Diazonium Ions

• Aryl diazonium salts can be converted into a range of systems as shown above.
• These reactions can be useful as the methodology can be complementary to other methods.
• Of particular note are the methods for introducing -F using HBF₄ (the Schiemann reaction) and -OH using H₂O.
• The reactions of copper salts (CuCl, CuBr and CuCN) are known as the Sandmeyer reactions.
• Coupling with phenols gives azo-compounds which are important as yellow-red dyes or pigments.
• Aryl diazonium salts are prepared from aryl amines (above) which in turn can be obtained by the reduction of nitrobenzene