Aromatic Diazonium Ions as Synthetic Intermediates

Aryl diazonium ions are usually prepared by reaction of an aniline with nitrous acid, which is generated in situ from a nitrite salt. Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Section 10.1), aryl diazonium ions are stable enough to exist in solution at room temperature and below. They can also be isolated as salts with non-nucleophilic anions, such as tetrafluoroborate. The steps in forming a diazonium ion involve 

Zollinger, Azo and Diazo Chemistry, Wiley-Interscience, New York, 1961 S. Patai (ed.). The Chemistry of Diazonium and Diazo Groups, Wiley, New York, 1978, Chapters 8, 11, and 14 H. Saunders and R. L. M. Allen, Aromatic Diazo Compounds, Third Edition, Edward Arnold, London, 1985. 

In alkaline solution, diazonium ions are converted to diazoate anions, which are in equilibrium with diazooxides.  

In addition to the classical techniques for diazotization in aqueous solution, diazonium ions can be generated in organic solvents by reaction with alkyl nitrites. 

The great usefulness of aryl diazonium ions as synthetic intermediates results from the excellence of N2 as a leaving group. There are at least three general mechanisms by which substitution can occur. One involves unimolecular decomposition of the diazonium ion, followed by capture of the resulting aryl cation by a nucleophile. The phenyl cation is very unstable (see Part A, Section 5.4) and therefore highly unselective. Either the solvent or an anion can act as the nucleophile. 

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