Reactions of amines : Reaction with nitrous acid

Nitrous acid with the chemical formula HNO₂ is unstable. Usually it is produced indirectly in a mixture of NaNO₂ and a strong acid such as HCl or H₂SO₄ in dilute concentration, so that the H⁺ ions will associate with the NO₂^– ions in solution.

Primary aliphatic amines with nitrous acid give very unstable diazonium salts which spontaneously decompose by losing N₂ to form a carbenium ion. The carbenium ion goes on to produce a mixture of alkenes, alkanols or alkyl halides, with alkanols as the major product. This reaction is of little synthetic importance because the diazonium salt formed is too unstable, even under quite cold conditions.

NaNO₂ + HCl → HNO₂ + NaCl

• Primary aromatic amines, such as aniline (phenylamine) forms a more stable diazonium ion at 0–5°C. Above 5°C, it will decompose to give phenol and N₂. Diazonium salts can be isolated in the crystalline form but are usually used in solution and immediately after preparation, due to rapid decomposition on standing even with little ambient heat. Solid diazonium salts can be explosive on shock or on mild warming.