Chemistry · Ch 9 — Amines
Reaction with Nitrous Acid
Reaction with Nitrous Acid
Reaction with Nitrous Acid
Nitrous acid () is too unstable to be stored, so it is always generated in situ, right inside the reaction mixture, by treating sodium nitrite with a mineral acid such as hydrochloric acid:
What happens next depends entirely on which class of amine is present — primary, secondary, or tertiary — and, for primary amines, on whether the amine is aliphatic or aromatic. This makes the nitrous-acid reaction one of the most reliable ways to characterise and distinguish amines, and it is also the doorway into diazonium chemistry (taken up in the next section).
Primary Aliphatic Amines
A primary aliphatic amine reacts with nitrous acid to form an alkyl diazonium salt. This intermediate is extremely unstable even at room temperature and decomposes essentially the moment it forms, releasing nitrogen gas quantitatively:
The alkyl diazonium salt is hydrolysed by water almost as fast as it forms, giving a primary alcohol as the chief product, together with a brisk, quantitative evolution of nitrogen gas. In practice the alcohol is often accompanied by an alkene and an alkyl halide as minor side products (via competing elimination and substitution of the highly reactive diazonium leaving group), so the reaction is not a clean way to make a specific alcohol. Its real value is the quantitative evolution of : because exactly one mole of nitrogen gas is released per mole of primary amino group, this reaction is used to estimate the number of free amino groups in a sample — for instance in the analysis of amino acids and proteins.
Primary Aromatic Amines
A primary aromatic amine, such as aniline, also reacts with nitrous acid to give a diazonium salt, but with one crucial difference: the arenediazonium salt is comparatively stable, provided the temperature is kept low (273–278 K, i.e., ice-cold conditions).
This controlled, low-temperature formation of a stable-for-a-while arenediazonium salt from a primary aromatic amine is called diazotisation. Benzenediazonium chloride, the product shown above, does not survive being warmed to room temperature — but at ice-bath temperature it is stable enough to be used directly, without isolation, as the starting material for the whole family of diazonium-salt reactions (dyes, nitriles, phenols, azo-coupling, and more) that follow in the next section.
The nitrous-acid outcome depends entirely on the class and type of amine — the single most tested distinction in this chapter.
| Amine class | Product with | Key observation |
|---|---|---|
| Primary aliphatic () | Alkyl diazonium salt | Unstable even at room temperature; decomposes at once, releasing gas quantitatively, alongside alcohol/alkene/alkyl halide |
| Primary aromatic () | Arenediazonium salt | Stable only at low temperature (273–278 K); the basis of diazotisation and all downstream diazonium chemistry |
| Secondary (aliphatic or aromatic) | N-Nitrosamine | Yellow, oily liquid; no nitrogen gas evolved |
| Tertiary aliphatic | Unstable N-nitroso ammonium salt | No N–H is available to lose, so only a simple (unstable) salt forms |
| Tertiary aromatic | Ring-nitrosated product (predominantly para) | The activated benzene ring itself is attacked; the nitroso group enters mainly at the para position |
Secondary and Tertiary Amines
Secondary and tertiary amines react with nitrous acid quite differently from primary amines, and neither pathway evolves nitrogen gas.
A secondary amine, whether aliphatic or aromatic, has one N–H hydrogen left on nitrogen. The nitrogen attacks the nitrosating species and that remaining hydrogen is lost, giving an N-nitrosamine — a yellow, oily liquid:
A tertiary amine has no N–H hydrogen at all, so it cannot be nitrosated at nitrogen the way primary and secondary amines are. What happens next depends on whether the tertiary amine is aliphatic or aromatic: …