Reaction with nitrous acid

Primary, secondary and tertiary amines each react differently with nitrous acid (HNO2), but ONLY the reaction of PRIMARY amines is examined in detail in this section. Because nitrous acid is itself an unstable compound that cannot simply be taken off a laboratory shelf, it is instead always prepared freshly, in situ, immediately before use, by adding aqueous sodium nitrite to hydrochloric acid that has already been mixed together with the amine substrate itself. (a) ALIPHATIC primary amines, reacting with nitrous acid, form an ALIPHATIC diazonium salt -- but this aliphatic diazonium salt is an extremely unstable intermediate, and it decomposes essentially immediately, reacting further with the surrounding solvent water, to give the corresponding alcohol as the actual isolated product, with nitrogen gas liberated as the accompanying byproduct. (b) AROMATIC primary amines, by contrast, reacting with nitrous acid, form an AROMATIC diazonium salt that has genuinely reasonable stability, provided the reaction is kept at a low temperature, specifically 273-278 K. These aryl diazonium salts are resonance-stabilised (the positive charge on the terminal diazonium nitrogen delocalised in part into the aromatic ring) and, because of this real stability, …