Oxidation of Amines to Nitroso Compounds
Oxidation of Amines to N-Nitrosamines: Mechanism
In the nitrosation of amines with nitrous acid, nitrosyl cation is the electrophilic reagent. The cation is generated by protonation of nitric acid with a strong acid (e.g. HCl).
Subsequently, the actual nitrosation process takes place.
The amine attacks the nitrosyl cation at the nitrogen atom. The resulting ammonium ion is stabilized by deprotonation, explaining the fact that the reaction does not proceed with tertiary amines. In this case, the intermediate cannot be stabilized by losing a proton.
However, primary amines are able to form nitrosamines which subsequently undergo tautomerization followed by protonation at the hydroxyl group and concomitant loss of water to form a diazonium ion.
Diazonium ions by themselves are labile and react further.