Amines - Reaction with nitrous acid
Nitrous acid (HNO2) is an unstable acid.
It must be freshly made.
Primary amines react rapidly with nitrus acid to form alcohol and evolved nitrogen.
Primary aromatic amines react with nitrous acid in the cold (below 273 K) to form diazonium salts. The process is called diazotisation.
If the temperature is more than 278 K, aromatic amines form phenol with the evolution of N2 gas.
Secondary aliphatic and aromatic amines react with nitrous acid slowly in the cold to form yellow oily nitroso amines.
The yellow only nitrosoamine gives a green solution when warmed with ohenol and conc. H2SO4. On dilution with water and addition sodium hydroxide, the colour changes to greensih blue to violet (Libermann's nitroso reaction. a test for secondary amines)
Tertiary amines dissolve in cold nitrous acid to form salts which decompose on warming to nitrosoamine and alcohol.
Aromatic tertiary amines react with nitrous acid to give coloured sustituted nitroso compound.