The reaction with Nitrous Acid ($HN{O}_2$) is used to distinguish between classes of amines:

1. Aliphatic $1^{c}irc$ Amines: React to form an extremely unstable diazonium salt that decomposes even at $0^{\circ }C$. $R-N{H}_2+HN{O}_2\to [R-N_2^{+}C{l}^{-}]\stackrel{H_{2}O}{\to }R-OH+N_2\uparrow +HCl$

• Observation: Rapid effervescence of Nitrogen gas and formation of an alcohol.

2. Aromatic $1^{c}irc$ Amines: Form stable diazonium salts at $0-5^{\circ }C$ with NO gas evolution.
3. Secondary Amines: Form yellow oily N-nitrosoamines.