1. Reaction with Nitrous Acid (HNO₂)

This is the most important reaction for distinguishing amines.

Amine	Reaction	Observation
1° Aliphatic Amine (RNH₂)	RNH₂ + HNO₂ → ROH + N₂↑ + H₂O	Brisk effervescence (N₂ gas)
1° Aromatic Amine (ArNH₂)	Forms diazonium salt (0–5°C)	Stable only at low temperature
2° Amine (R₂NH)	Forms N-nitrosoamine	Yellow oily liquid
3° Aliphatic Amine (R₃N)	Forms soluble ammonium nitrite	No gas
3° Aromatic Amine	Forms p-nitroso derivative	Green colour (often)

Examples

1° Aliphatic$$CH_3NH_2 + HNO_2 \rightarrow CH_3OH + N_2\uparrow + H_2O$$

1° Aromatic$$C_6H_5NH_2 + HNO_2 + HCl \rightarrow C_6H_5N_2^+Cl^- + 2H_2O$$

2° Amine$$(CH_3)_2NH + HNO_2 \rightarrow (CH_3)_2N-N=O + H_2O$$

1. 3° Aliphatic Amine (R₃N)

Example: Trimethylamine, (CH₃)₃N

Reaction:$$R_3N + HNO_2 \rightarrow R_3NH^+NO_2^-$$

• Forms a trialkylammonium nitrite (salt).
• No nitrogen gas (N₂) is evolved.
• No N-nitrosoamine is formed.

Example:$$(CH_3)_3N + HNO_2 \rightarrow (CH_3)_3NH^+NO_2^-$$