Q.How do primary, secondary and tertiary aliphatic amines differ in their behaviour towards nitrous acid ?

Nitrous acid (generated in situ from NaNO2 + HCl/dil. acid) reacts differently with primary, secondary, and tertiary amines, and this behaviour is used to distinguish between the three classes.

Primary aliphatic amines (R-NH2): react with nitrous acid at low temperature (0-5 degrees C) to form a highly unstable aliphatic diazonium salt, which decomposes immediately at that temperature with brisk evolution of nitrogen gas, giving mainly an alcohol (along with some alkene and alkyl halide as side products via carbocation rearrangement).

R-NH2 + HNO2 -> R-OH + N2(up) + H2O

(The vigorous, quantitative evolution of N2 gas at room temperature is used as a positive test for primary amines - this is distinct from aromatic primary amines like aniline, whose diazonium salt, C6H5N2+Cl-, is comparatively more stable at 0-5 degrees C due to resonance stabilisation by the ring.)

Secondary amines (R2NH, aliphatic or aromatic): react with nitrous acid to form a yellow, oily N-nitrosamine (N-nitroso compound), with no gas evolved.

R2NH + HNO2 -> R2N-N=O + H2O

Tertiary amines: …