HNO2 (generated in situ: NaNO2 + HCl) reacts differently with each amine class — the most important distinction reaction. 1° aliphatic (RNH2 + HNO2): Unstable diazonium RN2+ decomposes instantly -> ROH + alkenes + N2 (bubbles). 1° aromatic (ArNH2 + HNO2, 0-5°C): Stable ArN2+Cl- (arenediazonium salt) — clear solution, extremely versatile. 2° amine (R2NH + HNO2): N-nitrosoamine R2N-N=O (yellow oily liquid, insoluble). Both aliphatic and aromatic 2° amines give this. 3° aliphatic (R3N + HNO2): Only forms salt R3NH+NO2- (nitrite salt, no visible change). 3° aromatic (Ar-NR2 + HNO2): C-nitrosation — HNO2 acts as electrophile on the activated ring -> p-nitroso compound $\frac{green}{blue colour}$.