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Narrative
Aniline, a primary amine, N-methyl aniline, a secondary amine, and N,N-dimethyl aniline, a tertiary amine, are placed in separate test tubes. Each amine is dissolved in 6M HCl and cooled in an ice bath. A solution of 25% aqueous sodium nitrite is added to each tube. The solution is tested with starch iodide paper. Excess nitrous acid causes the white starch iodide paper to turn black. Aniline reacts with nitrous acid to form a diazonium ion. Secondary amines react to form N-nitroso compounds. Tertiary amines undergo reaction by ring nitrosation forming colored compounds. The aniline mixtures are removed from the ice bath. When the diazonium salt of aniline is warmed to room temperature it decomposes to release bubbles of nitrogen.

Discussion
The nitrous acid test is used to classify amines as primary (-NH₂), secondary (-NHR), or tertiary (-NR₂).

Primary aromatic amines react with nitrous acid to form diazonium salts which are stable at 0°C but decompose to nitrogen gas at room temperature. Secondary amines form N-nitroso compounds which are insoluble in the reaction mixture. Tertiary amines form products arising from electrophilic substitution of the nitroso group on the aromatic ring.