See image — Practical Organic Chemistry and Purification Chemistry Question

💡 Solution & Explanation

Concept: Lassaigne's test (sodium fusion test) for nitrogen works by fusing the organic compound with sodium metal, converting nitrogen into sodium cyanide (NaCN). The NaCN is then detected by forming Prussian blue (ferric ferrocyanide) upon reaction with FeSO4 and FeCl3. For this test to work, the compound must contain carbon AND nitrogen together, so that CN- ion can be formed during fusion. If a compound contains nitrogen but no carbon, no cyanide ion is formed, and the test fails. Reasoning: - Urea: (NH2)2CO — contains both carbon and nitrogen → forms NaCN on fusion → gives positive Lassaigne's test. - Hydrazine: N2H4 — contains nitrogen but NO carbon → cannot form NaCN during sodium fusion → does NOT give a positive Lassaigne's test for nitrogen. - Azobenzene: C6H5-N=N-C6H5 — contains both carbon and nitrogen → forms NaCN on fusion → gives positive Lassaigne's test. - Phenyl hydrazine: C6H5-NH-NH2 — contains both carbon and nitrogen → forms NaCN on fusion → gives positive Lassaigne's test. Why other options fail: Urea, azobenzene, and phenyl hydrazine all contain carbon along with nitrogen, enabling formation of CN- during sodium fusion, thus giving a positive result. Hydrazine (N2H4) is an inorganic/simple compound with no carbon, so no cyanide is formed and the test is negative. Therefore, the correct answer is B.