See image — Practical Organic Chemistry and Purification Chemistry Question

💡 Solution & Explanation

Lassaigne's test for nitrogen involves fusing the organic compound with sodium metal to convert covalently bound nitrogen into sodium cyanide (NaCN). The NaCN is then detected by converting it to Prussian blue (ferric ferrocyanide). For this test to work, the compound must contain nitrogen in a form that can react with sodium during fusion to produce CN- ions. Concept: The Lassaigne's test fails when nitrogen is present as a salt (i.e., in ionic form, such as -NH3+ or as a hydrochloride salt of a nitrogen base) because the nitrogen in ionic form (ammonium-type N-H bonds in a salt) does not fuse effectively with sodium to form NaCN. More specifically, when a compound is a salt of hydrazine (like hydrazine hydrochloride, NH2NH2.HCl), the sodium metal first reacts with the HCl (the acidic proton of the salt) to form NaCl and H2, consuming the sodium before it can react with the nitrogen to form NaCN. As a result, no cyanide ion is produced and the test fails. Reasoning for each option: (a) NH2CONH2 (urea): Contains nitrogen in covalent form. On fusion with sodium, it produces NaCN. Test succeeds. (b) NH2CONHNH2.HCl (semicarbazide hydrochloride): Although it is a hydrochloride salt, the compound still has sufficient nitrogen in covalent linkages (amide-type) that upon fusion, NaCN is formed. Test succeeds. (c) NH2NH2.HCl (hydrazine hydrochloride): This is a simple salt where sodium reacts preferentially with HCl to give NaCl + H2, leaving no effective sodium to convert the nitrogen into NaCN. Additionally, hydrazine itself does not have a carbon atom bonded to nitrogen (no C-N bond in the organic sense), so CN- cannot be formed. The compound has no carbon, so formation of NaCN is impossible. Test fails. (d) C6H5NHNH2.2HCl (phenylhydrazine dihydrochloride): Although it is a dihydrochloride salt, the phenyl ring provides carbon atoms bonded to nitrogen, and upon sodium fusion, the sodium neutralizes the HCl but sufficient C-N bond fusion can still produce NaCN. Test succeeds. The critical point for (c): NH2NH2.HCl has no carbon atom, so NaCN cannot possibly be formed regardless of the fusion conditions. Without carbon, no cyanide is produced, and the Prussian blue test gives a negative result. Therefore, the correct answer is C.