See image — Practical Organic Chemistry and Purification Chemistry Question

💡 Solution & Explanation

Lassaigne's test detects nitrogen (and sulfur) in organic compounds by fusing the compound with sodium metal to convert covalently bonded elements into ionic forms in the sodium fusion extract (NFE). When nitrogen is present, it forms sodium cyanide (NaCN) in the extract. When both nitrogen AND sulfur are present together in the same compound, the fusion produces sodium thiocyanate (NaSCN) instead of, or in addition to, NaCN. The test for nitrogen in the presence of sulfur involves adding FeSO4 and acidifying with H2SO4. The thiocyanate ions react with Fe3+ to give a blood-red color, but the Prussian blue test specifically involves a compound containing both N and S together forming Na4[Fe(CN)5NOS] (sodium nitroprusside-related complex). More precisely, when both N and S are present, the sodium fusion gives NaCNS (sodium thiocyanate), and upon reaction with freshly prepared FeSO4 followed by acidification, the complex formed is Na4[Fe(CN)5NOS], which is responsible for the Prussian blue coloration observed when both elements are present. The other options fail because: (a) Fe4[Fe(CN)6]3 (not Fe[Fe(CN)6]3) is Prussian blue formed when only nitrogen is present (no sulfur), (b) Na3[Fe(CN)6] is sodium ferricyanide, not the product of Lassaigne's test, (c) Fe(CN)3 is not a recognized product in this context. The question specifically asks about the compound responsible for Prussian blue when the Lassaigne's test detects nitrogen (with sulfur also present), giving Na4[Fe(CN)5NOS].