How to Systematically Identify Elements in an Unknown Organic Compound
Step 1 — Lassaigne's Fusion Start by fusing the unknown compound with sodium metal. The fusion converts all non-metallic elements to ionic forms. Dissolve fusion mass in distilled water.
Step 2 — Test for Nitrogen (First) Add FeSO4 + NaOH to the extract, boil, cool, acidify with H2SO4.
- Prussian blue → N present (and S absent)
- No blue color → N absent OR both N and S present (proceed to Step 3 to clarify)
Step 3 — Test for Sulfur Add sodium nitroprusside to the extract.
- Purple/violet → S present as Na2S (so N was absent in Step 2)
- No color → S absent
Cross-check Step 2 and 3:
- Step 2: no blue + Step 3: purple → S present, N absent (Na2S formed)
- Step 2: no blue + Step 3: no color → S absent, N possibly absent OR
- Step 2: no blue + Step 3: no color → could be N + S → NaSCN (test with FeCl3 for blood-red to confirm)
- Step 2: blue + Step 3: no purple → N present, S absent
Step 4 — Test for Halogens (if N or S present: FIRST boil with HNO3) Add AgNO3 to acidified extract.
- White ppt, soluble in dilute NH3 → Cl present (AgCl)
- Pale yellow ppt, partially soluble in conc. NH3 → Br present (AgBr)
- Yellow ppt, insoluble in NH3 → I present (AgI)
- No ppt → halogen absent (or F present, but AgF is soluble)
Step 5 — Functional Group Tests Based on known functional groups in organic chemistry, apply: 2,4-DNP (C=O), Tollens' (aldehyde), Fehling's (aliphatic aldehyde), I2/NaOH (methyl ketone), FeCl3 (phenol), NaHCO3 (carboxylic acid), carbylamine (primary amine).
Step 6 — Synthesis of Conclusion Combine elemental information with functional group results to narrow down or identify the compound.