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Lassaigne's test – detecting nitrogen, sulfur and halogens in organic compounds

ChemistryOrganic ChemistryAges 16–17Virtual lab

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Use with my class

Virtual lab for the sodium fusion (Lassaigne's) test. Fuse an organic compound with sodium in an ignition tube, plunge the red-hot tube into water, boil and filter to get the extract, then test portions of it: iron(II) sulfate and iron(III) chloride give Prussian blue with nitrogen; sodium nitroprusside (violet) and lead(II) ethanoate (black PbS) show sulfur; iron(III) chloride alone gives blood-red Fe(SCN)²⁺ when nitrogen and sulfur are both present; after boiling with nitric acid, silver nitrate gives white AgCl, pale yellow AgBr or yellow AgI, told apart with ammonia. Identify the elements in four unknowns from the observation table.

Lesson: Qualitative analysis of organic compounds: detecting nitrogen, sulfur and halogens (Lassaigne's test)

What it shows

Carbon and hydrogen in organic compounds are found by combustion, but nitrogen, sulfur and halogens are covalently bonded and give no ionic tests. Fusing the compound with sodium turns them into ions: cyanide, sulfide, thiocyanate and halide. Cyanide forms Prussian blue with iron salts, sulfide gives a violet colour with nitroprusside and black lead(II) sulfide, and thiocyanate gives blood-red Fe(SCN)²⁺. Halides are precipitated by silver nitrate only after boiling with nitric acid, because cyanide and sulfide would precipitate too. With too little sodium, nitrogen and sulfur stay together as thiocyanate.

How to use

Press Prepare extract to fuse the compound and filter the extract, then choose a Test and press Do test. Tap a tube in the rack or use the Tube buttons to switch between unknowns. Set Sodium to Small piece to see how thiourea behaves, or untick the nitric acid box before a halogen test. Enter your conclusions and press Check.

Parameters you can change

  • Mode Four unknowns A–D (random), Known compound (practice)
  • Compound (known mode) glucose, urea, N-phenylethanamide (acetanilide), thiourea, 4-aminobenzenesulfonic acid (sulfanilic acid), thiophene, chlorobenzene, bromobenzene, triiodomethane (iodoform), 4-chloroaniline, 4-bromoacetanilide
  • Amount of sodium Excess sodium, Small piece of sodium
  • Boil the halogen portion with conc. HNO₃ first
  • Show inferences (unknown mode)

Questions to explore

  1. Why must the extract be boiled with nitric acid before silver nitrate is added?
  2. Why can a compound containing both nitrogen and sulfur give a blood-red colour instead of Prussian blue?
  3. How does dilute ammonia tell silver chloride apart from silver iodide?

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