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Tests for organic functional groups – identify four unknowns

ChemistryOrganic ChemistryAges 16–17Virtual lab

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Use with my class

Virtual lab with 13 test-tube tests for organic functional groups: bromine water and Baeyer's reagent (cold, dilute alkaline KMnO₄) for C=C; sodium, ceric ammonium nitrate, neutral iron(III) chloride and acidified K₂Cr₂O₇ for alcohols and phenols; 2,4-DNP, Tollens' reagent, Fehling's solution and the iodoform test for aldehydes and ketones; NaHCO₃ for carboxylic acids; the carbylamine test and the azo dye test for primary amines. Tubes A–D hold four unknowns from four different classes: every test adds a row to the experiment–observation–inference table and a cell to the results grid, and students identify the functional group in each tube and name the compound in the starred tube.

Lesson: Practical: tests for organic functional groups

What it shows

A functional group gives an organic compound its typical reactions, so simple test-tube tests can identify it. Bromine water is decolourised at once by C=C. Sodium releases hydrogen from –OH groups, ceric ammonium nitrate turns red with alcohols and neutral iron(III) chloride gives a violet colour with phenols. Acidified dichromate oxidises primary and secondary alcohols and aldehydes and turns green. 2,4-DNP shows a C=O group, and Tollens' and Fehling's reagents tell aldehydes from ketones. NaHCO₃ fizzes only with carboxylic acids, and the carbylamine and azo dye tests detect primary amines.

How to use

In Mode choose Four unknowns A–D or Known compound. Pick a tube with the Tube buttons or by tapping the rack, choose a tab and a Test, and press Do test: the bench shows the result, a row is added to the observation table and a cell to the grid. When you have enough evidence, choose the class of each tube and the compound in the starred tube, and press Check. New set gives four new unknowns.

Parameters you can change

  • Mode Four unknowns A–D (random), Known compound (practice)
  • Compound (known mode) hexane, cyclohexene, ethanol, propan-1-ol, propan-2-ol, 2-methylpropan-2-ol, phenol, ethanal (acetaldehyde), propanal, benzaldehyde, propanone (acetone), pentan-3-one, ethanoic acid (acetic acid), butan-1-amine, phenylamine (aniline)
  • Show the inference column (unknown mode)

Questions to explore

  1. Why does phenol react with sodium but not with sodium hydrogencarbonate?
  2. Which two tests together tell an aldehyde from a ketone?
  3. Why does propanone give the iodoform test while pentan-3-one does not?

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