Required practical · A-level chemistry
RP 6: Test-tube identification of organic groups
Hazards
⚠Tollens' reagent — never store; explosive silver fulminate forms on standing. Make fresh, rinse away immediately
⚠Bromine water — toxic/corrosive; use dilute, in a fume cupboard if available
⚠2,4-DNPH — flammable when dry; use as supplied solution
Method
- Alkene: bromine waterShake a few drops of sample vigorously with ~1 cm³ bromine water. Orange → colourless = C=C present.
- Carbonyl: 2,4-DNPHAdd 3–4 drops of sample to 3–4 drops of 2,4-DNPH (Brady's reagent) at room temperature. Orange/yellow precipitate = aldehyde or ketone.
- Aldehyde vs ketone: Tollens' or Fehling'sMake Tollens' fresh (AgNO₃ + 1 drop NaOH, then just enough dilute NH₃ to redissolve the brown precipitate). Warm at 60–70 °C in a water bath — silver mirror (or grey/black deposit) = aldehyde. Alternatively Fehling's: mix equal volumes of A and B (clear deep blue), warm with the sample — brick-red precipitate of Cu₂O = aldehyde.
- Alcohol: sodium, then dichromateAQA's exemplar adds a rice-grain piece of sodium to ~1 cm³ of the dry liquid — effervescence (H₂) indicates the –OH group; destroy leftover sodium in ethanol. For 1° vs 2° vs 3°: warm with acidified potassium dichromate(VI) — orange → green means oxidisable (1°/2° alcohol or aldehyde); tertiary alcohols stay orange.
- Carboxylic acid: NaHCO₃Add solid NaHCO₃ or sodium carbonate solution — effervescence; the gas turns limewater milky.
- Haloalkane: the AgNO₃ ladderWarm with NaOH(aq) at ~60 °C, acidify with dilute nitric acid, then add silver nitrate solution — white ppt = chloro, cream = bromo, yellow = iodo.
Mistakes examiners see every year
| The mistake | Why it costs marks |
|---|---|
| Warming Tollens' over a flame or storing it | Water bath only, and rinse residues away at once — dried silver residues are explosive. |
| Calling a ketone negative 'no reaction observed' | State the observation: 'orange precipitate with 2,4-DNPH, no silver mirror with Tollens' → ketone'. Examiners want observations, not conclusions alone. |
4 more examiner traps for this practical
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Open it in ChemLabWhat to expect
Full result table: alkene decolourises Br₂ water; aldehyde: DNPH ppt + silver mirror + Fehling's brick-red + dichromate green; ketone: DNPH ppt only; 1°/2° alcohol: fizzes with Na, dichromate green; 3° alcohol: fizzes with Na, dichromate stays orange; carboxylic acid: fizzes with NaHCO₃. Bonus: the iodoform test (I₂/KI + NaOH, warm gently) gives a yellow CHI₃ precipitate for CH₃C=O compounds and CH₃CH(OH)– alcohols — it distinguishes ethanol (positive) from methanol (negative). Disposal: Tollens' mixtures go down the foul-water drain immediately after use (RSC) — stored residues form explosive silver compounds.