Simple test tube tests can identify the functional group in an organic compound. They form AQA Required Practical 6.Alkene (C=C): bromine water
Shake the sample with bromine water. An alkene turns the orange solution colourless.Primary or secondary alcohol (and aldehyde): acidified potassium dichromate(VI)
Warm with acidified potassium dichromate(VI) in a water bath. The solution turns from orange to green. Tertiary alcohols and ketones give no change.Aldehyde (-CHO): Tollens' reagent or Fehling's solution
Warm with Tollens' reagent: a silver mirror forms. Or warm with Fehling's solution: the blue solution gives a brick-red precipitate. Ketones give no change.Carboxylic acid (-COOH): sodium carbonate or sodium hydrogencarbonate
Add solid sodium carbonate or sodium hydrogencarbonate. Effervescence is seen, and the gas turns limewater cloudy (carbon dioxide):
2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O Warm using a water bath, never a naked flame, because many organic compounds are flammable.Worked example: three unlabelled liquids are propan-1-ol, propanal and propanoic acid. Plan a sequence of tests.
Add sodium hydrogencarbonate: only propanoic acid fizzes.
Warm the other two with Tollens' reagent: propanal gives a silver mirror.
The remaining liquid turns acidified dichromate(VI) green but gives no silver mirror, so it is propan-1-ol.Note that acidified dichromate(VI) alone cannot tell a primary alcohol from an aldehyde, because both are oxidised.
Shake the sample with bromine water. An alkene turns the orange solution colourless.Primary or secondary alcohol (and aldehyde): acidified potassium dichromate(VI)
Warm with acidified potassium dichromate(VI) in a water bath. The solution turns from orange to green. Tertiary alcohols and ketones give no change.Aldehyde (-CHO): Tollens' reagent or Fehling's solution
Warm with Tollens' reagent: a silver mirror forms. Or warm with Fehling's solution: the blue solution gives a brick-red precipitate. Ketones give no change.Carboxylic acid (-COOH): sodium carbonate or sodium hydrogencarbonate
Add solid sodium carbonate or sodium hydrogencarbonate. Effervescence is seen, and the gas turns limewater cloudy (carbon dioxide):
Add sodium hydrogencarbonate: only propanoic acid fizzes.
Warm the other two with Tollens' reagent: propanal gives a silver mirror.
The remaining liquid turns acidified dichromate(VI) green but gives no silver mirror, so it is propan-1-ol.Note that acidified dichromate(VI) alone cannot tell a primary alcohol from an aldehyde, because both are oxidised.