3.3.6.1 Identification of functional groups

• Fehling’s solution (Benedict’s solution) is used to test for aldehydes.

  • add Fehling’s solution to sample and heat in a hot water bath for a few minutes
  • blue solution produces a orange-red precipitate if aldehyde is present.
• Tollen’s reagent is used to test for aldehydes and ketones.

  1. add silver nitrate solution to test tube
  2. add a few drops of dilute sodium hydroxide solution
  3. add drops of dilute ammonia solution
  4. place test tube in a hot water bath and add sample of ketone or aldehyde
  5. if a aldehyde is present a silver mirror forms on walls of test tube.
• Test to identify alcohols

  • add acidified potassium dichromate. 
  • If the sample is an alcohol then it can be oxidised and the colour will change from orange to green. 
• Test to identify carboxylic acids

  • add sodium hydrogen carbonate solution
  • If a carboxylic acid is present, there will be fizzing (effervescence) as carbon dioxide is given off
• Test to identify haloalkanes (method)

  • Add sodium hydroxide solution
  • Warm the contents of the test tube using a water bath
  • Acidify the contents of the test tube by adding dilute nitric acid and then add silver nitrate solution.
• Test to identify haloalkanes (results)

  • Chloroalkanes form a white precipitate of AgCl
  • Bromoalkanes form a cream precipitate of AgBr
  • Iodoalkanes form a yellow precipitate of AgI

  However the colours can look very similar and they can be hard to tell apart. A further test can be used. 

  • Adding dilute ammonia will dissolve the AgCl precipitate.
  • Dilute ammonia will not dissolve the AgBr precipitate, however concentrated ammonia will. 
  • The AgI precipitate will not dissolve in dilute or concentrated ammonia. 
• Test to identify alkenes

  • add bromine water
  • if alkene is present, it decolourises