Aldehyde or Ketone

• The main reagents that can be used to distinguish between an aldehyde or ketone are: Fehling's solution, Benedict's solution and Tollens' reagent.
• Fehling's solution and Benedict's solution:

  They work in the exact same way.

  1. Add 2cm3 of the Fehling's / Benedict's solution (a clear blue solution) to a test tube.
  2. Add 5 drops of the aldehyde or ketone to the test tube.
  3. Put the test tube in a hot water bath to warm it for a few minutes.
  4. Observe the colour change.
• Results (Fehling's / Benedict's):

  • Aldehyde - blue solution forms a brick red precipitate.
  • Ketone - no change.
• Tollens' reagent:

  1. Put 2cm3 of 0.10 mol dm-3 silver nitrate solution in a test tube.
  2. Add a few drops of dilute sodium hydroxide solution. A light brown precipitate should form.
  3. Add drops of dilute ammonia solution until the brown precipitate dissolved completely.
  4. Place the test tube in a hot water bath and add 10 drops of aldehyde or ketone and wait for a few minutes.
• Results (Tollens'):

  • Aldehyde - a silver mirror forms on the walls of the test tube.
  • Ketone - no change.