CH4 - Qualitative Test for Carbonyl Compounds

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  • Qualitative Tests For Carbonyl Compounds
    • 2,4 - DNPH
      • 1. Add a few drops of 2,4-DNPH to the solution under test.
      • 2. A red/orange precipitate will form for a positive result.
        • This indicates that the solution under test is a carbonyl compound.
          • An aldehyde OR a ketone.
      • 3. The precipitate formed will have a well defined melting point.
        • The melting point can be measured and compared against known values to help with determining the name of the compound.
    • Tollen's Reagent
      • 1. Place 1cm3 of silver nitrate solution into a test tube.
      • 2. Now add 1 drop of sodium hydroxide solution.
        • A brown precipitate will form.
      • 3. Add drops of ammonium hydroxide solution until the brown precipitate just dissolves.
      • 4. Add a few drops of the solution under test to the freshly prepared Tollen's reagent.
      • 5. Place this into a water bath.
      • 6. A 'silver mirror' forming will indicate the presence of an aldehyde.
        • No 'silver mirror' indicates that the carbonyl compound is a ketone.
    • Fehling's / Benedict's Solution
      • 1. Place 2cm3 of Fehling's / Benedict's solution into a test tube.
      • 2. To this, add a few drops of the solution being tested.
      • 3. Place the test tube into a water bath.
      • 4. If a red precipitate forms this indicates the presence of an aldehyde.
        • No reaction means that the carbonyl compound is a ketone.
    • Iodine Solution & Sodium Hydroxide
      • 1. Place 1cm3 of Iodine solution into a test tube.
      • 2. Now, add 10 drops exactly of the solution being tested.
      • 3. Add 1cm3 of Sodium Hydroxide solution.
      • 4. A pale yellow precipitate will form which indicates:
        • A) The presence of RCOCH3
          • I.E. Methyl Ketone
          • I.E. Ethanal
        • OR B) The presence of a precursor to RCOCH3
          • I.E. Ethanol or a secondary alcohol with the -OH group and Carbon 2.
            • E.G. Propan-2-ol

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