PAG 7.2 The characteristic test for a carbonyl compound and the use of the 2,4-DNPH derivative to identify an unknown carbonyl compound

Give the method of this practical

1. Add 5cm depth of Brady’s reagent to a clean test-tube (in excess)

2. Using a dropping pipette, add three drops of the unknown compound and leave to stand

3. If a crystal precipitate does not form, add a few drops of dilute sulfuric acid until one does

4. A yellow / orange precipitate indicates the presence of an aldehyde or ketone

Explain how you would use the results to identify whether it is an aldehyde or ketone with the melting points

1. Put ethanol into a boiling tube and the boiling tube into a hot water bath

2. Filter the precipitate under reduced pressure using a small Hirsch/Buchner funnel. Wash with a minimum volume of cold methanol

3. Recrystallise the precipitate using a solvent of hot ethanol

4. Continue to warm the solvent using hot water and dissolve the crystals in a minimum volume

5. Stir up to cause the crystals to dissolve. Once dissolved place the boiling tube into ice to recrystallise

6. Crystals should form as the solution cools. Filter the pure crystals under reduced pressure using the Buchner funnel and dry at room temperature by plaing out a piece of filter paper

7. Find and record the melting point of the pure, dry product

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