Procedure:
Firstly test the unknown liquids to confirm that they are carbonyl compounds.
- Add six drops of aqueous 2, 4-dinitrophenylhydrazine to 6 test tubes. Place 4 drops of the unknown into a test tube each and shake gently.
An orange precipitate of 2, 4-dinitrophenylhydrazone should indicate all six to be carbonyl compounds.
Secondly separate aldehydes from ketones.
- A small amount of potassium dichromate (VI) solution is acidified with dilute sulphuric acid and a few drops of the unknown liquid is added to it.
A change of colour of solution from orange to green would indicate the aldehydes due to oxidation of the aldehyde by the dichromate. For the ketones there would be no colour change, solution remains orange.
The aldehydes are separated from the ketones and labelled.
Aldehydes are tested first then ketones.
Method 1:
Preparation of a crystalline derivative.
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Place 10 drops of the unknown liquid in a boiling tube. Add 5 cm3 of the 2, 4-dinitrophenylhydrazine solution and stir.
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If precipitation does not occur, carefully add 1 cm3 of dilute sulphuric acid.
- Using a Buchner funnel and suction filtration apparatus as shown in diagram1, filter the precipitate.
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Stop suction, either by lifting the funnel or by disconnecting the tubing, and soak the precipitate in about 1 cm3 of methanol. (Do not turn off the tap as you may get a “suck back” of water.)
- Resume suction and dry the crystals by drawing air through them for a few minutes.
- Recrystallize the solid using the following procedures.
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Transfer the crystals to a 150 cm3 beaker in a water bath.
- Dissolve the crystals in the minimum amount of hot ethanol.
- When the crystals have dissolved, cool the solution in an ice- water mixture until crystals re-appear.
- Filter the crystals as before. If necessary, rinse the beaker with the filtrate to complete the transfer. Wash the crystals with a few drops of cold ethanol.
- Press the crystals between filter paper to dry them thoroughly. Leave crystals in warm air until they are fully dry.
Having procured crystals of the unknown carbonyl, they are identified using the specific melting points of 2, 4- dinitrophenylhydrazone.
Method 2:
Determination of the melting point of the derivative.
- Take a melting-point tube and push the open end through a pile of the derivative until a few crystals have entered the tube.
- Tap the closed end of the tube vertically against a hard surface to make the solid fall to the bottom.
- Repeat the filling and tapping procedure until a total length of between 0.5-1 cm is compacted at the bottom of the tube. Prepare another tube in this way.
- Attach one of the prepared melting-point tubes to the thermometer as shown in daigram2.
- Half-fill the boiling-tube with dibutyl phthalate and position the thermometer with attached tube and the stirrer through the bung, as shown in diagram3.
- Place apparatus over low Bunsen flame and gently heat the apparatus, stirring the dibutyl phthalate all the time by moving the stirrer up and down.
- Keep an eye on the crystals and note the temperature as soon as signs of melting are seen. This first reading gives only a rough melting point but is a guide for the second determination.
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Remove the burner and the old tube containing derivative. Allow the temperature to drop about 100C before positioning a fresh melting point tube containing another portion of the derivative.
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Repeat the above procedure in order to obtain a more accurate value of the melting-point. Raise the temperature very slowly (about 20C rise per minute) until the crystals melt (take the formation of a visible meniscus as a sign of melting)
- Compare the melting-point of your crystals with the values given in the table which follows and identify the unknown compound.
Carbonyl Formula Melting point of 2, 4-dinitrophenylhydrazone
/0C
Aldehyde
Propanal CH3CH2CHO 156
Butanal CH3CH2CH2CHO 123
Ketone
Propanone CH3COCH3 128
Butanone CH3CH2COCH3 115
Pentan-2-one CH3CH2CH2COCH3 141
Pentan-3-one CH3CH2COCH2CH3 156
