Unit A23 4.7 Aldehydes and ketones

Question 1

4.7.8 - Preparing, recrystallising and determining the melting points of 2,4-dinitrophenylhydrazones; laboratory preparation of 2,4-dinitrophenylhydrazone

Overall method

Answer 1

• Place 5 cm3 of 2,4-dinitrophenylhydrazine solution in a suitable container.
• Add some drops of the test liquid e.g. propanal or the test solid dissolved in ethanol.
• If crystals do not form add some dilute sulfuric acid and warm the mixture.
• Cool the mixture in iced water.
• Filter off the crystals using suction filtration
• Recrystallise. - dissolve the impure crystals in the minimum volume of hot solvent. Filter when hot by gravity filtration using a hot funnel, or fluted filter paper, to remove insoluble impurities.
Allow filtrate to cool and crystallise
• Filter off the crystals using suction filtration.
• Dry by sucking air over the crystals in the Buchner and then in a low temperature oven/dessicator
• Determine the melting point.

Question 2

4.7.8 - Preparing, recrystallising and determining the melting points of 2,4-dinitrophenylhydrazones; laboratory preparation of 2,4-dinitrophenylhydrazone

Suction filtration method

Answer 2

• Place filter paper in a Buchner funnel
• Place Buchner funnel in a Buchner flask
• Attach the flask to a suction pump and suck air through the flask

Question 3

4.7.8 - Preparing, recrystallising and determining the melting points of 2,4-dinitrophenylhydrazones; laboratory preparation of 2,4-dinitrophenylhydrazone

Diagram of suction filtration should include:

Answer 3

• Buchner funnel and filter paper
• Buchner flask
• Apparatus complete with no gaps
• Suction indicated

Question 4

4.7.8 - Preparing, recrystallising and determining the melting points of 2,4-dinitrophenylhydrazones; laboratory preparation of 2,4-dinitrophenylhydrazone

Melting point method

Answer 4

• Place some solid in a capillary tube/melting point tube sealed at one end
• Heat slowly (using melting point apparatus)
• Record the temperature at which the solid starts and finishes melting
• Repeat and average the temperatures
• Compare the temperature(s) with known values in a data book

Question 5

4.7.9 - Using Fehling’s solution and Tollens’ reagent to distinguish between aldehydes and ketones

Answer 5

• Add a few drops of the unknown solution to 1 cm3 of freshly prepared Tollens’ reagent in a clean test tube
• Warm in a hot water bath
• Freshly prepare Fehling’s solution by mixing 1 cm3 of Fehling’s solution A with 1 cm3 of Fehling’s solution B
• Add a few drops of the unknown solution to 1 cm3 of freshly prepared Fehling’s solution reagent in a test tube. Warm in a hot water bath.