PAG 6- Synthesis of an organic solid

Question 1

Method (preparation)

Answer 1

• Mix 2.5cm^3 methyl benzoate with a measured vol of sulfuric acid in a 50cm^3 conical flask.
• Place conical flask into an ice bath.
• In a test tube, add 2cm^3 concentrated sulfuric acid and 2cm^3 concentrated nitric acid. Place test tube into ice bath.
• Once cooled, use glass dropping pipette to add mixture from test tube to conical flask dropwise.
• Stir continually, ensuring the temperature remains below 10°C.
• Leave the mixture at room temperature for about 15 minutes.
• Add about 25g of crushed ice to the conical flask and stir until it melts and forms a precipitate.
• Filter the mixture under reduced pressure using a Buchner flask, to obtain the crystals.
• Wash the crystals with cold water.

Question 2

Method (recrystallisation)

Answer 2

• Heat about 10cm^3 of ethanol to 60°C using a water bath.
• Dissolve the impure compound in the minimum volume of hot ethanol in a test tube.
• Filter the hot solution to remove any insoluble purities, heat prevents crystals reforming during filtration.
• Cool to room temperature.
• Use suction filtration to separate crystals from solution, using Buchner flask.
• Wash crystals with distilled water.
• Leave vacuum on to dry crystals.

Question 3

Ways in which product may be lost during recrystallisation

Answer 3

• Crystals lost when filtering or washing.
• Some product may stay in solution after recrystallisation.
• Other side reactions may occur.
• If crystals are not dried properly, the mass will be larger than expected, leading to a percentage yield greater than 100%.

Question 4

Method (thin-layer chromatography)

Answer 4

• Draw a pencil line about 1cm above the bottom of the TLC plate. Mark 3 equally spaced crosses along the line.
• Using a capillary tube, spot small drops from part 1 and 2, and the pure methyl-3-nitrobenzoate on the TLC plate. Each spot should be small, as otherwise spots may merge. Leave to dry before spotting again.
• Add about 10cm^3 solvent into a 100cm^3 beaker and cover with a watch glass, prevents evaporation of solvent.
• Place TLC plate into beaker, making sure level of solvent is belobe pencil line. Make sure watch glass is placed on top of beaker.
• When solvent is about 1cm from top of plate, remove plate and mark solvent front with a pencil. Leave to dry in a fume cupboard, as it is toxic solvent.
• Mark spots with pencil once visible.
• Calculate Rf values.