Core practical 16 - Aspirin Synthesis

Question 1

Steps in RECRYSTALISATION

Answer 1

1. Prepare a solution of hot solvent (Ethanol 3:1)
2. Dissolve the crude aspirin crystals in the MINIMUM AMOUNT OF hot solvent (stir and make sure it is dissolved)
3. Perform hot gravity filtration with a funnel and fluted filter paper
4. Recrystalise the filtrate in an ice bath allowing it to cool
5. Filter the recrystallised aspirin under reduced pressure (Buchner funnel) and wash with water to remove soluble impurities
6. To check purity measure the melting point

Question 2

How do you prepare aspirin? (Steps)

Answer 2

1. Add Salicylic acid, Ethanoic anhydride to a pear-shaped flask
2. Add 5 drops of conc H2SO4 to the mixture
3. Heat the mixture under reflux (WATER BATH) in a fume cupboard (10 mins)
4. Add crushed ice to stop the reaction and break down ethanoic anhydride
5. Stand the flask in iced water until crystals appear
6. Filter the product using a Buchner funnel under reduced pressure
7. Perform recrystallisation to purify the sample (melting temp for purity)

Question 3

Why do we preform HOT filtration?

Answer 3

• Removes insoluble impurities

- Heat prevents crystals from forming

Question 4

Why do we recrystalise the sample after hot filtration?

Answer 4

• To remove soluble impurities

- They stay in the solution while aspirin crystalises due to ice

Question 5

Why do we use ethanoic anhydride?

Answer 5

• Cheaper
• Less vulnerable to hydrolysis
• XS will be hydrolysed and boil away

Question 6

Loss of yield in recrystallisation examples

Answer 6

• Crystals lost when filtering or washing
• Some product stays in the solution after recrystallisation
• other side reactions occurring