CPAC 16

Question 1

Describe the process of aspirin synthesis

Answer 1

1. weigh 2g of 2-hydroxybenzoic acid and put it in a pear-shaped flask
2. add 5cm^3 ethanoic anhydride and 5 drops of concentrated sulfuric acid
3. fix a condenser to the flask and heat under reflux using a bunsen burner until all or almost all the solid has dissolved and heat for a further ten minutes
4. remove the flask from the heat and add 10cm^3 crushed ice and distilled water to break down any unreacted ethanoic anhydride (produces 2 moles of ethanoic acid)
5. stand the flask in a beaker of iced water until precipitation appears complete
6. filter the produce using a Buchner funnel and suction apparatus
7. wash crystals with minimum volume of iced water
8. recrystallise the aspirin in a minimum volume of 1:3 ethanol and water
   - heat solvent mixture and dissolve hot crystals in it
   - filter hot through heated funnel to remove insoluble impurities
   - then cool in ice bath until precipitate appears
   - wash with cold solvent
9. filter and dry in warm oven/dessicator
10. measure mass of crystals
11. measure melting temperature using melting temperature apparatus

Question 2

why do impure solids often melt at a lower temperature than pure solids?

Answer 2

impurities disrupt the regular crystalline arrangement of particles so intermolecular forces are weaker

aspirin is a crystalline solid made up of molecules

Question 3

How is aspirin produced?

Answer 3

esterification of alcohol group in 2-hydroxybenzoic acid with ethanoic anhydride

Question 4

why are the crystals washed with iced water?

Answer 4

wash away any impurities without dissolving the aspirin