RP 10, recrystallisation

• Recrystallisation

  -Dissolve impure compound in minimum vol hot water

  -hot filter through filter

  -cool in ice

  -filter under reduced pressure, Buchner flask

  -wash with distilled water

  -dry with filter paper
• Why is minimum hot solvent needed?

  so saturated solution
• Why is hot filter important?

  to remove insoluble impurities, heat will prevent crystals from forming
• Why is cooling in ice important?

  reform crystal, but soluble impurities remain in solution because they are present in small quantities
• why is using Buchner funnel important?

  speeds up filtration, air is passed through, not just around it
• Why do you need to wash with distilled water?

  to remove soluble impurities
• Why do you need to dry the crystals?

  water is an impurity
• What leads to loss of yield?

  -crystal lost when washing, filtering

  -some product stay in solution after recrystallisation

  -other side products, reaction occurring
• What happens when crystals are not dry?

  mass will be larger than expected, more than 100% yield
• How does pure and impure aspirin differ?

  -whiter, larger crystals

  -lower bp, range of values
• Preparation of aspirin

  -add to pear shaped flask, 2 hydroxybenzoic acid and ethanoic anhydride

  -add phosphoric acid, swirl flask

  -Fit flask with reflux condenser and heat mixture on boiling water bath for a bout 5 minutes

  -Without cooling add 2 cm3 of water in one portion down condenser

  -when vigorous reaction ended, pour mixture into cold water, stir to induce recrystallisation

  -cool in ice to complete recrystallisation

  -collect by suction filtrate and wash with a little water
• What is aspirin made up of?

  2 hydroxybenzoic acid containing phenol group, turns into ester when reacting with ethanoic anhydride
• Why is ethanoic anhydride used instead of acyl chloride?

  -less exothermic reaction, less corrosive
• What kind of organic compounds can undergo reflux?

  compounds with low boiling points