RP10: Aspirin

• Ethanoic anhydride is used instead of ethanoyl chloride:

  • Cheap
  • Less corrosive
  • Less vulnerable to hydrolysis
  • Less dangerous to use
• Re-crystallisation of crude product:

  1. Dissolve the sample in a minimum amount of hot solvent
  2. Filter the solution whilst hot
  3. Cool in ice-water bath AND filter the sample using a Buchner funnel
  4. Wash the same thoroughly with cold water AND dry in oven
• Using a minimum amount of solvent creates a saturated solution and therefore increases the yield
• Filter solution whilst hot so prevents any re-crystalisation whilst filtering and to remove insoluble impurities
• Cool the filtrate in an ice-water bath to increase the amount of crystals formed
• Wash sample thoroughly with cold water to remove any soluble impurities
• Choosing a suitable solvent for re-crystallisation:

  • The product is soluble in the hot solvent
  • The product is insoluble in the cold solvent
• An impure solid will melt over a range of temperatures which are lower than the melting point of the pure solid
• Measuring melting point:

  1. Use melting point apparatus to heat sample gently
  2. Heat slowly close to the melting point of aspirin
  3. If the aspirin is pure, melting point close to data book value
  4. Impurities will cause the melting point to be lower than expected and have a large range
• Aspirin synthesis - preparation:

  1. Weigh out given mass of salicylic acid
  2. Transfer the contents of this into the pear shaped flask using a glass rod
  3. At fume cupboard add ethanoic anhydride and phosphoric acid (should feel warm)
  4. Add anti-bumping granules to prevent violent boiling
• Aspirin synthesis - heating mixture:

  1. Reflux the mixture for 10 mins
  2. Prepare beaker 1/3 filled with deionised water and a water ice-bath
  3. Detach pear-shaped flask and QUICKLY pour mixture into water containing beaker (crystallises immediately)
  4. Rinse inside of pear-shaped flask and ensure all contents transferred to small beaker for filtration
• Aspirin synthesis - crystallisation:

  1. Stir the small beaker in the ice-water bath and allow complete crystallisation to occur (5 mins)
  2. Filter the sample using the Buchner funnel under reduced pressure
  3. Wash the sample thoroughly with deionised water to remove soluble impurities
  4. Pat the filtered sample dry using filter paper and place in fume hood for further drying
• Describe how a crystalline solid is separated and purified:

  • Filter
  • Dissolve in minimum volume of solvent
  • Use a hot solvent
  • Cool so crystallises and then filter
  • Wash with cold solvent
• A cold solvent is added to remove any soluble impuritites
• Method of recystallisation:

  • Dissolve product in minimum volume of solvent
  • Add hot solvent
  • Allow solution to cool so crystals form
  • Filter off product under reduced pressure
• Method of checking sample purity:

  • Use melting point apparatus
  • Measure melting point
  • Melting point should match data sources
• Why percentage yield may not be 100%

  • Product left in beaker or glassware
  • Sample lost during recrystallisation
  • Sample was still wet
• Filtration using a separating funnel:

  1. Place crude sample in separating funnel
  2. Add an equal volume of NaCl to solution
  3. Shake and open separating funnel periodically
  4. Let two layers separate out (immiscible)
  5. Use the tap at the bottom of the separating funnel to run off lower layer
  6. Add a drying agent to remove any remaining water
• Higher density liquids will form the lower layer in a separating funnel
• Sample may have been lost during recrystallisation so percentage yield will not be 100%
• Liquids in the separating funnel will be immiscible if two layers formed
• Describe how to separate a crystalline solid:

  • Filter
  • Dissolve in a minimum volume of hot solvent
  • Cool to crystallise and filter under reduced pressure
  • Wash with cold water
• Purified aspirin will have larger crystals and appear lighter in colour compared to crude aspirin
• Flask may be cooled to room temperature before crystals filtered as yield of crystals would be lower if warm
• Crystals were compressed in a funnel because air passes through the sample so better drying
• A likely impurity could be water so press sample of crystals between filter papers to ensure melting point is reliable