Required practical 10a - Preparation of Aspirin

The theory (key idea behind the practical):

The preparation method:

• Weigh a known mass of salicylic acid in a conical flask

• Add a measured volume of ethanoic anhydride, and a few drops of concentrated sulfuric acid

• Heat the mixture using a water bath, to avoid naked flames, since these organic chemicals are flammable

• Crystalization: Pour mixture into cold water, so this causes the crude aspirin to precipitate out as a solid

• Filter under reduced pressure, using a Buchner flask, to separate the solid

Purification, recrystallization:

1. Dissolve the impure crystals in minimum volume of hot solvent, usually ethanol

   • ensures a saturated solution, so maximum amount of product crystallizes when cooled.

2. Filter hot through filter paper

   • To remove soluble impurities, because if cold, then the aspirin will dissolve

3. cool the filterate in an ice bath.

   • The aspirin is less soluble at cold temperatures, so crystals reform, and cooling in ice icreases the yield.

4. Filter under reduced pressure (Buchner funnel) to collect the purified crystals

5. Wash the crystals with ice-cold solvent

   • This is to wash away the soluble impurities, while minimising the loss of aspirin product

6. Dry the product in warm oven

Testing for purity:

• Using melting point apparatus, place small amount in capillary tube

• Heat slowly near the melting point

• Pure sample: Sharp melting point that matches the data book value

• Impure sample: Melting point is lower, and occurs over a range of temperatures

  • Lower BP, becasuse impurities are between the aspiring molecules, so reduces the intermolecular forces of attraction, so less energy required to overcome

  • OR u can use thin layer chromotography, run a TLC plate with 3 spots, salicylic acid, synthesized aspirin and pure aspirin standard => Rf value same as salicylic acid then the reaction is incomplete// Rf value matching standard aspirin the sample is pure

Percentage yield formulea// the ratio between salicylic acid and aspirin// why yield not 100%?