Preparation of aspirin

Aspirin is a human-made, synthetic version of salicylic acid, used to reduce fever and inflammation and relieve pain. Originally sold as a powder, and now in tablets, aspirin was a trademarked name until 1921. Researchers keep finding new ways that aspirin promotes and protects human health

It is an ester and can be prepared by the condensation reaction between

2-hydroxybenzoic acid (or salicylic acid) and Ethanoic anhydride. Purity of aspirin can be checked by determining its melting point.

Aim

* To produce synthetic aspirin.

* To calculate the percentage yield of the aspirin produced.

* To compare the aspirin produced in the lab with tablet aspirin and aspirin from the lab by there melting points

Method

Glassware Chemicals

Conical flasks Measuring cylinders 2- hydroxybenzoic acid

Beakers Thermometers Ethanoic anhydride

Dropper Hot plate Conc. Sulphuric acid

Balance Buchner funnel and flask Ethanol

Vacuum pump Clock glass Anti-bumping granules

Oven Melting point apparatus Deionised water

Capillary tubes Stirring rod Ice

> A 50 cm3 conical flask was weighed and ~10g of 2- hydroxybenzoic acid was added.

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The flask and its contents were reweighed.

> In a fume cupboard, 20cm3 of ethanoic anhydride was added by a measuring cylinder, to the 2- hydroxybenzoic acid. The flask was swirled as it was added to ensure thorough mixing.

> 5 to 10 drops of concentrated sulphuric acid was added to the mixture.

> The flask was placed on a hot plate, in a fume cupboard and the mixture heated to about 85oC it was kept at this temperature for about 10 minutes, it did not go over this temperature as if it did the compound may of decomposed.

> The mixture was cooled in an ice/water bath, and then the mixture was poured in to ~150 cm3 of cold deionised water in a 250 cm3 beaker.

> The precipitate was filtered of using a water pump, and was washed thoroughly with several washings of cold water.

> The crude product was transferred to a 100 cm3 conical flask containing ~20 cm3 of ethanol. A couple anti-bumping granules were added and the mixture was gently heated on a hot plate until the solid dissolved.

> The solution was poured into a 100 cm3 conical flask containing ~ 50cm3 of deionised water. Oil formed, so the mixture was reheated on a hot plate to dissolve it. It persisted so a few drops of ethanol were added and the mixture was reheated.

> The mixture was set aside and allowed to cool to room temperature.

> The crystals of aspirin were filtered of using a vacuum pump and washed of with a small volume of cold water. Air was drawn through the crystals for a few minutes to allow them to partially dry.

> A clock glass was weighed and the crystals were transferred on to it. The crystals were dried in an oven at about 100 oC and then the clock glass and crystals reweighed.

> The melting point of the aspirin was determined, and the percentage yield of the aspirin was calculated.

Results

C7H6O3 + C4H6O3 � C9H8O4 + CH3COOH

G.F.M 138.12 g 102.09g 180.16g

Actual mass 10g 1.082g/ cm3 4.307g

20* 1.082g

= 21.64 g of C4H6O3

(2- hydroxybenzoic acid is the limiting factor.)

Theoretical Mass 10g 21.64g 13.04g

Actual percentage yield = 4.307 * 100 = 33 %

13.04

Type of aspirin

Melting point � C

The synthetic aspirin

147

Tablet aspirin

151

High purity aspirin

148

Discussion

When the aspirin was produced it formed small white crystals, although the method was followed as instructed only a 33% yield was obtained. This will be because of human error and impurities in the aspirin. Also, no experiment is 100% efficient due to the rate of reactions as they approach equilibrium. The experiment could perhaps be improved by heating the reaction mixture longer to allow equilibrium to be reached.

Crystallisation does not recover the entire product from the solution. This process can be helped by leaving the crystallising mixture in the dark, undisturbed.

The melting point of pure aspirin is 138 - 140� C. The tablet aspirin which is sold in shops as medicine has a melting point of 151� C which is above pure aspirin, but this is because there are other compounds within the tablets. As for the high purity aspirin stored in the lab it has a melting point of 148� C which shows there are impurities in the compound.

The synthetic aspirin produced in the lab is not pure aspirin as it melted at 147� C which is above the melting point for pure aspirin. The aspirin produced has got some impurities in it.

Conclusion

The aspirin produced in the lab is not pure aspirin as impurities are present as detected by the melting point.

Due to human error and some of the desired compound was lost and along with the impurities in crystallisation.

This made the percentage yield relatively low at only 33%. Also you can read about Ka of ethanoic acid