Core practical 16 - Synthesise aspirin from 2-hydroxybenzoic acid

Which functional group of the 2-hydroxybenzoic acid reacts with the ethanoic anhydride?

The hydroxyl group.

Draw the structural formulae for the reactants and product involved in the formation of aspirin from 2-hydroxybenzoic acid.

Calculate the relative molecular masses of 2-hydroxybenzoic acid and aspirin.

2-hydroxybenzoic acid = 138; aspirin = 180

Calculate the theoretical yield.

2.6g

Calculate the percentage yield.

This will vary if you use your own results: the yield / 2.6 × 100; using sample data gives 81%.

Why might the apparent yield be higher?

Because of impurities in the sample; the crystals may not be dry

What would you expect to be the main impurity in your sample?

Unreacted 2-hydroxybenzoic acid.

The actual melting temperature of aspirin is 136 °C. Is this similar to the value you recorded? Why do you think there might have been a difference?

You should record a melting temperature range rather than a single temperature. This is because impurities in the sample cause the solid to melt over a temperature range, rather than sharply at one temperature. The narrower the range and the closer your value to 136°C, the purer the sample.