Preparation of aspirin

The experimental details given here are given in good faith and are believed to be safe and workable methods. However, the authors cannot take responsibility for the consequences of performing these experiments.

The experiments are written for experienced science teaching staff to use as instructions for a supervised class of students. The experiments are not designed for students or inexperienced members of the public to perform without supervision. If you wish to attempt the experiments, ensure that you have completed a legally adequate risk assessment beforehand and that you work within the constraints of the risk assessment.

2-hydroxybenzoic (‘salicylic’) acid reacts with ethanoic acid and its derivatives to form 2-ethanoylhydroxybenzoic acid (‘aspirin’)

Chemicals and apparatus

Round-bottomed flask
Vacuum filtration apparatus
Reflux condenser
Ice-bath
Beaker, 100 cm³
Heating mantle
Balance
Watch glass
2.0 g of 2-hydroxybenzoic (‘salicylic’) acid
3.0 cm³ of ethanoic anhydride
5 drops of 85% phosphoric (V) acid

Precautions

Wear eye protection and laboratory coat at all times.
Tie back long hair when using Bunsen burners.

Procedure

Weigh out approximately but accurately 2.0 g of 2-hydroxybenzoic (‘salicylic’) acid and add it to a round-bottomed 'Quickfit' flask.
With care, add 3.0 cm³ of ethanoic anhydride to the flask contents.
Again with care add 5 drops of 85% phosphoric (V) acid and swirl gently to mix the flask contents.
Using a heating mantle reflux the mixture gently for five minutes.
Immediately after 5 minutes carefully add 2.0 cm³ of water down the vertical condenser. Note: this will produce a vigorous reaction.
When the reaction has subsided carefully pour the flask contents into 40 cm³ of cold water in a small beaker.
Allow the beaker contents to cool, scratching the inside of the beaker with a glass rod if crystallisation does not occur immediately.
Once crystallisation has started cool the beaker and contents in an ice-bath.
Filter the solid product using suction (vacuum) filtration and wash the solid with a little cold water.
Recrystallise the product from the minimum of hot water, filtering and washing as before.
Dry the material in air, scrape it onto a pre-weighed watch glass and determine the mass of product.

Treatment of the product

Determine the percentage yield.
Carry out a melting point determination.

Conclusions and comments

What sort of reaction is occurring? What sort of bond is formed?
What is the purpose of the phosphoric acid?
What is the relationship between ethanoic anhydride and ethanoic acid?
Look up and write a short account of how aspirin is manufactured. How is the 2-hydroxybenzoic acid made?

References