In this activity, learners use ethanoic anhydride to convert 2-hydroxybenzoic acid into aspirin
The reaction takes place easily in acidic solution, but the product is formed as part of a mixture containing several other compounds.
- Access to a fume cupboard
- Pear shaped flask, 25 cm3
- Hot water bath
- Measuring cylinder, 10 cm3
- Bath of iced water
- Glass stirring rod
- Buchner funnel and suction
- Watch glass
- 2-Hydroxybenzoic acid, 1 g
- Ethanoic anhydride, 2 cm3
- Eight drops of concentrated phosphoric acid
Health, safety and technical notes
- Read our standard health and safety guidance here.
- Wear eye protection.
- Sulfuric acid can be used in place of phosphoric acid, but may give lower yields.
- Some teachers have reported problems which were due to using ethanoic anhydride that had already been hydrolised to ethanoic acid. Add a drop to water to ensure it is still reactive.
- If no precipitate appears, scratch the inside of the beaker with a glass rod or add a seed crystal of aspirin.
- As much as 40% of the mass of product after filtering may be water. Overnight drying is preferable to oven drying.
- Students should obtain about 0.9 g of crude product from 1.0 g of 2-hydroxybenzoic acid.
- For more information on hydroxybenzoic acid, see CLEAPSS Hazcard HC052
- For more information on ethanoic anhydride, see CLEAPSS Hazcard HC039
- For more information on phosphoric acid, see CLEAPSS Hazcard HC072
- Take about 1 g of your sample of 2-hydroxybenzoic acid and weigh it accurately.
- Put it into a dry pear shaped flask and add 2 cm3 of ethanoic anhydride followed by 8 drops of concentrated phosphoric acid.
- Put a condenser on the flask.
- In a fume cupboard, warm the mixture in a hot water bath, with swirling, until all the solid has dissolved and then warm for another 5 minutes.
- Carefully add 5 cm3 of cold water to the solution.
- Stand the flask in a bath of iced water until precipitation appears to be complete. It may be necessary to stir vigorously with a glass rod to start the precipitation process.
- Filter off the product using a Buchner funnel and suction apparatus.
- Wash the product with a little cold water, transfer to a weighed watch glass and leave to dry overnight.
- Weigh your product.
- Formation of an iron-phenol compound with Fe3+ gives a definite colour. Does the crude product contain any phenol type impurities?
- Draw the structural formulae of phenol, 2-hydroxybenzoic acid and aspirin. Identify the functional group most likely to be reacting with the Fe3+ ions.
- The crude product may contain 2-hydroxybenzoic acid, as well as water or ethanoic acid as impurities. 2-Hydroxybenzoic acid can be formed either from incomplete reaction or from hydrolysis of the product during its isolation.
The OH group attached to the benzene ring produces a purple colour with Fe3+(aq) ions. The OH group in aspirin is part of the carboxylic acid group and does not react in the same way.
These resources were compiled by David Lewis and edited by Colin Osborne and Maria Pack.
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The preparation of aspirin