The preparation of aspirin

The reaction takes place easily in acidic solution, but the product is formed as part of a mixture containing several other compounds. 

Equipment

Apparatus

• 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

Chemicals

• 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

Procedure

Stage one

1. Take about 1 g of your sample of 2-hydroxybenzoic acid and weigh it accurately.
2. Put it into a dry pear shaped flask and add 2 cm³ of ethanoic anhydride followed by 8 drops of concentrated phosphoric acid.
3. Put a condenser on the flask. 
4. 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.

Stage two

1. Carefully add 5 cm³ of cold water to the solution.
2. 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. 
3. Filter off the product using a Buchner funnel and suction apparatus. 
4. Wash the product with a little cold water, transfer to a weighed watch glass and leave to dry overnight.
5. Weigh your product. 

Questions

1. Formation of an iron-phenol compound with Fe³⁺ gives a definite colour. Does the crude product contain any phenol type impurities?
2. Draw the structural formulae of phenol, 2-hydroxybenzoic acid and aspirin. Identify the functional group most likely to be reacting with the Fe³⁺ ions.

Answers

1. 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.

2. 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.