Organic synthesis
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I would like to know how to conduct an experiment for the synthesis of aspirin by the acylation of salicylic acid using ethanoic anhydride.
Igloo writes ...
Here are instructions for the preparation of about 2 grams of aspirin. See the health and safety notes at the bottom.
Add to a pear-shaped flask 2.0 grams of salicylic acid (2-hydroxybenzoic acid) and 4 cm3 of ethanoic anhydride (take care – this liquid is corrosive).
To this mixture add 5 drops of 85% phosphoric(V) acid [this acts as a catalyst] and swirl to mix thoroughly. Fit the flask with a reflux condenser and heat the mixture on a boiling water bath for about 15 minutes. Without cooling the mixture, carefully add about 2 cm3 of cold water in one portion down the condenser. The excess ethanoic anhydride will hydrolyse (to ethanoic acid) and the contents of the mixture will gently boil, since this reaction is exothermic.
When this reaction has subsided, pour the mixture into about 40 cm3 of cold water contained in a 100 cm3 beaker, stir and rub the sides of the beaker with a glass stirring rod if necessary to start the crystallisation process. Then allow the mixture to stand in an ice-bath to complete crystallisation.
Collect the crystals using suction filtration and wash the crystals through the filter paper with a small quantity (say 10 cm3) of cold water. This removes most of the ethanoic acid since it is water soluble.
Finally the product should be recrystallized using the minimum volume of hot water. The melting point should then be measured to check its purity.
Health and Safety. We are not qualified to give safety advice on React. Do not rely on what is said here. Always carry out a risk assessment and check with your teacher before starting any practical work. Make sure that you understand all of the procedures mentioned above, use appropriate safety precautions, and do not start until you have permission from your teacher.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 18 January 2007
