Practical investigations
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I am currently synthesising aspirin for my A2 coursework. However, during the recrystallisation of my crude aspirin, an oil formed that is preventing the aspirin from falling out of solution. I am using ethanoic acid, ethanoic anhydride and ethanoyl chloride to produce the aspirin and ethyl ethanoate as a solvent to recyrstallise. I need to remove this oil and have tried adding hexane and heating with no success. What do you suggest I try?
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Igloo writes ...
I am unsure as to what the oil is, but I do note that you are using a somewhat unconventional technique to manufacture your aspirin and to recrystallise it. Your oil may be due to spurious impurities in one of the many chemicals you are using, or it may have something to do with the ethyl ethanoate you are using for the recrystallisation. Adding hexane is simply going to contaminate your product and, incidentally, so is the ethyl ethanoate you are using for the recrystallisation itself. I don’t know what’s happened, but your methods are far more complicated than they need to be.
The method I am describing below definitely works – I can vouch for it – and none of my students have ever had any problems with an oil appearing.
Add to a pear-shaped flask 2.0 g of 2-hydroxybenzoic acid (salicylic acid) and 4 cm3 of ethanoic anhydride, followed by 5 drops of 85% phosphoric acid. DO NOT USE ETHANOYL CHLORIDE AT ALL. Swirl to mix. Fit the flask with a reflux condenser and heat the mixture on a water bath for about 10 minutes. Without cooling the mixture, carefully add 2 cm3 of cold water in one portion down the condenser. The excess ethanoic anhydride will hydrolyse and the contents of the mixture may boil briefly. After this, remove the condenser and pour the mixture into 40 cm3 of cold water in a 100 cm3 beaker surrounded by ice contained in a second larger beaker. Stir, and rub the sides of the beaker with a glass rod to induce crystallisation. Finally allow the mixture to stand to complete the crystallisation process.
Collect the solid by suction filtration (ordinary filtration, if suction filtration isn’t possible) and wash it with a little cold water.
The solid can now be recrystallised from hot water.
This method of recrystallisation involves only water, so the crystals only need to be dried – there is no other contaminant to worry about.
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updated: 11 February 2008
