Chemistry experiment: Aspirin

It is that time of year that we come to do the Chemistry practical in class which I did last year as part of my Crest project. At one point I will dedicate a post to my Crest as I find it  a really interesting topic (I’m just waiting for a few things to sort out first) but for now I will simply talk about the experiment.

This is an experiment all about producing aspirin from the oil of wintergreen. The oil of wintergreen is a strong ‘minty’ smelling substance that is made by the distillation of the leaves of a plant named Gaultheriae procunbers (more commonly known as wintergreen). This substance contains roughly 98% methyl 2-hydroxybenzoate, a compound that can be hydrolysed by heating with sodium hydroxide and adding an acid to form 2-hydroxybenzoic acid (a solid product) and sodium chloride. This is also known as salicylic acid (a product which I made from willow bark in my Crest and a form of the active ingredient found in aspirin). Next the 2-hydroxybenzoic acid is esterified by a reaction with ethanoic anhydride to form aspirin. We then recrystallised the aspirin to try to form a purer product.

Since the first step was hydrolysis which required 30 minutes of heating, the technique of reflux is used to prevent any vapours from escaping the reaction mixture as the condenser attached to the reaction vessel condenses any vapours which then drop back into the flask. This allowed prolonged heating without drying out or losing hazardous volatile substances to the environment.

Fig.1 Refluxing
Fig. 1 The equipment set up to reflux the oil of wintergreen.

The first part of the experiment went well for us as we manged to a achieve an 86.5% yield of the 2-hydroxybenzoic acid. We then also managed to achieve a 65.1% yield of the actual aspirin although this was still the crude product and we had a high rate of product loss through the aspirin being left behind on filter paper and any other equipment.

From then things went a little downhill.

The re-crystallisation did not go well as although our 2g of crude aspirin happily dissolved, it then refused to re-crystallisation in the cold environment. When it finally did, the re-crystallised product was a far cry from the pile of crude aspirin we used and after filtration under reduced pressure nothing was left behind on the filter paper. Our teacher suggested we may have got carried away with adding the solvent and so decided to heat our solution on a hot plate in hope to evaporate off the ethanol. Unfortunately some rubber bung was present in the solution (?? we think it came from the tubing on the filtration under reduced pressure equipment) and proceeded to be melted into a lovely charcoal colour. Nice crystals eventually formed although marred by the black colouring. However there was a suspicious ‘minty’ smell to me although our chemistry teacher described it as an ‘acidy’ smell…

Pure aspirin
Fig. 2 Our apparently ‘pure’ aspirin with a hint of burnt rubber.

Our initial theory was that somehow our reaction conditions forced a wrong OH group on the 2-hydroxybenzoic acid to be replaced by the methyl group meaning oil of wintergreen was made rather than aspirin. As a result the chromatography results were much anticipated to try to solve the mystery of the ‘pure’ aspirin.

aspirin chromatography
Fig. 3 The solvent rising up the TLC (thin layer chromatography) paper which separates out the different components present in a sample.

As hoped for the chromatography did shed so much-needed light, UV light to be specific. Although our crude aspirin had a Rf value similar to the shop-brought aspirin, the ‘pure’ aspirin  had a figure closer to the Rf value of the school’s 2-hydroxybenzoic acid and this may explain the ‘acidy’ smell our teacher identified.

We believe that the issues occurred because either excess ethanol was added or it was too hot. In addition, heating on a hot plate to remove the excess ethanol may have heated the aspirin ‘to oblivion’ (as our teacher kindly put it as they turned the heat up) and the outcome of this is that the aspirin decomposed-clearly back into 2-hydroxybenzoic acid.

We also carried out melting point analysis however the results showed only a weak correlation with the results expected for aspirin and 2-hydroxybenzoic acid.

Overall although the experiment in effect failed because we did not end up with pure aspirin, a form of aspirin was still present at one point and even though we then did not manage to purify it, we instead made a different product and so it was interesting to see how even a minuscule error in our method proved to change the results so much.

As they say, we had a learning curve.

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