Synthesis of Aspirin

Aspirin has been with us for long time – the first documented reports of its use occurred over 2000 years ago. That said, the aspirin may not be in the form that you instantly think of (ie, a tablet) but it has been used all the same.

Aspirin is an analgesic, which means it has pain killing effects and it has been well documented that the ancient Egyptians used a special tea made out of willow bark to treat a number of ailments.

It is also a substance that is remarkably easy to synthesize in the lab. Synthesis of it is also a good way to learn some useful chemical techniques where 2-hydroxybenzenecarboxylic acid (salicylic acid) is esterified using ethanoic anhydride:

The technique used to synthesize aspirin is refluxing, filtering under reduced pressure, recrystalization and melting point determination.

Refluxing is, in some ways similar to distillation but the condenser is placed vertically and not horizontally. It is used to return the vapours of the reactants back into the reaction vessel and also where heating over long periods of time needs to be carried out.

Filtering under reduced pressure involves a Buchner funnel. The funnel is connected, via a T-shaped valve to a tap, where water is flush through the tap. This causes a reduction in air pressure in the funnel and more effectively draws to the solution through the filter paper. It is faster than the more traditional method of filtering under gravity.

Recrystalisation involves dissolving a product in the minimum amount of hot solvent and then quickly cooling down this solvent. The hot solution will be saturated, meaning that crystals will form out if the solution as it cools down and can therefore hold less solute. It is common practice to scratch the inside of the vessel with a glass rod, below the level of solvent to encourage crystals to grow. The scratching process gives the aspirin a crystalline surface on which it can grow.

The melting point determination gives an idea of purity as the purer a substance is the more defined it’s melting point is. Pure aspirin melts at 135^oC. If you have manufactured pure aspirin it will melt at this temperature. If, however, it is not so pure, it will melt over a range of temperatures (eg, 130 – 140 ^oC). The greater the range, the lower the purity.

Have you synthesized aspirin in your lab? Did it work? How did you know? And did you make it using the above steps or did you use a different method?