Got a headache?
Well, why don’t you go and find some trees and chew on bark for a while?
Sounds mad, doesn’t it?
But at some point in human history, someone must have tried it – and guess what?
It works!
The bark of the willow tree contains a chemical called salicylic acid.
You might know one of its derivatives – aspirin.
Aspirin is one of the most commonly used medicines in the world (it’s on the WHO list of essential
medicines), and it is very simple to make!
First, we take salicylic acid that has been isolated from willow bark – it’s a white
crystal.
We add to the salicylic acid a small amount of ethanoic anhydride – a colourless liquid
– and then a few drops of sulphuric acid to act as a catalyst.
We need to heat this reaction to force it to happen.
If we let it heat in an open container, the liquid reactants and solvents would evaporate
and leave the powders.
To stop this, we can set up our equipment for reflux.
When we heat a liquid at reflux, we use a condenser – this long piece of glassware,
which has a water stream running through an outside chamber – to cool down any vapour
leaving the reaction flask and condense it back into a liquid.
The liquid then drops back into the flask.
Now that we have refluxed for a while, the reaction should be finished.
In order to quench the reaction and stop everything going in reverse back to the starting material,
we can add ice.
Adding ice makes the aspirin product precipitate, and we can see a solid at the bottom of the
flask.
We are going to filter the mixture in this flask to get our solid product.
I’m going to fold this filter paper so that it fits into a funnel – this is known as
a fluted filter paper.
By folding the paper this way and that, we can make a sort of concertina that makes the
paper into a cone.
This speeds up the filtration massively since there is so much surface area for the liquid
to filter through.
So, we have our solid.
But what else might be in it?
To purify our compound, we can recrystallize it.
This means we will dissolve everything into a hot solvent.
The product and all the impurities will dissolve in the solvent while it is hot, but when we
cool the solvent down again, the product will crystallise out and leave the impurities in
solution.
In this case we are using a mixture of ethanol and water to recrystallize our product.
We can test the purity with thin layer chromatography.
What we will do here is put some small spots of the starting material and the product on
a sheet of foil which is covered in a silica powder.
When we put this sheet into some solvent, the solvent will slowly creep up the silica
like water soaking into tissue.
The solvent will carry our chemicals up the plate – but at different rates depending
on the polarity of the molecules.
We can then use a stain to see how far our spots have moved up the plate – and how
many spots there are!
One spot, in theory, means a pure product.
Some materials might not show up on a TLC plate.
Another test of purity is to use infrared spectroscopy.
IR radiation interacts with the bonds of molecules (depending on the wavelength of the radiation),
so molecules with different functional groups will absorb different amounts of IR radiation.
We will get a different IR spectrum for each molecule we examine.
Here, we have the IR spectrum of the starting material.
The big peaks we will look at are the carboxylic acid here – a very sharp peak – and the
alcohol peak here – a very broad peak.
After the reaction, our product looks like this: we still have the very sharp peak for
the carboxylic acid, but we have lost the big broad peak for the alcohol – now we
have another sharp peak next to the carboxylic acid.
The peaks are close together and look similar because they are caused by similar functional
groups.
One carboxylic acid and one ester!
We have esterified salicylic acid to make our product, aspirin!
We have done this reaction on about a 1 gram scale.
This reaction is actually scaled up massively in the real world and 35,000 tonnes are made
each year.
Next time you’ve got a headache, and you’ve run out of aspirin tablets – you know what
to do!
Find a willow tree… and a chemistry lab.