salicylic acid

Chemistry A level revision resource: Making aspirin in the lab

Aspirin has come a long way from its humble origins. The willow bark from which the active component, acetylsalicylic acid, can be isolated has been used since before the time of ancient Egypt to brew teas and chew as a fever or pain remedy. It was not until 1897 that a chemist, who wanted to help his father with joint pain, produced a method to make the pure and optimised form of aspirin which sells 16 tonnes of tablets in America alone each year.

Making aspirin

As part of your A Level course you will be expected to prepare an organic solid, so even if you are not specifically making aspirin, a lot of the techniques and safety considerations are similar. The first step of the synthesis involves mixing salicylic acid with ethanoic anhydride. This reaction would be very slow without a catalyst. In this case just a few drops of sulphuric acid added to the mixture means that there are some free positive hydrogen ions in the solution which can bind to the ethanoic anhydride. This makes it more active and speeds up the reaction. 

To ensure the reaction goes as quickly as possible we must heat it. But how do you heat a reaction to boiling point without losing most of the product by evaporation? A reflux condenser is a long-sealed glass tube surrounded by a water jacket through which cold water is pumped. The hot gas produced rises up the tube and is cooled by the running water causing the gas to condense and return to the mixture as liquid thus saving your precious aspirin.

After the reaction has been heated for some time, we add ice to stop it. Slowing or stopping a reaction is known as quenching. The ice has another role to play. Aspirin is only slightly soluble in water so if you add it to the reaction mixture the product would rather clump together than be in the water phase. This causes the solid to begin to form or ‘precipitate.’ We then need to separate the solid from the rest of the unwanted mixture. This can be done by pouring the entire contents of your flask through a fluted filter. Fluting increases the surface area available for the liquid to seep through, speeding up the filtration. 

Before aspirin can be used it needs to be as pure as possible. No one wants to put a tablet in their mouth if there are some unknown impurities in it! There are various ways of finding out how clean your final product is, and these are discussed in the video below.

The acylation of salicylic acid

A man standing with a blue shirt
Acylation of salicylic acid

Working safely in the lab

One of the biggest considerations that any scientist has is safety. Is what you are doing safe for you and everyone around you? If not what measures can we put in place to reduce the risk?

Lets start with the obvious things. If you think anything is dangerous or spot someone or something that is dangerous it is your responsibility to let people know immediately. By simply telling someone about the problem you could save a lot of pain or damage. Your partner could be adding the wrong chemicals; tell them to prevent unnecessary explosions! You might spot some broken glass or have dropped something yourself, don’t be embarrassed or worried, just tell the person in charge so it can be dealt with safely and quickly.

Wearing personal protective gear such as lab coat and safety goggles is essential in a lab environment. Even if what you’re using is safe, people around you may be doing something with different hazards that you are not aware of. Unfortunately, we are all human and spills and splashes can be common! Lab coats are made out of a chemical resistant material which allows liquids to run off easily so they are less likely to settle on or burn through your sleeve.

Then there are the chemicals themselves. Every company that supplies a chemical is required by law to provide a material safety data sheet or MSDS. This includes details such as the dangers of using that chemical, what to do in an emergency and how to dispose of it. In this experiment there are a number of hazards to consider. Salicylic acid and sulphuric acid are both corrosive and a skin irritant at high concentration. You should only be using a small amount but wear gloves to protect your hands. As well as being an irritant, ethanoic anhydride is known to combust easily so keep it away from open flames.

However, we can never know whether there are any other dangerous side products being formed so always treat every chemical and mixture with care.

Laboratory Confessions

In the Laboratory Confessions podcast researchers talk about their laboratory experiences in the context of A Level practical assessments. In this episode we look at water baths, heaters, and melting point apparatus, thin-layer chromatography and the purification of organic solids.

How does aspirin work?

As long as you remember to be aware of your surroundings and other people, follow the instructions given carefully and ask if you have any questions you should end up with a beautiful, pure sample of aspirin (though don’t give it a try - just in case!). This unassuming white powder has a number of effects on the body which can be utilised for medical purposes. When you have a headache aspirin helps deactivate the enzyme which is produced when your body is in pain. This prevents pain messages being sent to your brain. Aspirin is also used in patients with coronary heart disease as it stops platelets from clumping together. This prevents blood clots from blocking the diseased arteries so all in all, a very useful thing to know how to make! And remember, even if you’re not making aspirin, all the techniques and safety tips above carry over to other experiments where you have to make an organic solid.