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Aspirin is a widely used drug which helps reduce fevers as well as reduce pain. It was originally just salicylic acid, which can be found naturally in the bark of willow trees. It is one of the oldest natural medicines used to treat fevers, illnesses as well as reduce pain. However, it was harsh on the stomach taking the acid by itself, which is why chemists in the 19th century began experimenting to find a softer version which does the same thing.
To create aspirin, salicylic acid is often mixed with an excess of an anhydride, which synthesises to create aspirin along with an acid.
Salicylic acid is converted to aspirin through the process of esterification. This esterification reaction occurs between the acid and the compounds containing a hydroxyl group and only takes place in acidic environments. Hence, the use of a concentrated acid is required.
The mixture is then heated gently, before being it is cooled to form crystals. Aspirin has very low solubility in cold water, so this process removes the soluble acids but not the aspirin. The aspirin is then purified by recrystallisation, where the impure crystals are dissolved in a small volume of hot ethanol. Water is then added, and the mixture is yet again cooled. The aspirin will recrystallise, and the unreacted salicylic acid remains dissolved in the solution. The liquid is filtered out using a Buchner funnel and the crystals left out to dry. To test the purity of the aspirin, a drop of low concentrated iron chloride is able to react with the product. It will change to a purple colour in the presence of salicylic acid and remain the same yellow colour in the presence of aspirin.
Lab Vs Industrial
In order to manufacture aspirin industrially, it is important to take in consideration of the cost of the synthesis and the physical properties of the materials involved (The Manufacture of Aspirin, n/a). There are two main ways to synthesise aspirin, using acetic anhydride or acetyl chloride. Both ways have its pros and cons, but there is an obvious preferred method. If you were to use 400kg of salicylic acid, you would need 200L of acetyl chloride or 400L of acetic anhydride.
It is much cheaper to be using acetic anhydride than acetyl chloride. The densities of the two materials are also very similar, making the price estimates pretty accurate of the actual cost.
As there will be the same amount of salicylic acid and toluene in both reactions, those prices can be ignored. Acetic anhydride will cost approximately $430, while acetyl chloride costs $3900.
In addition to this, the Material Safety Data Sheet reveals that acetyl chloride is very toxic and dangerous. It also produces hydrochloric acid in large quantities and has many undesirable properties, which is not wanted.
Synthesising aspirin in a lab is very much similar to industrial. Salicylic acid is reacted with an excess of acetic anhydride, or acetic acid, as that is less toxic than acetic anhydride and safer to use. The equipment used will be much simpler and less advanced than industrial production companies, but the method still has the same concepts in them.
Importance of Reaction
Aspirin is important because it is an anti-inflammatory drug and a common pain reliever for minor pains and also reduces fevers. Although new pain killers are now produced that are much more effective, aspirin can be used long-term in low doses to prevent blood clots, stroke and heart attacks.
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