Chemistry in Aspirin
Before we have discovered the appearance of the Aspirin, we found the Salicin as a natural substance and can be used as a painkiller. After the discovery of the Salicin few chemists twist the structure of the Salicin a bit and formed the salicylic acid which is even a better and more effective painkiller. (Figure 1) However, there is a problem because Salicylic acid is an acid thus it can irritate the stomach of the patient who took the drug. In order to solve this problem, the chemists synthesized another type of acid to solve this problem and this acid is called acetylsalicylic acid which is also called as Aspirin.
Figure 1 From Salicin to Salicylic acid
At first the Chemists were trying to synthesise the Acetylsalicylic acid by using the Salicylic acid and the Ethanoic acid, but they soon found a problem, the reaction will be super inefficient, because the reaction keeps going backwards and here is the reason: Because the reaction between the Salicylic acid and the Ethanoic acid is a condensation reaction which means they will form water by combining the OH- ions and the H+ ions. Meanwhile, the reaction is also forming the Acetylsalicylic acid. According to the structure of the Acetylsalicylic acid, there is Ester bond in it, and as we know that ester bond can be broken down by the water molecules thus in the solution of the product water molecules are keep breaking down the Acetylsalicylic acid by opening up the ester bond and form a backward reaction. (Figure 2) This makes the method becomes slow and pointless.
Figure 2 This is the reaction happens between the Salicylic acid and the Ethanoic acid and the backward reaction will take place as well
Soon the Chemists come up a new method, use the anhydride Ethanoic acid instead of the normal Ethanoic acid. This small change makes the reaction much easier because the byproduct will become acetic acid instead of the water and this will avoid the backward reaction takes place which makes the reaction much faster and more efficient. (Figure 3)
Figure 3 Salicylic acid reacts with Anhydride Ethanoic acid to form Acetic acid acetylsalicylic acid
Apart from the basic structure of the Aspirin the Chemists always try to play around with it and form some other types of Aspirin. For example, in order to make the aspirin more soluble, the Chemists change the H+ into Na+ which forms a Sodium salt which definitely has a higher solubility than the normal Aspirin. (Figure 4) The reason behind this is because when normal aspirin dissolves into water it can only form hydrogen bonds with water molecules. However, for sodium salt aspirin there are ionic bonds formed between sodium ion and oxygen ion, thus it is much easier to form permanent dipoles between ions and polar water molecules compare to form hydrogen bonds between two polar molecules; and here is my sodium salt aspirin is more soluble compared to the normal aspirin.
Figure 4 The Sodium salt of Aspirin is forming
Here is another example of the variation structure which the Chemists have created. This one is called Polyaspirin. As we can tell from its name it is a polymer type of Aspirin. In each single isomers there are two Die acid one at the front one at the back, and the Die acid means there is hydroxyl group in them; in between them we have got a long chain of carbons, on both ends of the carbon chain there are two hydroxyl groups as well one on each side. There will be condensation reactions take place between the carbon chain and the acids which will link them together into a longer chain and at the same time there will be two water molecules have been formed. On the other side of the acid, there will be another acid which has the same structure. In order to link them together, the condensation reaction will take place. (Figure 5)
Because it is a polymer so when it goes into the body it takes a long time to break down thus we can control the release of the drug which means we have got a controlled releasing painkiller.
Figure 5 The formation of Poly Aspirin
So how actually Aspirin works in our body?
Well once the Aspirin went through our stomach and goes into out intestine it starts to react with the water, when the ester bond meets water it is going to break and the Salicylic acid and the Ethanoic acid are going to be formed. As I said the Salicylic acid is acting as the painkiller so once the Salicylic acid is released the painkiller effect is going to show.