30 Jan 2018
Aspirin is a drug which is widely used today to deal with the symptoms of many minor illnesses ranging from general relief from aches and pain; muscle, tooth, headaches, and arthritis; to prevent blood clotting and reduce the risk of stroke and heart attack; lower fever and potentially prevent certain cancers.
History of Discovery
Aspirin has a great history of development which could be considered to have begun in the 3000BC where it was recorded that remedies from the willow tree were the most useful for the treatment of many ailments. Though it was not for another 1500 years that it was more specifically noted in the Elbers papyrus for its usage for pain relief and inflammation. For the next few thousands of year the willow tree bark was used for in much the same way, although it would not be until 1828, 4800 years after the discovery of its usefulness, that the active ingredient of willow bark was isolated by the German chemist Joseph Buchner who managed to obtain salicin, from the latin Salix, crystals. At the time the crystals were considered fairly pure but Henri Leroux, of France, managed to obtain them in purer forms a year later. Several other chemists/pharmacists made discoveries in the years following, all of which we now know contained Salicylic Acid. It was the Italian Chemist Raffaele Piria who managed to devise a method of containing a more concentrated acid form of salicin from the willow extract, which he named Salicylic Acid (SA).
After these discoveries the usage of Salicylates, esters and salts of salicylic acid, became more widespread by doctors for treating pain, fever and inflammation but with limited success due to adverse side effects such as gastric irritation to the lining of the mouth and stomach. It was not until around 1897 that Felix Hoffmann of Bayer Pharmaceuticals developed a method of synthesising acetyl salicylic acid after he had be tasked to find a substitute to SA, that did not cause gastric irritation, 4 years earlier; by Eichengrunalong with Dreser(the pharmacist who developed heroin).
Early clinical trials found aspirin to be a potent treatment and today it is the best known and most widely use medicine in the world where it is estimated that over 100 billion tablets or 40,000 tonnes are consumed each year.
Unsurprisingly shortly after clinical trials finished acetyl salicylic acid (ASA) became the over the counter drug under the name of Aspirin, though in some countries Aspirin is no-longer holds trademark and has become a generic term for ASA.
The method of synthesis today involves the reaction of salicylic acid and acetic anhydride in the presence of an acid catalyst such as sulphuric acid or phosphoric acid to give the products ASA and acetic acid. The process is a form of esterification reaction and thus is reversible.
Salicylic acid is placed into a conical flask along with acetic anhydride while inside a fume hood wearing goggles and gloves as acetic anhydride is an irritant and the vapour is harmful is inhaled or reaches the eyes. The acid should then be added in a small quantity and the flask should be swirled to mix thoroughly. The mixture should then be warmed in a beaker of water for around 10 minutes to increase the rate of reaction. Alternatively the mixture could be refluxed over heat. The mixture can then be left to cool, and dried leaving an impure sample of ASA which can be purified. (Continue to Purification)
The oxygen atom of the phenol group initiates a nucleophilic attack on a carbon in a carbonyl bond of the acetic anhydride. As a result the oxygen of the phenol group becomes positively charged and the electrons forming the carbonyl bond in the acetic anhydride are pushed onto the oxygen of the bond and thus becomes negative. The bond between the hydrogen and oxygen in the phenol group breaks where the oxygen takes both electrons and becomes stable. The proton is ejected from the structure and the carbonyl bond then reforms as the oxygen delocalises the electrons it gained previously and becomes stable.
The process of purification is relatively simple and is done by the process of hot and cold filtering.
Distilled water should be added to the impure ASA crystals. Using the properties of ASA and the fact that it is soluble in hot water but virtually insoluble in cold water, any insoluble impurities can be removed by heating the mixture of water and ASA crystals to the point that the ASA crystals are just dissolved in the solution. This hot solution should then be filtered into a clean beaker. Any impurities which are insoluble are caught by the filter paper but the ASA which is soluble passes though. The hot filtered mixture should then be put into an ice bath to recrystallize it. When cold, using a Hirsch funnel, filter paper, a side arm flash, and a vacuum pump the mixture can be filtered once again. As the ASA will be crystalline it will be stopped by the filter paper and thus any soluble impurities are removed. What is left in the filter paper can be washed with distilled water and then dried. Providing the synthesising reaction was done to completion, the dried crystals should be a pure form of ASA or Aspirin.
Method of Action of ASA
As with many drugs which were developed early in the 20 century, the actual method that the drugs worked inside the body was not known. In the 1970’s a British Scientist Professor John Vane discovered that it blocked the enzyme Cyclooxygenase which is needed for the production of hormones related to the body’s natural inflammatory and pain responses. Vane won the Nobel Prize for Medicine for this work. (Aspirin-foundation.com, n.d.)
One enzyme involved in inflammation is cyclooxygenase (COX). COX is responsible for the formation of a group of inflammatory mediators known as prostaglandins. COX activates a chemical known as thromboxane A2 that causes platelets to stick together to form a 'plug' over the damaged area. The aggregation of platelets (plug), in concert with the clotting process, results in a fibrin clot which stops bleeding and aids repair of the blood vessel.
Aspirin inhibits COX, in turn stopping the formation of prostaglandins; hence aspirin acts as an anti-inflammatory agent in this process. [In regards to thromboxane A2] thereby reducing the ability of platelets to aggregate. This is why aspirin is known as a ‘blood thinner’ or anti-platelet agent.
As previously stated Aspirin inhibits COX which inhibits prostaglandins. Prostaglandins are produced by cells in the lining of the stomach and form majority of the barrier that protect the stomach wall from the concentrated Hydrochloric acid it contains. As a result ASA can result in stomach ulcers. Other side effects would likely to be irritation of the gut, indigestion and nausea.
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