Aspirin: Better Version of Salicylic Acid
Sumerians 4,000 years ago identified the pain remedies of the willow tree and Mesopotamians used the same extract to treat a variety of maladies. In fact, Hippocrates, known as the father of medicine, recommended chewing on willow tree bark to patients suffering from fever and pain. These early people found salicylic acid, a natural substance that has been used for the last millennia. However, it was only during the 19th century when scientists discovered aspirin, a “better” salicylic acid.
In 1895, Felix Hoffmann, a chemist from the Bayer Company, approached the production of aspirin with a special interest. His father, suffering from joint inflammation and pain, was talking salicylic acid, but he could not consume the drug without vomiting. Hoffman found a way to chemically change salicylic acid through the modification of the hydroxyl group on the benzene ring. The unintentional key to his discovery was that this chemical change provided a new molecule that the body could break down without severe gastrointestinal pain. Once ingested, the new molecule was converted back to salicylic acid in the stomach, liver, and blood, therefore providing the same effects without the undesirable side effects.
Modern aspirin can be attributed to the same technique: a delivery system for a natural product that has been in medical use for thousands of years. However, aspirin does not stop the problem causing the pain in the first place. It works by sticking to the cyclooxygenase 2 enzymes, which are responsible for creating a chemical called prostaglandin. A dearth of prostaglandin equates to fewer pain signals being sent to the brain. Though these days, aspirin is not only used for pain relief. It also prevents heart attacks by stopping platelets from sticking together and forming artery-blocking clots.
The first step to purifying the crude aspirin product was to add sodium bicarbonate to it. The sodium bicarbonate, a base, reacted with the crude product, an acid, and caused an acid-base reaction. The HCO₃ pull off the H from the product while Na+ replaces its spot, so the product is now charged and therefore dissolves when mixed. Water and carbon dioxide, which were lost through the fizzing bubbles, was also created.
When the solution was filtered, the salicylic acid was left behind as it was an unreactive solid polymer. Following that, hydrochloric acid was added to remove the filtrate of sodium bicarbonate because of the negative chlorine ions attached to the positive sodium ions. Plus, the acetic acid was removed when it was dissolved by cold distilled water. This reaction required the water to be cold to ensure the aspirin did not dissolve in the solution, as the aspirin is much less soluble compared to acetic acid. Filtering this solution again only removed the hydrochloric acid as it merely helped the aspirin to precipitate.
This leaves the product, water, and acetic anhydride, which was responsible for the vinegar odor. Acetic anhydride was removed through the final recrystallization step using hot ethyl acetate. The ethyl acetate, when heated, combined with the acetic anhydride but did not dissolve the aspirin. The acetic anhydride, ethyl acetate, and water were removed through evaporation, leaving only pure and odorless aspirin behind.
Conclusion
The first objective to create aspirin was met as 0.99 grams of acetylsalicylic acid was the end product. Although 0.99 grams is not close to the theoretical yield of 2.61 grams, ultimately, pure aspirin was synthesized. The ferric chloride tests proved the purity of the aspirin with its yellow color, signaling that minimal salicylic acid, an impurity, remained.
Ferric chloride forms highly colored solutions with phenolic compounds like salicylic acid. So when the ferric chloride is added to the product, it is obvious if the product mixture contains any salicylic acid by the color of the solution. Solutions containing salicylic acid are dark purple and ones without are yellow. After the addition of the ferric chloride, the salicylic acid solution in tube #2 turned dark purple, and both the crude and purified aspirin solutions were yellow. Because both the aspirin solutions remained yellow, there was not any unreacted salicylic acid present in the product to turn it purple.
The final objective was to achieve a yield within reach of the maximum yield. Unfortunately, many factors such as losing partial product after every relocation from beakers, funnels, and filter paper; washing the product with suboptimal water temperatures, and the rush caused by limited time periods might have contributed to a low yield. Ultimately, only 37.95% of the potential product was synthesized.
