Synthesis of Aspirin
Sypnopsis In this experiment, acetylsalicylic acid was synthesized from the acidification of salicylic acid and acetic anhydride. The objective was to convert a specific amount of salicylic acid into the same amount of aspirin that was high in purity. Furthermore, the other objectives were to enable students to conduct the synthesis of aspirin, reinforce skills or recrystallisation and the technique of melting point determination. The amount of each compound should be the same because there is a 1:1 ratio between them.
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The purity of the synthesized aspirin was measured by determining its melting point and percent yield. Soluble impurities increase the range over which a compound melts and often decreases its overall melting point temperature1. If the experiment went as expected, a pure sample of aspirin with a high percent yield would have been obtained. The percent yield obtained was 56. 1% and the melting point was 134. 7- 136. 8. Introduction Aspirin (acetylsalicylic acid) is a versatile drug that is consumed in huge quantities worldwide.
It is a non-steroidal anti-inflammatory drug (NSAID) with a wide range of physiological effects. The first discovery of aspirin occurred in England, in 1963, believing that the bark of willow trees with a beneficial effect in alleviating distress due to fevers, aches, and pains2. Salicylic acid was later extracted from the willow bark and it proved to be an active ingredient. Salicylic acid was synthesized from basic starting materials by 1860 which was helpful to the medicinal field but there were some problems2. Salicylic acid turned out to be irritating to the membranes of the throat, mouth, and stomach.
The product had a high acidity of the compound but fortunately, successful amendments were done, namely, the replacement of the acidic phenolic hydrogen atom with an acetyl group. In 1893, a effective synthesis of acetylsalicylic acid was created, patented in 1899, commercialized under the trade name of ‘aspirin’ by the Bayer Company in Germany3. The name ‘aspirin’ was invented by the chemist, Felix Hoffman, who originally synthesized acetylsalicylic acid for Bayer3. At very low doses, aspirin is used to treat and prevent heart attacks and blood clots.
At higher doses, it is used as an analgesic to reduce pain and as an antipyretic to reduce fever. At very high doses, it is an effective anti-inflammatory agent used to treat rheumatic fever, gout and rheumatoid arthritis. It is also an anticoagulant, it dissolves corns and calluses, and it provokes loss of uric acid (a toxin) but promotes retention of fluids in the kidneys. It kills bacteria and induces peptic ulcers. When ingested, acetylsalicylic acid remains intact in the acidic stomach, but in the basic medium of the upper intestinal tract, it hydrolyzes forming the salicylate and acetate ions.
When ingested, acetylsalicylic acid remains intact in the acidic stomach, but in the basic medium of the upper intestinal tract, it hydrolyzes forming the salicylate and acetate ions. The exact mechanisms of its pharmacological actions are still under study. In many plants, salicylate can induce flowering. However, aspirin may cause side effects for example, nausea, vomiting, stomach pain and heartburn. Theory Aspirin is prepared from salicylic acid and acetic anhydride with the help of an acid catalyst. Concentrated sulphuric acid acts as a catalyst.
After preparation, the product is purified. This is especially important for chemicals that are used as food additives and pharmaceuticals. The most common method of purifying solid organic compounds is by recrystallization. When an impure solid compound is dissolved in a solvent, it is then allowed to slowly crystallize out as the solution cools. As the compound crystallizes from the solution, the molecules of the other compounds dissolved in solution are excluded from the growing crystal lattice, giving a pure solid. Crystallization of a solid is different from a precipitation of a solid.
In crystallization, there is a slow, selective formation of the crystal framework resulting in a pure compound. In precipitation, there is a rapid formation of a solid from a solution that usually produces an formless solid containing many trapped impurities within the solid’s crystal framework. For this reason, experimental procedures that produce a solid product by precipitation always include a final recrystallization step to give the pure compound. Figure: The esterification of salicylic acid by acetic anhydride5. The -OH group of salicylic acid that reacts with acetic anhydride to form an ester-like product.
The carboxylic acid group of salicylic acid remains unchanged. Acetic anhydride is used because it is cheap and forms a by-product, acetic acid. Acetic acid is non corrosive and can be recovered to produce more acetic anhydride. Procedure 2. 4g of salicylic acid was weighed and poured into a 100ml conical flask. The actual weight was then recorded. In the fume hood, 6ml of acetic anhydride was added to the salicylic acid in the flask. To this mixture, 3 to 4 drops of sulphuric acid was added, swirled to mix and then heated in a water bath for 10-15 minutes to complete the reaction.
The mixture was removed from the water bath while it was still hot and then 1ml of distilled water was added from a dropper carefully to decompose the excess acetic anhydride. An additional 40ml of cold water was added and stirred with a stirring rod to induce crystallisation. The crude product was collected by suction filtration and washed with a little cold water. The crude product is relatively impure; hence it was needed to be purified by recrystallisation. A solvent suitable for this recrystallization process would be a mixture of ethanol and water.
The crude product was dissolved in approximately 5ml of ethanol in a 100ml conical flask and heated on a hot plate. To the solution, 30ml of hot distilled water was added in. The solution was then warmed till all solid has dissolved. The solution was allowed to cool. A clean, dry watch glass together with a filter paper was weighed and the weight was recorded. The recrystallised product was obtained by suction filtration using the weighed filter paper. The crystals and filter paper was transferred onto the weighed watch glass and dried in to the oven (100°C) for 15 to 20 minutes.
The crystals, filter paper and watch glass are then placed in desiccators for 5 to 10 minutes. The dried crystals, together with the filter paper and watch glass were weighed. The weight was recorded and the weight of dried, recrystallised aspirin was calculated. The expected yield of aspirin was calculated from the amount of salicylic acid used. The percentage yield of dried, recrystallised aspirin was also calculated. The melting point of aspirin was determined. Results Mass of salicylic acid Mass of filter paper and watch glass
Mass of dried, recrystallised aspirin, filter paper and watch glass Mass of dried, recrystallised aspirinMass of dried, recrystallized aspirin, filter paper and watch glass Mass of filter paper and watch glass Percent yield Number of moles of salicylic acid used (mol wt of salicylic acid = 138) Expected number of moles of aspirin Expected mass of aspirin Percent yield Melting point Temperature Range134. 7 – 136. 8 Appearance The crystals are white and shiny, shaped needle-like. The crystals resemble glass wool. Discussion
The results and calculations shows how much acetylsalicylic acid was synthesized compared to how much salicylic acid was used. Since there is a 1:1 ratio between the two, then the amount of salicylic acid used should be equal to the amount of acetylsalicylic acid recovered at the end of the experiment. As the result shows, this is not the case, for 1. 83g of acetylsalicylic acid was harvested when 2. 40g of salicylic acid was used. This could have some effect on the purity; for the melting point of the harvested acetylsalicylic acid which is 134. 7 – 136. 8 was not very close to the theoretical melting point.
The percentage yield is only 56. 1%. Since the final product was determined not to be very pure, it is highly likely that less than 1. 83g of the product was acetylsalicylic acid. Further analysis of the product will have to be done to determine exactly how pure the product actually was. There were several problems that could have contributed to the low purity or mediocre percent yield. When dissolving the initial amount of salicylic acid in the solution of acetic anhydride and concentrated sulphuric acid, it did not completely dissolve into the solution, even when it was heated. There could also be loss of product on the filter paper.
Some crystals would be stuck onto the filter paper and this might have affected the mass of the crystals. The sample may not have been completely dried out before weighing. This could have a slight impact on the results of the overall yield of aspirin because it was possible that not all of the salicylic acid was synthesized. To determine if this affected the synthesis of aspirin at all, the experiment should have been ran a second time to see if the same thing occurred. Conclusion The experiment did not go completely as expected. The reaction yielded only 56. 1% of its expected product.
The purity of the aspirin was not very high, because there might be impurities in the acid used. Reference 1. Henry, Dr. Geneive. 2004. Susquehanna University. Synthesis of acetylsalicylic acid (aspirin). 2. The history of Aspirin, accessed 25th May 2009, 3. History of Aspirin, viewed on 13th Dec 2010, ; http://inventors. about. com/library/inventors/blaspirin. htm; 4. Theory, accessed 15th Dec 2010, 5. Dr. Carman. 2002. University of Nevada, accessed, 15 Dec 2010, ; http://tooldoc. wncc. nevada. edu/aspirin. htm; 6. Synthesis of Aspirin, accessed 13th Dec 2010, 7. Aspirin, accessed 14th Dec 2010,