- Theory
- Formation of Aspirin
Aspirin is formed in a process known as esterification. In this process, a carboxylic acid is reacted with an alcohol in the presence of a strong acid, such as sulfuric acid. Esterification is a reversible process. However, to form complex esters such as aspirin, alcohol is reacted with its anhydride. In the case of formation of aspirin, salicylic acid is reacted with acetic anhydride to form acetylsalicylic acid, also known as aspirin.
Acetic anhydride is used as it as cheap and it is readily available. Drug manufacturers often use acetic anhydride to acetylate alcohols and phenols. This is how they prepare aspirin from salicylic acid.
- Theory of Recrystallisation
When purifying solids, a characteristic of the solid which is important is its’ solubility in a different solvent. When crystals are formed from a reaction, such as the reaction of an alcohol with an acid, there is the likelihood of impurities being trapped on the crystals. Even though washing it with cold water may eliminate some of the purities, this does not ensure that the crystals are fully pure. This is where recrystallisation comes into practice.
Organic solids are generally more soluble in hot solvents as compared to cold solvents. Just enough hot solvent is added to dissolve an organic solid, which would form a saturated solution. When the solution cools, crystals would form in the funnel. However, there would be a slight deviation of the mass of crystals formed compared to the theoretical value of crystals formed. This is due to the solid dissolving in the cold solvent, which cannot be recovered.
Having different solubilities is an important factor in separating the crystals from its’ impurities. For impurities with very different solubility from its crystal, they can be separated by hot gravity filtration. As for impurities that are just as soluble as the crystal, recrystallization has to be carried out, or as an alternative, a different solvent may be used.
- Procedure
4.1 Preparation of Aspirin
Firstly, 2.4g of salicylic acid was approximately weighed out in a dry 100ml conical flask. The actual weight was recorded. 6ml of acetic anhydride was then added into the flask in the fume hood. The mixture was then swirled after adding 3 to 4 drops of concentrated sulfuric acid. To complete the reaction, it was heated in a water bath for 10 to 15 minutes.
After removing the flask from the water bath, 1ml of distilled water is added carefully with a dropper while the mixture is still hot. This is to decompose the excess acetic anhydride. 40ml of cold distilled water was then added into the mixture and left to stand to induce crystallization. Afterwards, suction filtration was then used to collect the crude product, which was washed with a little cold water.
4.2 Recrystallisation of Aspirin
The crude product which was collected by suction filtration had to be purified further by recrystallisation. The crude product was first dissolved in approximately 5ml of ethanol in a 100ml conical flask and warmed. 30ml of hot distilled water was then added into the solution. The solution was then warmed until the solid dissolves completely. The solution is then left to cool.
A clean, dry watch glass along with a filter paper was weighed, and the weight was then recorded down. Using the filter paper that was weighed, the recrystallized product was obtained using suction filtration. The crystals and filter paper was then transferred onto the weighed watch glass. The watch glass was placed in the oven for 15 to 20 minutes to dry under 100oC temperature.
Afterwards, the watch glass was placed in the desiccator for 5 to 10 minutes. The crystals, filter paper and watch glass were then weighed and recorded. The weight of the recrystallized aspirin was calculated. The expected yield of aspirin was then calculated from the amount of salicylic acid that was used earlier.
- Results and Calculations
- Mass
Mass of salicylic acid (a) = 2.39g
Mass of filter paper and watch glass (b) = 29.43g
Mass of dried, recrystallized aspirin, filter paper and watch glass (c) = 30.66g
Mass of dried, recrystallized aspirin (d) = (c) – (b)
= 30.66 – 29.43
= 1.23 g
- Percent Yield
Number of moles of salicylic acid used (e) = 0.0174 mol
Expected number of moles of aspirin (f) = 0.0174 mol
Expected mass of aspirin (g) = no. of moles of aspirin x Mr of Aspirin
= (e) x 180
= 0.0174 x 180
= 3.132g
Percent yield = x 100%
= x 100%
= 39.27% yield.
- Melting Point
Temperature range = 133.4oC to 136.9oC.
- Appearance
The appearance of aspirin is a white crystalline powder.
- Discussion
From the experiment that was carried out, the temperature range of aspirin’s melting point was found to be between 133.4oC and 136.9oC. The theoretical melting point of aspirin is shown to be 134oC. The slight deviation in the temperature range may be due to the presence of impurities in the crystals. As the impurities cannot be completely eliminated from the final product, it may affect the results that were obtained in the experiment. As for the melting point, the presence of impurities would lower the melting point of a sample.
Another experimental error which would have affected the results that were obtained was due to the use of sulfuric acid for the experiment. Sulfuric acid, a strong acid, would react strongly with organic molecules. This would affect the percentage yield of aspirin. A good substitution for sulfuric acid would be phosphoric acid as it is a weak acid; therefore it would react less strongly with the organic molecules and not affect the percentage yield at the end of the experiment.
- Conclusion
The experiment was unsuccessful as the percentage yield of aspirin that was obtained fell below the expected percentage yield. From calculations, the expected mass of aspirin that should be obtained was 3.123g. However, the mass obtained at the end of the experiment was only 1.23g, making the percentage yield of aspirin only 39.27%. This is probably due to the impurities that were still present in the solution, causing it to affect the yield obtained.
- References
Brown, W.H. and Foote, C.S., 2002. Organic Chemistry. 3rd ed. USA: Thomson Learning
McMurry, J., 2003. Fundamentals of Organic Chemistry. 5th ed. USA: Thomson Learning.
McMurry, J. and Castellion, M., 1995. Fundamentals of General, Organic and Biological Chemistry. 2nd ed. New Jersey: Prentice-Hall, Inc.
Makie, RK. Smith, DM and Aitken, RA. , 1995. Guidebook to organic synthesis. 2nd Ed. England: Longman Group UK Limited 1990.
Ouellete, R.J. and Rawn, J.D., 1996. Organic Chemistry. Upper Saddle River, New Jersey 07458 (United States of America): Prentice Hall
Allen, MS.,Barbara, AG. And Melvin, LD.,2004. Microscale and Miniscale ORGANIC CHEMISTRY laboratory Experiments.2nd Ed. New York: McGraw-Hill
