Preparation and Recrystallisation of Aspirin

The aims of this experiment are to conduct the preparation of aspirin, perform recrystallisation to purify the compound using suction filtration and reinforce the skill of determination melting point of recrystallised compound. Aspirin is a crystalline compound derived from salicylic acid. It is widely used in assuagement of fever and as a wonderful painkiller and anti-inflammatory drug and, to prevent strokes and heart attacks. Esterification and recrystallisation are the two main process needed. Esterification which is the combination of a carboxylic acid and alcohol to form an ester, is involved in the synthesis of aspirin.

Recrystallisation is a process of purifying solid organic compounds by the difference in solubility at different temperature. The procedure is being separated into two parts, 1) Preparation of Aspirin and 2) Recrystallisation of Aspirin. 1, we need to put salicylic acid in 100ml conical flask, then add acetic anhydride and followed by conc. sulphuric acid. Secondly, we need to heat the mixture until the reaction is complete and add distilled water to it while it is still hot. Lastly, add an additional of cold water to the mixture and collect crude products by suction filtration. 2, we need to dissolve crude product in ethanol. Secondly, add hot distilled water to it and allow it to cool.

Lastly, perform suction filtration, put crystal on watch glass, put into oven then desiccator. And Crystal will be obtained in the end of the experiment. Overall, the experiment is successful. The percent yield and the melting point I had falls within the range of the theoretical results. The crystal has a light flaky appearance and it is white in colour. With a narrow range of melting point, it shows that my product do not contain much impurities.

1. OBJECTIVE OF EXPREIMENT

The objectives of this experiment are to conduct the preparation of aspirin and perform recrystallisation to purify the compound using suction filtration. Besides, this experiment also requires reinforcing the skill of determination melting point of recrystallised compound.

2. INTRODUCTION

2. 1 Definition of aspirin.

“Aspirin is a white, crystalline compound, CH3COOC6H4COOH, derived from salicylic acid” and is commonly use in tablet form to relieve pain, reduce fever and inflammation. It is also use as an anti platelet agent (Yahoo! Inc, 2009). The chemical name of aspirin is acetylsalicylic acid (Aspirin. foundation 2010). It has analgesic, antipyretic, and anti-inflammatory properties, and belongs to the class of non-steroidal anti-inflammatory drugs (Hubpages 2010). An -OH group (an alcohol) and a carboxyl group -COOH (an organic acid) are the functional groups that found in aspirin and the alcohol group undergoes esterification to form an acetylated ester (Yahoo! Inc 2010).

2. 2 History of Aspirin.

Patients in 400BC suffering from fever and inflammation were recommended to bite on willow bark (figure 1), a natural form of salicylic acid. It used to cure fever, pain, headaches and inflammation of people in China and Europe. In 1853, acetylsalicylic acid was being produced by chemist Charles Frederic Gerhardt (figure 2) as he reacted acetyl chloride with sodium salicylate(Wikipedia 2010). Making of aspirin was not successful. But, 44 years later in 1897, a more chemically unmixed, unchangeable and tasty type of aspirin was produced by a young chemist who worked for Bayer called Felix Hoffman (Figure 3). It was also the first anti-inflammatory drug in the world (Aspirin. foundation 2010). Aspirin was then trademarked under the Imperial Office of Berlin two years later on 3 March 1899 (Aspree 2010). Figure 1: Figure 2 : Figure 3 Willow bark Charles Frederic Gerhardt Felix Hoffman Pictures : yahoo

2. 3 Uses of Aspirin.

Aspirin is widely used in assuagement of fever and as a wonderful painkiller and anti-inflammatory drug and, to prevent strokes and heart attacks. Aspirin can solve the problems that some gardeners faced, for example like fungal infections on soil. Since aspirin also perform as an astringent, which can cause the pimples to be smaller in size and will not be as red in appearance as before. There are more and more evidences that often use of aspirin may decrease the risk of developing certain cancers and decrease the speed of the development of Alzheimer’s disease. Aspirin may also reduce the danger of pregnancy difficulty in women with pre-eclampsia and in those with antiphospholipid antibody syndrome (APS, Hughes syndrome) (Aspirin. foundation 2010).

2. 4 Side effect of Aspirin.

The most common side effects being reported are those related to the gastrointestinal tract and in some individuals aspirin can cause stomach bleeding and dyspepsia. It can also caused asthma to be worse and hence it must be taken with cautions (Aspirin. foundation 2010).

2. 5 Functional group of Aspirin.

Aspirin is also a carboxylic acid (?? COOH) and an ester (?? COOCH3) (Figure 4) (Chemistry in the World Around Us 2010) Figure 4 – Functional group of aspirin. In the experiment, the crystal should be having white flaky appearance. And the theoretical melting point fall within the range of 135. 7? c ~ 138. 7? c and a percent yield of 40. 6%.

3. THEORY

3. 1 Recrystallisation.

In chemistry, to purify compounds, we need to carry out recrystallisation. The most common situation is that a desired “compound A” contained a small amount of “impurity B”. There are a few methods of purification that may be attempted; one of the methods is recrystallisation. There are also different recrystallisation methods that can be performed such as: Single-solvent recrystallisation, Multi-solvent recrystallisation and hot filtration-recrystallisation. It can be used in numerous processes, such as the creation of aspirin (Wikipedia 2010). Recrystallisation is completed by placing the impure substance in a solvent, heating the solution so the compound dissolves, and impurities were being filtered. In various cases, it may be essential to use carbon to remove coloured contaminants from the compound.

Crystals were allow to form when the mixture is allowed to cool. The major foundation behind recrystallisation is the fact that substances will more often higher in solubility when the solvent is hot than when it is cold. Therefore, a compound will dissolve in a warm liquid but be insoluble at room temperature. During the recrystallisation, a person should only use a small amount of solvent to dissolve the target compound. If large amount is used, the compound may not recrystallise when the time comes. When the target compound has been fully dissolved, a person can filter out any insoluble impurities. The solution should then be allowed to cool slowly so that crystals can form. If the solution is being cooled down too quickly, the crystals may catch dissolved impurities and render impure crystals (Wisegeek 2010).

3. 2 Esterification.

Esters can be formed by performing a chemical reaction called esterification, which are compounds of the chemical structure R-COOR’, where R and R’ are either alkyl or aryl groups. The most ordinary method for preparing esters is to heat a carboxylic acid, R-CO-OH, with an alcohol, R’-OH, while the water that is formed will be removed. A mineral acid catalyst is often needed to make the reaction be performed at a useful rate. The esterification process has a wide spectrum of uses from the preparation of highly specialized esters in the chemical laboratory to the production of millions of tons of commercial ester products.

These commercial compounds are manufactured by either a batch or a continuous synthetic process. The batch method involves a single pot reactor that is being filled with the acid and alcohol reactants. The acid catalyst is added and the water removed as the reaction proceeds. This procedure is most common used by chemists in the laboratory, but in some cases, it is used by industry to make large quantities of esters (Net industries and its licensors 2010).

4. PROCEDURE.

4. 1 Preparation of Aspirin

First, 2. 4g of salicylic acid was being weighed in a dry 100ml conical flask and the weight was being recorded. Secondly, a 6ml of acetic anhydride was being added in the fumehood to the salicylic acid which was being contained in the flask. Next, 3 to 4 drops of concentrated sulphuric acid were added to the mixture and swirl it gently. The mixture was being heated in a water bath for 15 minutes to complete the reaction. Afterwards, the flask was then being removed from the water bath and around 1 ml of distilled water was being added into the hot flask by dropper to decompose the excess acetic anhydride. Then, an additional 40 ml of cold water was added to the mixture and was being stirred to induce crystallisation. Lastly, suction filtration was preformed and the crude product was being collected and washed with a little cold water. (Use spatula to scoop up the crude product. )

4. 2 Recrystallisation of Aspirin

First, the crude product was being dissolved in approximately 5ml of ethanol in a 100ml conical flask. Secondly, a 30ml of hot distilled water was added to the solution. Then, the solution was warmed to allow the solid to dissolve. Next, the solution was allowed to cool by placing it inside the ice bath for 10-15mins. A clean, dry watch glass was being weighed together with a filter paper and the weight was being recorded. The suction filtration was being carried out to obtain the recrystallised product with weighed filter paper.

Next, the crystals and filter paper were being transferred onto the weighed watch glass and were being put in the oven (100? C) for around 15-20 minutes to dry them. Next, the crystal, filter paper and watch glass was placed in a desiccator for 5-10 minutes. Afterwards, the dried crystal was being weighed together with the filter paper and watch glass. The weight was recorded and the weight of dried, recrystallised aspirin was being calculated. Lastly, the expected yield of aspirin from the amount of salicylic acid that was being used and the percentage yield of dried recrystallised aspirin was being calculated. And then, the dried crystals were being put inside the Optimelt (Automatic melting point system) to determine the melting point of aspirin.

5. RESULTS AND CALCULATION

Mass Mass of salicylic acid (a) = 2. 42g Mass of filter paper and watch glass (b) = 36. 04g Mass of dried, recrystallised aspirin, filter paper and watch glass (c) = 37. 32g Mass of dried, recrystallised aspirin (d) = (c) – (b) = 37. 32g – 36. 04g = 1. 28g Percent yield Number of moles of salicylic acid used (e) = Mass of Salicylic acid / Mr of Salicylic acid (mol wt of salicylic acid = 138) = 2. 42 / 138 = 0. 0175 mol. (3 sig. fig.) Expected number of moles of aspirin (f) = 0. 0175 mol. (3 sig. fig. ) Expected mass of aspirin (g) = No. of moles of aspirin x Mr of aspirin (mol wt of aspirin = 180) = 0. 017536 x 180 = 3. 16g (3 sig. fig. ) Percent yield = (d) / (g) x 100% = 1. 28g / 3. 1565g x 100% = 40. 6% yield (3 sig. fig. ) Melting point Temperature range = 135. 7 to 138. 7 °C Appearance White, thin, fine and needle-like crystals.

6. DISCUSSION

1. Safety goggles and disposable gloves were to be worn.
2. Acetic anhydride is a strong irritant and it is also corrosive and volatile. And hence, avoid contact with the skin and do not inhale its vapour.
3. While handling hot glassware, cotton glove was to be worn.
4. Concentrated sulphuric acid is very corrosive. And hence, avoid the contact with skin and do not inhale its vapour.
5. And during the preparation of aspirin, we need to try our best to put all salicylic acid in to the conical flask.

Do not let it stick on the side of the conical flask. Percent yield “In chemistry, the percent yield measures the amount of a chemical compound that was produced compared to the amount that should have been produced in theory. This calculation compares the amount that should have been created with how much of a chemical was actually created. ”The percent yield was being calculated to determine the success of a chemical reaction and the efficiency of usage. Percent yield will also show how successful a chemist is. And if the result shows a low percentage yield, it also means that the chemist is not efficiently using the chemicals. The theoretical yield is to show the remains of chemicals and the amount of limiting reactant that will be used up in the process. And for a pure aspirin, the percent yield is accepted to fall within 40% – 60% (Answerbag,2010). Melting point Melting point is direct relationship to determine the purity of the substances.

The melting point of a pure Aspirin is 140? C. Temperature range obtained : 135. 7? C – 138. 7? C. My percent yield is fall in the range of 40% – 60%. This also means that my percent yield falls within the typical/accepted range and hence, my chemical reaction is quite successful and had good efficiency of the usage of chemicals. But it is still not that perfect as if the percent yield closer to 60% will be better. And for the melting point, my result is very near to the melting point of a pure aspirin and the temperature range of my result is very narrow. From this, it can show that the recrystallised aspirin do not contain much impurities and it can also say that it is nearly pure.

7. RECOMMENDATION

Hence, in order to make the melting point of the crystal I obtained same as the pure aspirin, I will not use too much distilled water for the suction filtration as large amount of distilled water will dissolve the crystal and hence, results will be affected. Next, I will put my crystals in the oven longer as water will make the crystals impure. And I will also cover my crystals with a piece of paper, just to prevent dust from causing crystals to be impure. With this, it can increase the percent yield and the melting point will be the same as the theoretical range.

8. CONCLUSION

In conclusion, I can say that my experiment is successful. As the aim of this experiment is to perform recrystallisation to purify the compound and the crystals that I have obtained in the end of the experiment is pure. The supporting evidences are the percent yield and the melting point of the recrystallised aspirin. As the melting points in my results is very narrow and it is very near to the melting point of the pure aspirin. This shows that my aspirin do not contain many impurities. And my percent yield fall in the range of 40%-60%, which also means that my percent yield is quite high. High percent yield shows that my chemical reaction is successful and had a good efficiency of the usage of chemicals. Therefore, I can say that my experiment is successful.

REFERENCES

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