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To synthesize the drug aspirin, the purity of prepared of aspirin is determined by a chemical test. In addition, the percentages of aspirin made are determined (CHM 1024 Organic Chemistry, practical manual).
Aspirin also know as acetylsalicylic acid, in the history of medicine, aspirin is the first drugs that ever come into common usage which as medical treatments worldwide. It is known as a pain killer, fever reducer and anti-inflammatory agent. As a matter of fact, an anti-inflammatory agent has become a widely known and utilized treatment for arthritis.(1)
Aspirin is also an anti-clotting agent. It is because it can inhibiting the production of thromboxane which will bind with platelet to create a patch of damaged walls of capillaries and platelet will become large enough to block the blood flow, locally and also downstream, aspirin also can use in long-term at low doses for helping prevent heart attacks, strokes and also blood clot formation.(2)
The synthesis of aspirin is actually an esterification reaction. By acid derivative, salicylic acid is treated with acetic anhydride, which will giving a chemical reaction that change hydroxyl group in salicylic acid into ester group (R-OH → R-OCOCH3). This esterification reaction will yields aspirin as well as acetic group which is a byproduct in this reaction. This process is speed up by using catalyst which is concentrated sulphuric acid.(3)
Figure 1: salicylic acid reacts with acetic anhydride to form acetyl salicylic acid which is aspirin and a by-product, acetic acid.
Overdose of aspirin will lead to acute or chronic poisoning. If a single large dose of aspirin is taken then it is acute poisoning; if taking higher than normal dose of aspirin over some period of time then it is chronic poisoning. The mortality rate for acute overdose is low, but for chronic overdose is more generally fatal. Other than that, overdose of aspirin also will have some bad side effects which is stomach ache and headache. These side effects can be reduced through the addition of buffer solution.
The presence of any unreacted or excess salicylic acid is detect by 1% iron III chloride(FeCl3). As salicylic acid got phenol group, the FeCl3 test will giving a positive result that show colour change to violet colour if there have molecule possessing phenol group. Hence, the commercial aspirin will given negative result because it does not have phenol group with it.
When salicylic acid reacts with acetic anhydride will yield aspirin and acetic acid.
Preparation of aspirin
A bath was prepared by using 400mL beaker was filled with half with water and it was heated until it boiling.
2.00g of salicylic acid was weighed and placed in 125mL dry conical flask. This quantity was used to calculate the theoretical yield of aspirin.
3mL of acetic anhydride was measured and added into the conical flask with gently swirled. The 3 drops of concentrated sulphuric acid was added into the flask during swirling. After that, the conical flask was placed in a beaker of warm water for 15 minutes.
The reagents were mixed and then placed in a beaker of boiling-water bath for 30mins until the solid was completely dissolved. The solution was swirled occasionally.
The conical flask was removed from the bath and it was cooled down to approximately room temperature which is 27°C.
The solution was slowly poured into a 150mL beaker which containing 20mL of ice water and mixed completely then placed in an ice bath.
During the ice bath, glass rod was used to mix the solution and scratching along the bottom of the beaker vigorously.(be aware no to poke a hole through the beaker.)
The crystal were formed and collected by using Buchner funnel through filtration and suction.
The crystals were washed by 5mL of cold water and the followed by 5mL of cold ethyl acetate.
Suction was continued for at least 5mins for drying the crystals.
The crystals were transferred to a 250mL beaker by using spatula and 5 drops of ethanol were added into the beaker. The beaker was stirred and warmed inside a water bath to dissolve the crystal. Re-crystallization process was performed.
After the crystals dissolved 25mL warm water was added to the alcohol solution. The solution was let to cool down and then crystals were formed. Once the crystallization started, the beaker in the ice bath was set to complete the re-crystallization.
The contents in the beaker were poured into Buchner funnel and suction and filtration was applied.
Filter papers were used to remove all the excess water by moving the crystals on the filter paper anf placed it between several sheets of filter papers and gently pressed to dry the solid.
Petri dish was weighed. Then crystal added on it and reweigh of petri dish. The weight of the purified aspirin was obtained by simple calculation and then this is used to obtain the percentages yield of aspirin.
Analysis of Aspirin.
Compare the quality of the synthesized aspirin with commercial aspirin and salicylic acid.
1% of iron III chloride was added to separate the test tubes containing aspirin crystals of each substance. The colour was observed. There will be no colour showed if aspirin is pure. Purple colour was showed if salicylic acid or traces is present in impure aspirin. Then the intensity of colour qualitatively tells how much salicylic acid was present.
The aspirin crystals were observed under the microscope and were drawn on the result sheet.
The purity of sample was tested by determining its melting point range. Reduce the sample to a fine powder by using mortar and Pestle. A melting point capillary tube with 1-2cm depth of fine crystals was loaded. The melting point f purified aspirin varies between 130-135 °C, temperature was noticed at the aspirin begins to melt and the sample has completely melted.
When dealing with acetic anhydride and sulphuric acid need to use gloves and do it in fume hood, because acetic anhydride will irritate the eyes; sulphuric acid will causes burn to skin because it is concentrated and corrosive. The aspirin prepares is not pure enough and cannot use as a drug as well as cannot consume.
Table 1: Iron(III) Chloride, FeCl3 test on aspirin
Table 2 : Experimental results
1. Weight of the salicylic acid
2. Weight of petri dish + filter paper
3. weight of aspirin + petri dish + filter paper
4. Weight of aspirin [3-2]
5. Melting point of purified aspirin
6. Melting point of commercial aspirin
The molar molecular mass of acetylsalicylic acid = 180g/mol
The molar molecular mass of salicylic acid = 138g/mol
The molar molecular mass of acetic anhydride = 102g/mol
Number of moles of salicylic acid, C7H6O3
= (weight of salicylic acid) / (molar molecular mass of salicylic acid)
= 2.0100g / (138g/mole) = 0.0146 mole
Number of moles of acetic anhydride, C4H6O3
= (weight of acetic anhydride) / (molar molecular mass of acetic anhydride)
= 3.246g / (102g/mole) = 0.0318 mole
By comparing the number of mole of the salicylic acid and acetic anhydride, the mole of salicylic acid is lesser. Therefore, the acetic anhydride is in excess. Hence the limiting agent in this experiment is salicylic acid.
By comparison of the chemical equations:
C7H6O3 + C4H6O3 ƒ C9H8O4 + C2H4O2
one mole of C7H6O3 reacts with one mole of C4H6O3
to produce one mole of C9H8O4 and one mole of C2H4O2
Theoretical yield of aspirin = 0.0146 mole - (180g/mole)
Actual yield of aspirin = 0.8953g
Percent yield of the aspirin
= (actual yield of aspirin) / (theoretical yield of aspirin) -100%
=0.8953g/2.628g - 100%
Diagram of Commercial Aspirin
In this experiment, aspirin crystals obtained were white crystals and is in pure form which is white crystalline solid. Then the percentages yield of aspirin obtained was 34.07%, and there are so many reasons that could not get a 100% yield of the aspirin.
The product can be lost while transferred from one container to another. Other than that, when during suction through filtration, some of the aspirin might remain in the dissolved filtrate and this causes the yield of the product to be less than theoretical yield. Besides that, when weighing salicylic acid and acetylsalicylic acid there might have zero error on the weighing machine and this will also affects the result and inaccuracy in the yield of the product.
The melting point of purified aspirin obtained will have error be found as well, because the instrument that using for measuring the melting point of aspirin was not totally cool down during measuring the crystals. Furthermore, aspirin might contain impurities that causes melting point to be higher or lower than the actual melting point.
During the Iron (III) Chloride test, the commercial aspirin showed no colour change, it is because there have no impurities and suitable for ingestion, but for the purified aspirin it gave a positive results which mean the solution turns violet colour due to the impurities in the product and it proved that the purified aspirin is not suitable for ingestion.
This experiment was required 3 drops of concentrated sulphuric acid, because concentrated sulphuric acid was act as a catalyst for speeding up the reaction without affecting the result. The reaction will still take place if the concentrated sulphuric acid were omitted.
This experiment used ice water during the work up is due to the characteristic of aspirin. Aspirin is insoluble in water and cold water will reduce the solubility of aspirin in water and it can be isolated when undergo suction through filtration with chilled reaction solution. Then the water is used to hydrolyse the acetic anhydride to acetic acid and can be easily removed by filtration. Therefore, all the impurities can be removed.
The principle of recrystallization is the amount of solute that can he dissolved by a solvent increases with temperature. Recrystallization only works when a proper solvent used. In this experiment the solvent that used to dissolved the impurities is ethyl acetate and then using hot filtration to remove inpurities.
The purity of the synthesized acetylsalicylic acid was tested by FeCl3, if there is colour change (violet colour) that means there have impurities in acetylsalicylic acid. The purity of acetylsalicylic acid was determined by melting point. In this experiment the melting range for acetylsalicylic acid is 144°C-172°C which is higher than commercial aspirin which is132.8°C-138.9°C, meaning that acetylsalicylic acid obtained in this experiment contained impurities.
There have some improvement for this experiments as in used all the salicylic acid powder and ensured that there is no drop on the table. During transferring the reactants from one to another container make sure that there is no any remaining in the container. Furthermore, during filtering the aspirin by using the suction ensure that all holes on the Buchner funnel are covered for preventing the reduction of yield product.
This experiment was supported the hypothesis suggested. When salicylic acid reacts with acetic anhydride will yield acetylsalicylic acid which known as aspirin. In this experiment, the limiting agent is salicylic acid. The melting point range for the acetylsalicylic acid obtained is 144°C-172°C, it is because it contains impurities. The theoretical yield of aspirin is 2.628g, but the actual yield of aspirin obtained is 0.8953g. therefore, the percentage yield of aspirin is 34.07%.