Preparation of Recrystallization of Aspirin
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Published: Fri, 08 Jun 2018
This experiment serves 2 objectives – the production of Aspirin from esterification of salicylic acid with excess acetic anhydride and obtaining it in a purer state by recrystallisation, hence determines the melting point of the acetylsalicylic acid (ASA) that was synthesised and purified.
To achieve this, salicylic acid is reacted with excess acetic anhydride in the presence of the sulphuric acid as the catalyst, which produces precipitate ASA and aqueous acetic acid. I applied suction filtration to collect the ASA as the residue having acetic acid as the filtrate.
Pure aspirin is a white crystalline solid. The aspirin initially produced by synthesis may be light tan, indicating the presence of impurities. The ASA synthesis earlier is purified by recrystallization. In recrystallization, the impure solid is dissolved in a hot solvent (aqueous ethanol) and the resulting solution is allowed to cool slowly. As the solution cools, crystals of the product will form and soluble impurities will remain in solution.
From this experiment, the percentage yield of ASA is around 49%, using capillary test the melting point of ASA falls in the rage of 137.9-140.5 0 C. The appearance of ASA is an odourless, white, needle-shaped and shiny solid. In conclusion, experiment is a success. Despite product had a low percent yield and was found to be impure due to its low melting point.
Aspirin is the trade name for the molecule acetylsalicylic acid.
The earliest known use of this molecule has been traced back to the fifth century B.C. The Greek physician Hippocrates described an extract of willow tree bark, a bitter powder that could be used to reduce fevers. In 1829, Salicin was isolated from willow bark and used as a pain reliever. Unfortunately Salicin was not very popular since it was found to be very acidic and a stomach irritant.
In 1897 Felix Hoffman, a German chemist, was working for the Bayer chemical company. Hoffman wanted to formulate for a less acidic pain reliever as his father could consume for his arthritis. His studies led to the synthesis of acetylsalicylic acid (ASA) or aspirin. Bayer patented the name and commenced to market the product in 1899. It was a huge success and sales grew rapidly. In fact, the company set up by Friedrich Bayer & Company is generally considered to have been the first pharmaceutical company, and the production of aspirin has indisputably laid the base of the modern pharmaceutical industry.
Only until the 1970’s that scientists starts to comprehend the concept on how aspirin function as a pain reliever. Today 80 billion aspirin tablets are taken every year over the al the countries to reduce fevers, relieve pain, and even help prevent heart attacks. Bayer In commercial aspirin products, a small amount of ASA (300 to 400 mg) is bound together with a starch binder and sometimes caffeine and buffers to make an aspirin tablet. The basic conditions in the small intestine break down the ASA to yield salicylic acid, which is absorbed into the bloodstream. The addition of a buffer reduces the irritation caused by the carboxylic acid group of the aspirin molecule.
Esters are organic compounds with the general formula RCOOR’, where R and R’ can be an alkyl group or an aromatic group. Alkyl group is an alkane that is short of one hydrogen since it needs one bond to be branched from the parent chain. Aromatic compounds are a class of molecules containing benzene, a six-membered carbon rings with delocalized pi electrons. This type of group is found in salicyclic acid and ASA.
Esters are readily synthesized by the reaction between a carboxylic acid, RCOOH, and an alcohol, R’OH, as shown in the following reaction.
The reaction above is also known as esterification. It is a condensation reaction, whereby two molecules combine to form one single molecule, while removing a small molecule (e.g. H2O) in the process.
The reaction to I have used synthesis ASA is very similar to the one above. Salicylic acid a phenol consisting of a phenyl bonded to the hydroxyl (-OH) which is very much alike to alcohol and acetic
Anhydride is a dehydrated carboxylic acid (acetic acid). In the below reaction we use concentrated sulphuric acid as the catalyst. After the warm bath to complete the reaction, we added cold water to quench the reaction (hydrolysis of acetic anhydride) to prevent further reaction to take place. The cold water must be added quickly as to allow fast crystal formations due to the drastic drop in temperature. Stir and rub the walls can create uneven surfaces hence inducing crystallization. ).
As such, in this experiment, 2.4 grams of salicylic acid should yield 3.13 grams of Aspirin, it is stated that acetic anhydride is in excess. This is proven by the following calculation:
Mol of salicylic acid in this experiment:
2.4(2d.p)/138 = 0.0174 (3 s.f.)
Mol of salicylic acid : Mol of Aspirin
1 : 1
Therefore, Mol of Aspirin:
0.0174/1 x 1 = 0.0174 (3 s.f.)
Expected mass of Aspirin:
0.017391 x 180 = 3.130 (3 dec.)
The solid ASA formed contains impurities and should be recrystallised to achieve a purer state. Recrystallization is possible because most solids are more soluble in hot solvents than in cold solvents.
The solubility of ASA increases as temperature increases. This means that if aspirin is dissolved in ethanol to produce a saturated solution and that solution is cooled, the aspirin will crystallize during the cooling.
Ethanol has been chosen as the solvent because the polar nature of the hydroxyl group causes ethanol to dissolve many ionic compounds, moreover the ethanol molecule also has a non-polar end, and it will also dissolve non-polar substances.
While cooling, crystallization takes place. In crystallization, there is a slow, selective formation of the crystal framework resulting in a pure compound. Instead in precipitation, due to the rapid formation it will trap impurities in the solid’s crystal framework. For this reason, we should include crystallization to get a purer solid substance.
The alternate way of synthesising the aspirin can be produced by replacing the acetylating agent from acetic anhydride to an even more acidic acid known as acetyl chloride CH3COCl. This can increase the percentage yield of ASA, since acetyl chloride is more acidic. The reaction of acetyl chloride with salicylic acid is showed below. However, the by-product is hydrochloric acid (HCl) instead of acetic acid.
However, Acetic anhydride is preferred because it is less hazardous to use and less expensive than acetyl chloride. In industry, the acetic acid produced in this reaction can be recovered and converted back into acetic anhydride by the process known as dehydration:
We may also create or Salicylic acid from Kolbe’s reaction whereby we start with a phenol:
Approximately 2.4 grams of salicylic acid is weighed and placed in a dry, 100ml conical flask.
6ml of acetic anhydride is added into the same conical flask, along with 3-4 drops of concentrated H2SO4 as catalyst.
The conical flask is then heated at 80-100 degree C in a water bath for 10 to 15 minutes to hasten the reaction.
1ml of distilled H2O is added into the conical flask immediately after it is removed from water bath.
40ml of cold distilled H2O is then added to the conical flask.
A stirring rod is used to gently ‘rub’ the side of the conical flask. This is to scratch off crystals which have formed.
Suction filtration is then carried out to remove the crystals from the solution.
The crystals are removed from the filter paper. To ensure accuracy, cold distilled H2O is used to wash away remaining crystals from the filter paper.
The crystals are then dissolved in 5ml of ethanol.
30ml of hot, distilled H2O is added into the solution, and slowly cooled.
Crystallisation of Aspirin will take place as the solution is cooled to room temperature.
Suction filtration is done to remove the Aspirin from the solution. The residue is then placed on a watch glass along with the filter paper.
Aspirin is then dried by placing it in the oven for 20 minutes, along with the watch glass and filter paper.
It will then be transferred to the desiccators for 15 minutes to further dry it.
The crystals remained on the filter paper would be pure, dry, Aspirin.
The following measurements are taken:
- Actual weight of salicylic acid is weighed at (1).
- Weight of filter paper and watch glass is taken.
- Weight of Aspirin, along with watch glass and filter paper, is taken after (15).
After obtaining Aspirin, the melting point of Aspirin is then determined using the capillary method.
5. Results and Calculation
Mass of salicylic acid (a) = 2.39 g
Mass of filter paper & watch glass (b) = 33.11 g
Mass of dried, recrystallised aspirin, filter paper & watch glass (c) = 34.64 g
Mass of dried, recrystallised aspirin (d) = (c) – (b)
= 1.53 g
Number of moles of salicylic acid used (e) = 0.0173 mol
(Molecular weight of salicylic acid = 138)
Expected number of moles of aspirin (f) = 0.0173 mol
Expected mass of aspirin (g) = 3.12 g
(Molecular weight = 180)
Temperature range 137.9-140.5°C
Needle shaped, white, and shiny.
My percentage yield ASA is relatively low since it is only 49%. This may occur due to several factors:
While transferring the salicylic acid, some of the solid may stay at the side of the conical flask and even your spactula, and resulting in lesser salicylic acid participating in the process of esterification compared to what have been weighed earlier. Thus, this will result in lower yield of the crystal ASA. It will be advisable to wash the walls with distilled water to ensure more salicylic acid will participate in the reaction.
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. 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. The iron (III) chloride test also could have been ran to determine if any of the aspirin degraded to salicylic acid or never converted from salicylic acid.
During the suction filtration we should rinse the solid ASA formed in the conical flask and the glass rod during the synthesis before pouring into the Büchner funnel repeatedly. After the final suction filtration, another error causes the percentage yield to drop. This is due to the crystals not being fully collected on the filter paper, but rather, remains on the side of the Buchner funnel. This loss can be minimized by the crystals from the side of the Büchner funnel to the filter paper moving using a metal spatula before collecting the filter paper and placing it on the watch glass.
During recrystallization, one common error of adding too much solvent (ethanol) will result in less percentage yield while cooling. We should refrain from moving or shifting the solution while it is cooling as it will interrupt the crystal formation since the particles are generally held by weak dispersion forces, dipole-dipole forces, and hydrogen bond. We should also allow slow crystal formation as fast crystal formation will cause the trap of impurities and giving higher percentage yield.
With the presence of impurities the melting point will decrease and vary in a larger range.
In conclusion, the experiment is a success since I have managed to synthesis and recrystallise the ASA although we have relatively low percentage yield of 49% that may arise due to several factors as stated in the discussion. I was able to analysis the melting point of the recrystallised aspirin and it is generally low in temperature which implies the presence of impurities.
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