Introduction
Aspirin is a widely used drug worldwide, and it is regarded as one of the most important achievements in pharmacy and medicine in the 20th century. medical historians have traced the birth of pharmaceutical aspirin to 1897, it dates to approximately 3500years back when willow bark was used as a painkiller and an antipyretic by the early Sumerians and Egyptians and then in Greece and Rome by the physicians. In modern medicine aspirin has had precursors like salicylates in the 17th century, then in 1897 a bayer chemist first synthesized aspirin and about 70 years later a pharmacologist john vane expatiated its mechanism of action in inhibiting prostaglandin production [1]. Aspirin is a non-steroidal anti-inflammatory drug and was originally used as an antipyretic and an anti-inflammatory drug then it became popular and was used for its antiplatelet properties which is useful in preventing cardiovascular and cerebrovascular diseases, it in recent developments could be used in cancer chemoprevention [2], in recent years newer uses for aspirin are getting researched some progress has been seen in the possible reduction in pre-eclampsia, dementia, cataracts and some types of cancer, and because of aspirin’s various uses and benefits its been called the miracle drug in recent years [3].
The synthesis of aspirin (Fig.1)
To prepare aspirin, salicylic acid is reacted with acetic anhydride which is in excess see(fig.1)with a little amount of acid catalyst used, and the excess acetic anhydride would be quelled by the addition of water, the aspirin product obtained is not very soluble in water and would precipitate when water is added, but acetic acid is very soluble in water so it is easily separated from the aspirin product. The aspirin obtained after the reaction is called the crude product and can be purified by its recrystallization in hot ethanol.[4]
In this experiment, aspirin would be synthesized by reacting salicylic acid with acetic anhydride in the presence of phosphoric acid and after synthesis, the product would be purified by recrystallization of the sample. Then the purity would be tested by various chemical tests and by analysis by usage of a TLC (thin layer chromatography).
Experimental
Refer to the laboratory manual [5]
Results and discussion
In this experiment into the synthesis of aspirin, the product recovered from the reaction is not pure so after recrystallization, it should be as pure as its pharmaceutical equivalent assuming the procedure was carried out as it should have.
Table 1: mass of salicylic acid and crude aspirin
- Mass of salicylic acid
- Weight of watch glass and crude aspirin
- Weight of watch glass
- Mass of crude aspirin
In the table 1 above the mass of the crude aspirin obtained from the experiment is obtained from weighing by difference the weight of the watch glass and aspirin and the weight of the watch glass, the result obtained by this might not be accurate due to the fact that a series of errors occurred, such as when the product was transferred some was spilled and all the product was not completely collected after the experiment was completed and also the inaccurate weighing of the watch glass before the aspirin was added, all these could contribute to the inaccuracy of the values obtained.
The purpose of recrystallization is to purify the aspirin, recrystallization is a separation technique in the solid is dissolved in a hot liquid until there is saturation then the mixture is then allowed to cool and then form crystals, during the recrystallization in the lab the crystals didn’t form while cooling so a glass rod was used to scratch the sides of the beaker this was done to stimulate crystal formation by giving the crystals something to form round which sped up the formation of the crystals.
Table 2: mass of purified aspirin
- Weight of watch glass and purified aspirin
- Weight of watch glass
- Mass of purified aspirin
After the recrystallization of the crude aspirin was successfully completed the mass of purified aspirin was recorded, as seen in the table above. During the recrystallization process, a sizeable amount of product is lost due to inefficient lab procedures so the mass of the purified aspirin may not be accurate.
For the ferric chloride test to work, a complex must be formed between an electronegative oxygen atom and electropositive iron atom. Ferric chloride reacts with the phenol group to cause a colour change. The melting point of the products were determined using the melting point apparatus, small samples of the products were placed in the apparatus then was watched until they each melted, and the points were recorded. The melting points of the salicylic acid, crude aspirin, and purified aspirin and compared to the literature values which were gotten from the Merck index[6]. The recrystallized product should be higher than the crude aspirin but both were lower than the literature due to impurities mainly due to the solvent in the recrystallized and also left over in the crude product, but the lower melting point might be also due to human error when reading the temperatures off the melting point apparatus.
Table 3: melting points of salicylic acid, crude and purified aspirin.
Melting point
- Salicylic acid
- Salicylic acid (literature)
- Crude aspirin
- Purified aspirin
- Aspirin (literature)
(fig 2.)
After the recrystallization was completed and the melting points were recorded the samples underwent some chemical tests to indicate the purity of the crude and recrystallized aspirin and then the results were compared to the starting material in the synthesis, salicylic acid. The test reagent used was ferric chloride FeCl3 solution and this gave a highly coloured complex when reacted with phenols so for a standard we performed the ferric chloride test against phenol itself. To perform the test 5mL of water was added to each of the test tubes containing the crude aspirin and recrystallized aspirin samples and 5mL solutions of the salicylic acid and phenol were made in two further test tubes and then 10 drops of 1% FeCl3 solution and then colour changes were observed and recorded.
Table 4: chemical tests
- Substance
- Initial colour of aqueous solution
- Colour after adding the fecl3 solution
- Phenol
- Colourless
- Dark purple
- Salicylic acid
- Solution with white percipitate
- Very dark purple
- Crude aspirin
- Whitish gel with percipitate
- Light purple
- Recrystallized aspirin
- White solution with percipitate
- Purple
FeCl3 reacts with phenol groups so a purple complex, forms with both phenol and salicylic acid, however aspirin does not contain a phenol and therefore it should not react, the phenol must be converted to an ester and change colour.
To calculate the percentage yield of the recrystallized aspirin obtained the actual yield is divided by the theoretical yield then the percentage was taken, firstly the moles of salicylic acid was calculated by dividing the mass of the salicylic acid used by the molecular mass then the number of moles of salicylic acid calculated was multiplied by the molecular mass of aspirin to give the theoretical yield which is now divided by the actual yield and multiplied by a hundred to give the percentage yield.
- Percentage yield = Actual yield/ theoretical yield X 100
- Number of moles of Salicylic acid= mass/GFM = 5.00/142 = 0.00352
- Moles of salicylic acid x GFM of aspirin = 0.0352 x 180 = 6.33g
- (3.91/6.34) x 100 = 61.67%
The yield that was obtained is less than 100% due to the loss of recrystallized product during the experiment and inefficient lab skills in weighing and basic lab procedures.
BP Assay for aspirin
In a flask with a stopper 1g was dissolved in 10mL of ethanol and 50mL of 0.5 NaOH then was closed and allowed to stand for an hour, this was done in duplicates and in a third flask, ethanol was measured, and NaOH without aspirin and then also allowed to sit for an hour, after the hour elapsed 0.2mL of phenolphthalein indicator was used to titrate 0.5M HCl. This assay is a back titration which involves the addition of an excess of NaOH to the sample followed by the titration with HCl to determine the amount of excess so the amount of base equivalent to the sample is calculated but NaOH reacts with the CO2 in the atmosphere so the exact amount of NaOH has to be calculated by performing a blank titration.[7]
Table 5: mass of each sample of recrystallized aspirin taken
- Mass of weighing bottle + aspirin
- Mass of weighing bottle
- Mass of aspirin taken
- Mass of weighing bottle + aspirin
- Mass of weighing bottle
- Mass of aspirin taken
Table 6: burette readings
- Sample 1
- 29.40 cm3
- 0.00 cm3
- 29.40 cm3
- Sample 2
- 31.40 cm3
- 0.00 cm3
- 31.40 cm3
- 49.30 cm3
Calculations
Some calculations were performed after the BP assay had been completed in order to test for the purity of the recrystallized products, these calculations were done on the two samples collected and can be seen below.
- Number of moles of aspirin = [(b-a) x molarity of HCl x 0.5]/ 1000
- Mass of aspirin = number of moles x molar mass of aspirin
- Percentage purity = mass calculated/mass taken x 100%
First sample:
- Number of moles of aspirin = [(49.30 – 29.40) x 0.5 x0.5]/1000 = 0.004975moles.
- Mass of aspirin = 0.004975 x 180.2 = 0.896g.
- %purity = 0.896/1.04 x 100 = 86.5%
Second sample:
- Number of moles of aspirin = [(49.30 – 31.40) x 0.5 x0.5]/ 1000 = 0.004475moles.
- Mass of aspirin = 0.004475 x 180.2 = 0.806g
- %purity = 0.806/0.98 x 100 = 82.24%
Although the two samples had high percentage purities they can not be acceptable because for a sample to be considered pure it must have a percentage purity of not less than 99.5% and no more than 101%, so none of the samples are acceptable because the experiment was only repeated twice further replication would have reduced the uncertainty in the results as well as making the experiment more reliable.
Another way of determining the purity of a substance is by thin layer chromatography. For this experiment during the one hour standing time for the BP assay a TLC analysis was performed on the synthesised aspirin also on the references of salicylic acid and pure aspirin, then it was visualised with ultraviolet light and then sprayed with ferric chloride solution.
(Fig 3.)
After the TLC analysis was completed and placed under the ultraviolet light the RF values were calculated as seen below;
- RF values = aspirin (A) = 0.5
- Salicylic acid (S.A) = 0.59
- Pure aspirin (P.A) = 0.48
When reacted with the ferric chloride solution a black spot appeared on the S.A spot and did not appear on the rest of the spots, this indicates that the sample of the synthesised aspirin is as pure as the pure sample of aspirin as it was also unreacted and also was around the same distance travelled on the solvent front so this also proves that the synthesised aspirin can be regarded as pure. [8]
Conclusions
For this experiment , the actual yield of aspirin was 3.91g but given that the theoretical yield for the experiment was 6.34g , the percentage yield of the aspirin was 61.67%, the percentage yield was slightly low due to the fact that a part of the product was lost. Also the Rf values of the recrystallised aspirin, salicylic acid, and pure aspirin were 0.5,0.59 and 0.48 respectively , furthermore from the TLC analysis we can conclude that the salicylic acid travelled the furthest as it is the least polar amongst all the samples as displayed by the Rf values. A consensus of all the tests performed show that the recrystallised product although of acceptable yield is not regarded as clinically pure so to prevent error in the future , a number of steps could be taken including ensuring that the water bath is sufficiently heated and also conducting the experiment under a fume cupboard to avoid exposure to atmospheric air along with better lab procedures and techniques. [9]
References
- Maria M, Sergio M, Raffaele C. the first 3500 years of aspirin history from its roots. vascular pharmacology. 2019; 113: 1-8.
- Peter E, Michael S. New uses for old drugs: aspirin the first miracle drug. The pharmaceutical journal.2001.
- Sarah M, Brian F. Aspirin. reference module in biomedical sciences.2018.
- Lewis. Aspirin: a curriculum resource for post-16th century chemistry courses. 2nd edition. Royal society of chemistry. london.2003.
- The laboratory manual. The Robert Gordon university. Synthesis of aspirin. 2019 [accessed November 17th ,2019].
- Maryadele J O’Niel. The merck index. 13th edition. Cambridge; royal society of chemistry, 2013.
- British Pharmacopeia Commission. British Pharmacopeia 2011. London: TSO; 2011.
- Williamson K, Masters K. macroscale and microscale organic experiments. 6th edition, cengage learning. Belmont, 2011.
- Marian E. the synthesis and analysis of aspirin. Journal of chemical education. 2014.