In addition heat is used to initiate a reaction, provide energy for molecules to react with another. The water is added to purify the sample and wash away any excess acetic anhydride. Using vacuum filtration the sample is filtered and weighed.
The reaction taking place in the reaction is:
Balance symbol equation:
C7H6O3+C4H6O3→C9H8O4+ C2H4O2
Diagram 1 shows the skeletal formula of the changes in shape that takes place in the molecules. The limiting reactant, structure of salicylic acid contains a benzene ring and a carboxylic group. It shows the addition of water allows the excess acetic anhydride to become hydrated break in to two molecules, one of which is added to the salicylic acid to produce the aspirin, forming an ester. For the breaking of bonds heat is required which is how the reaction is initiated under gentle heating. The H+ is obtained either from concentrated H2SO4 or H3PO4 and is used to catalyse the reaction.
There are numerous amounts of equipment that would help produce Aspirin in a lab, however suction filtration is a common way to filtrate and the draining of the water works by a creating a vacuum and isolating the substance.
The heating mantle is a much more efficient way to heat the round bottom flask, as the mantle encloses the flask and circulates heat within it. Also the temperature of the heating mantle can be controlled more closely as there are numbers used to adjust the heat.
To find the melting point a small cylinder was inserted into the beaker full of Aspirin. This was then inserted in to a larger cylinder which then applied heat on to the small tube and became even hotter recording the temperature.
Method
All reactions were completed in a fume cupboard for safety. The first step was to add 50g of salicylic acid with 80ml of acetic anhydride in a round bottom flask. As it is very tricky to get exactly 50g of the acid exactly the sample used was 50.07g, as close as possible to the preferred amount. In addition to these two reactants a maximum of 0.5ml of concentrated sulphuric acid was added to the flask.
During the heating process the temperature was to be maintained between 60-70°c. If the temperature had exceeded 70°C the acid would have been burnt and effect the final product. To heat the flask a heating mantle was used. After that to guarantee the sulphuric acid was dispersed throughout the product, loosening the round bottom flask from the clamp and swirling it around helped ensure this.
The sample should have been gently heated for a period of 30 minutes to ensure a better quality product and then cooled to 50°c. 750ml of cold water should be added to the acid after the heating process and mixed it. To help filter and recrystallize the product, the Buchner funnel was used which helps sucks the remaining water from the product. Lastly measure the yield and the melting point of the crystals.
Diagram of experiment:
On paper.
Hazard and Risk Analysis
Results and Calculations
The formula equation for this reaction will be used to calculate the number of moles of Aspirin formed which will help calculate the percentage yield.
C7H6O3+C4H6O3→C9H8O4+ C2H4O2
Mr of Salicylic acid= 138Mr
Mr of Aspirin=180Mr
Mass of salicylic acid=50.07g
50.07g/138mr = 0.36231mol of Salicylic acid
Theoretical yield of Aspirin should be:
Ratio of salicylic acid to Aspirin is 1:1 so
0.36231mol x 180mr= 65.22g
Actual yield of Aspirin:
Mass of aspirin= 153.8746g-101.337g=52.538g
Percentage yield:
(52.54/65.22)x100=80.56%
Discussion and Recommendations
The experiment consisted of heating the salicylic acid, with the acetic anhydride. Cold water is added after the reaction, after adding 750ml of cold water, a white precipitate was formed since aspirin is insoluble in cold water; this step is necessary for the purification of the crystals and avoids any impurities.
The results collected for the melting point showed the crystals started to melt at 119°c which then completely finished melting at 122°c. The mass of wet product was 82.82g and the mas of the dry product was 52.39g and the mass of the theoretical yield is 65.22g. Finding these results made it possible to calculate the percentage yield which was 80.56%. As the experimental objective was to produce Aspirin, measure the melting point but also in addition to this the amount of yield collected meant the procedure was successful.
The yield calculated was very high and showed the experiment is feasible and as Aspirin is already produced in industry this confirms how the yield collected from each batch will always have minimal loses. However as the percentage yield was not closer to 100% there are still ways in which more sample could have been collected and stop any aspirin from going to waste.
Firstly during suction filtration procedure, Aspirin was left behind on the Buchner funnel and filter paper. As they were quite small particles it was very tricky to try and remove each grain of aspirin. To avoid this we could allow time for the filter paper and sample to dry then slowly remove the left over product. However this would take time to do and slow down the experiment.
Extra product could also be lost during the dissolving and recrystallization. Ethanol was used to dissolve the crystals, as well as the impurities including salicylic acid. When cold water is added after the entire product has been dissolved, not all may have reformed consequently some may need cooler water to become insoluble. To try and avoid this trying colder temperatures of water could be used.
The salicylic acid and acetic anhydride that didn’t react are all dissolved by the addition of ethanol. As this reaction is Endothermic, using Le Chatliers principle; the presence of excess acetic anhydride would cause a favoured forward reaction and want the desired product. Although if reactants do not react enough the product made would still be minimal, and misused by being dissolved and not completely reacting.
Another source of error was the weighing scale which is sensitive to air movement and easily fluctuates the mass measured, thus would create an increased amount of errors that would build up. Therefore to avoid mini errors, weighing the substance should be done in a non-drafted area.
A source of error would be the apparatus which measured the melting point which would have limited accuracy, hard to control the temperature and rate of melting, along with this experiment being very complex any unforeseen variables that are hard to control or cannot be controlled may pop up. The equipment used will have left over product on them, which would be a potential source of error and a low the yield. Such as trying to transfer all the sample from the round bottom flask in to the beaker, or residue left on the glass rod. Again some errors are unavoidable and are just minimised as in this situation.
Measuring the melting point came to 135°C (Haynes, 2014) but the experimental melting point was 122°C.
The melting points show a small range, but a larger difference between the theoretical and actual melting points. There can be many reasons as to why there was a difference between the melting points. As the yield is not 100% some of the product may have been lost when transferring the product from one container to another. All reactants may not have all reacted.
Conclusion
The experiment as a whole was successful and the reactants react together to form Aspirin, however if the experiment was to be done again a way to increase the percentage yield would make it feasible would make the experiment more commendable. For example when measuring the melting point the temperature there should be more control over the temperature and humidity of the surroundings, and kept at a minimum to stop the reactants reacting with vapour in the air.
Nomenclature
2-Acetoxybenzoic acid=Aspirin
2-Hydroxybenzoic acid=Salicylic acid
RSoC=Royal society of Chemistry
References and Acknowledgements
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Ling, G (1994) Aspirin (Online) Available from: [Accessed 13th October 2015]
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Greenlaw, E (2005) Your guide to over the counter pain relief [ Accessed 14th October 2015]
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Royal Society of Chemistry (2014). ChemSpide-Aspirin [Online]. Available from: http://www.chemspider.com/Chemical-Structure.2157.html [Accessed 13th October 2015].
- Haynes, W.M (2014) Handbook of Chemistry AND Physics. 94th Edition. Gaithsberg: CRC PRESS.
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Chemlatech (2012) Chemistry 104: Synthesis of Aspirin [Online]. Available from: [Accessed 15th October 2015]
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Ali (2014) Characterization of Aspirin [Online] Available from : [Accessed 14th October 2015]
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