Aspirin Synthesis

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Molar mass of salicylic acid

138.1200 g/mole

Molar mass of acetic anhydride

102.0900 g/mole

Density of acetic anhydride

1.0800 g/mole

Molar mass of aspirin

180.0000 g/mole

Molar mass of acetic acid

60.0000 g/mole

Number of moles of salicylic acid = [pic 5]

= 0.0145 mole

Mass of acetic anhydride = 3 x 1.08

= 3.2400 g

Number of moles of acetic anhydride = [pic 6]

= 0.0317 mole

Salicylic acid acts as a limiting reagent in this experiment as it only has the no of moles of 0.0145 moles which is lesser than the no of moles of the acetic anhydride which is 0.0317 moles. Therefore only 0.0145 moles of acetic anhydride will react with 0.0145 mole of salicylic acid.

Theoretical value of aspirin = molar mass x number of moles of aspirin

= 180 x 0.0145

= 2.6100

Percentage yield of aspirin = x 100 [pic 7]

= [pic 8]

= 50.4521 %

Analysis of aspirin

Table 3 shows the result of the aspirin analysis

Type of aspirin

Result

Commercial aspirin

White powdered solution turned pale purple when 1% iron (III) chloride solution is added. This indicates that salicylic acid is present in the tablet and the intensity of the pale purple shows that little amount of salicylic acid is present in the tablet.

Synthesized aspirin

The fine white crystals did not show any colour change s when the 1% iron (III) chloride solution is added. This shows that the synthesized aspirin is pure and there are no traces or presence of salicylic acid in the sample.

Melting point of synthesized aspirin

Table 4 shows the melting point range of synthesized aspirin

Melting point range of synthesized aspirin

127.3 ºC -134.7 ºC

Discussion

At the end of the experiment, when the percentage of yield of synthesized aspirin is calculated, it gives us about 50.4521 %. This percentage of yield is relatively very low. The melting point obtained for the synthesizing aspirin is in between the range of 127.3ºC and 134.7ºC. Although the melting point of the synthesized aspirin did not reach the theoretical value, the melting point range of the synthesized aspirin was closer to the theoretical value. It showed that the synthesized aspirin was not in its purest state. The low value of percentage yield for the synthesized acetylsalicylic acid could have occurred due to some errors happened during the conduction of the experiment.

For example, when the salicylic acid was transferred from the weighing boat to the Erlenmeyer flask, not all the salicylic acid powder was transferred and there were some traces of the powder on the weighing boat. This actually affected the final percentage yield of the synthesized acetylsalicylic acid. Besides that, the percentage yield of the aspirin sample was relatively low due to the loss of some of the crystals during repeated process of crystallisation. The crystals were not completely transferred to the Buchner funnel. Incomplete reaction of the crystallisation or overheating could also contribute to the low percentage yield of synthesized aspirin.

The purity level of the synthesized aspirin was higher than that of the commercial aspirin provided. When the synthesized aspirin was mixed with two to three drops of 1 % iron (III) chloride solution, there was no colour change observed while when the commercial aspirin powder was mixed with two to three drops of 1 % iron (III) chloride solution, the white powder changed colour to pale purple. This observation showed that the commercial aspirin was not pure. The intensity of the purple colour produced by the mixture of the 1% iron (III) chloride and the commercial aspirin showed the level of the purity of the commercial aspirin which was relatively lower than the synthesized aspirin. The synthesized aspirin has a higher purity level as there were no traces of impurities such as salicylic acid in the filtered acetylsalicylic acid crystals.

Concentrated sulphuric acid (H2SO4) was added at the initial stage of the experiment. This acid was added in order to speed up the rate of reaction by providing an alternate way which lowers the activation energy. Concentrated sulphuric acid acted as a catalyst in this experiment. The reaction might have taken a longer time or may not have occurred if the concentrated sulphuric acid was not added to the mixture of salicylic acid and acetic anhydride solution. This is because of the presence of stabilised salicylic acid structure due to the localised benzene ring by the lone pairs from the carbonyl group and hydroxyl group.

The salicylic acid structure is made up of benzene ring with the carboxylic group and phenol group attached to it. The pure salicylic acid causes some irritation towards the internal environment of the consumer such as thinning the stomach and oesophagus layer. The use of aspirin reduced the side effects of the salicylic acid as the acetylsalicylic acid will be hydrolysed into salicylic acid in the small intestine. This hydrolysed salicylic acid will then be absorbed into the bloodstream without causing serious damage to the internal environment of the consumer.

Addition of ice water destroys the unreacted acetic anhydride in the mixture of salicylic acid and acetic anhydride. Acetic anhydride acts as excess reagent in this experiment. Ice water was added to separate the acetic acid from the acetylsalicylic acid as acetic acid is water soluble whereas aspirin is water insoluble organic compound. This reaction satisfies the