Organic Synthesis Aspirin Aim Biology Essay


To synthesized aspirin, to determine the purity of aspirin using chemical test, and to determine the percentage of yield aspirin.

Aspirin is usually used as pain reliever or analgesic, fever reducer, and as anti-inflammatory agent. Aspirin also is one of the most widely used for non-prescription drugs. Aspirin was found back in 18th century, they found aspirin from the extract of the bark of willow trees which are useful especially in reducing pain and fever. It was then later found that the active ingredient in willow bark was salicylic acid. These are the structure of the salicylic acid and aspirin. Although they are an effective at reducing pain, it also has unpleasant side effects which are irritation to the lining of the mouth, oesophagus, and stomach. For aspirin image, in 1899, the Bayer company in Germany patented a drug which they named is as Aspirin, which was a modification of salicylic acid.

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Aspirin is an acid. A mixture of salicylic acid and acetic anhydride with H2SO4 is used for aspirin. The synthesis of aspirin is an esterification, in or in other words when an acid and an alcohol mix together in order to form a product which in this case is an aspirin. Besides that, esterification can also be reversible. The strength of aspirin varies depending on their ingredients. The image below shows how aspirin is synthesized.


Preparation of Aspirin

A half-filled bath was prepared using a 400-mL beaker. A few boiling stones was added and the beaker was heated to warm the water.

2.00g of salicylic acid was placed in a dry 125-mL conical flask.

A 3-mL of acetic anhydride was added to the flask and the flask was swirled slowly. 3 drops of concentrated sulphuric acid was also added during the swirl of the flask.(sulphuric acid is dangerous and gloves and goggles is needed for this steps and so on)

The flask was gently swirled to mix the solution and the flash was placed in a beaker of warm water for 15 minutes. A plastic stopper was also used to cover the flask

The reagent was then mixed and placed in the boiling-water bath; it was also heated for 30 minutes. The solid was then dissolved and the solution was swirled again.

The conical flask was removed from the bath and cooled to about the room temperature.

Solution was then poured into a 150-mL beaker containing 20 mL of ice water and the beaker was then placed in a water bath.

A glass rod was used to mix the solution while in the water bath and the glass rod was also rubbed vigorously to the bottom of the beaker.

The solution was then poured into the Buchner funnel for filtration and the crystals were collected on the filter paper.

Collected crystals were then washed with 5-mL of cold water and followed by 10mL of cold ethyl acetate.

The crystals were then again continued to dry by suction through the Buchner funnel for at least 5 minutes.

Re-crystallization was then performed in order to purify the product. By using a spatula, the crystals were then transferred into a 250mL beaker. 5mL of ethanol was then added. The solution was then stirred in order to dissolve the crystals while the beaker was placed in a warm water-bath.

After the crystals have dissolved, 15 mL of warm water was added to the alcohol solution. Cover was then covered and crystals were formed as the solution cools. The beaker was then set to ice bath to complete the re-crystallization

The content was then again poured into the Buchner funnel and suction filtration was applied.

The crystals should be placed between several sheets of filter paper and press dry the solid only if the contents were wet.

Petri dish was weighed and the crystals were then added to reweigh. Weight of the aspirin was calculated and the percent yield then determined. (The aspirin is not pure to be digest, do not consume it)

Analysis of Aspirin

One drop of 1% iron III chloride was added to separate test tubes that contained a few crystals of each substance. Color was then observed; pure aspirin would not show color, while salicylic acid or traces of it in impure aspirin will show a purple color. The intensity of the colour qualitatively tells how much salicylic acid is present.

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The aspirin crystals were examined under a microscope and the morphology of the crystals was recorded.

The purity of the sample determined by its melting point range. A mortar and pestle was used to reduce the sample to a fine powder. A capillary melting point tune was loaded with a 1-2 cm depth of fine crystals. The melting point of pure Aspirin varies between 130-135°C.


Preparation of Aspirin

Theoretical yield of aspirin:

Table below shows the data used to calculate the theoretical yield of aspirin.


Mass of salicylic acid used (g)



Relative molecular mass of salicylic acid (g/mol)



Moles of salicylic acid used (mol)



Moles of aspirin formed (mol)



Relative molecular mass of aspirin (g/mol)



Expected mass of aspirin yield (g)


So, theoretical yield of aspirin = 2.6100 g

Experimental yield of aspirin:

The table below displays the results obtained after the re-crystallisation process.


Mass of empty Petri dish (g)



Mass of Petri dish + Crystals (g)



Mass of aspirin crystals (g)


Thus, experimental yield of aspirin =

Analysis of Aspirin

Test tube

Colour before adding iron(III) chloride

Colour after adding iron(III) chloride

Salicylic Acid

Colourless solution

Dark purple solution

Commercial Aspirin

Colourless solution

Light purple solution

Prepared Aspirin

Colourless solution

Light purple solution

Based on the prepared aspirin, the observed melting point for aspirin is at range of 97-107°C. The theoretical melting point for aspirin is 131-134°C. Each of prepared aspirin, salicylic acid and commercial aspirin was tested with iron (III) chloride. Pure aspirin that tested with iron (III) chloride will give no purple colour solution. Purple colour formed when iron (III) chloride combines with the phenol group that was found in salicylic acid. Based on the above table, salicylic acid gives out dark purple solution when reacted with iron (III) chloride; both commercial and prepared aspirin give our light purple colour solution. It shows that, in commercial and prepared aspirin, the impurity (salicylic acid) is present as unreacted reactant.

Morphology of aspirin

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Based on the experiment, synthesis of aspirin involves esterification of salicylic acid. Esterification occurs when a carboxylic acid is being reacted with an alcohol to form an ester. For the reaction to take place, concentrated sulphuric acid is being added. Concentrated sulphuric acid acts as catalyst as to speed up the reaction. If concentrated sulphuric acid was not added, the reaction will take a long time to take place as the reaction may take too long to reach an equilibrium stage.

The limiting reactant for the aspirin synthesis is salicylic acid. According to stoichiometry, 1 mol of salicylic acid will react with 1 mol of acetic anhydride to give out 1 mol of acetylsalicylic acid (aspirin) and 1 mol of acetic acid. 3mL is approximately to 3.0000g, so in this reaction, acetic anhydride is in excess. Based on the calculation (refer results), the experimental result for yield of acetylsalicylic acid is 0.0041 g. The theoretical yield of acetylsalicylic acid is 2.6100 g, in which 1 mol. Based on the data obtained, the yield percentage yield is . Possible causes that lead to this yield percentage is errors done during the experiment.

Unreacted acetic acid was destroyed by the addition of ice water.


(acetylsalicylic acid)




salicylic acid


acetic acid

(ethanoic acid)

Ice cold distilled water is added to the flask until crystallization of aspirin is complete. Ice cold distilled water is being used as aspirin is insoluble in cold water so, this will cause aspirin not to dissolve. Acetic acid and sulphuric acid are water soluble, so they can be removed by washing the aspirin with cold water. Ice cold distilled water is being used because acetic anhydride will decomposes to water soluble acetic acid and during the washing, the impurities will be removed. So, pure acetylsalicylic acid is expected to get.

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Recrystallization involved in this reaction is for purification of the synthesized acetylsalicylic acid by dissolving the crude acetylsalicylic acid in the ethanol and pour it into beaker with warm water. When the solid separated, the mixture need to be warm and allow clear solution to cool slowly. Crystalized acetylsalicylic acid can be seen when the beautiful needle-like crystal separated. Recrystallization is the primary method to purify organic compounds. Compounds obtained might have impurities. So recrystallization is needed in order to remove the impurities.

The purity of the synthesised aspirin was tested with two separate tests. First, when it was tested with iron (III) chloride, a purple colourisation was formed. This indicates that the sample had a certain amount of salicylic acid which did not react and was not removed after purification. However, when compared to a sample of pure salicylic acid, the intensity of the purple colourisation in the synthesized aspirin is much less than pure salicylic acid. So, it can be said that there is a relative amount of salicylic acid traces in the synthesised aspirin is much less than the pure salicylic acid. When sample was compared to a sample tested with commercial aspirin, it is found that both had equal intensity of purple colourisation. It shows that the relative amount of salicylic acid in the synthesised aspirin is acceptable. However, the sample obtained had a significantly lower melting point than the literature value of 135C. This might be due to the weak structure of the crystal than it should be or the reaction of esterification reaction was not well completed.


The reaction theory proposed in the Introduction section in order to synthesise an aspirin is correct. The reaction that takes place is an esterification which requires a strong acid to proceed. For this experiment, we used concentrated sulphuric acid. It can be seen that the experimental yield was lower that the theoretical yield. This might had been caused by errors in techniques during the experiment.

The iron (III) chloride test had successfully indicated the relative amount of trace salicylic acid in the final product and was found to be at an acceptable level when compared to commercial aspirin.