Purity of Aspirin from Salicylic Acid and Acetic Anhydride


To synthesize and determine the purity of aspirin from salicylic acid and acetic anhydride along with calculating the yield of product (Acetylsalicylic acid).

Introduction to Aspirin Synthesis and Analysis

Aspirin is also known as acetylsalicylic acid(C9H8O4) acting as an analgesic drug used for a pain reliever, anti-inflammatory and reducing fever [1], Felix Hofmann was first synthesised in 1893, had the similar properties of salicylic acid which is extracted from willow bark tree, can cause stomach ingestion problem due to high acidic conditions (pKa =2.97) than normal human stomach pH conditions (pH ~4) due by two diprotic acid consists of two acidic functional groups such as carboxylic acid[-COOH]and a phenol[-OH], comparatively monoacidic protonated acetylsalicylic acid has less acidic behaviour((pKa =4.57) has of ester replaced at phenolic -OH group. The acidic compound (-COOH) replaced by an ester(-COOR) at the position of phenolic(-OH) group at the ring in salicylic acid catalysed by phosphoric acid(H+) the process is called esterification.

The initial step of esterification reaction proceeds via suspension of salicylic acid with excess of acetic anhydride which acting as a both of reactant and solvent as well, the reaction speed up by the catalyst of H3PO3 which donates the H+ ions to complete the reaction, when the reaction proceeds then salicylic acid (solid) is disappeared and the product of aspirin is completely dissolved in the solution, once the solids of salicylic acid completely disappeared then we could understand, the reaction is complete, in this reaction the excess of acetic anhydride must be hydrolysed by water into acetic acid for that water is added a drop by drop condition into acetic anhydride due by more reactive with water[2]. The purpose of the experiment was to prepare aspirin and analyse the product with spectroscopic techniques.

The whole reaction is proceeded with reflux condenser method to collect the aspirin without losing precious amount, once the product is formed then purified with recrystallisation techniques to remove the impurities from the crude product, then filtered and then dried, the %yield will be calculated for this reaction to check the quality of the product by a melting point as one of the methods and spectroscopic assay as done.


The objective of the whole experiment mainly focused on the cost-effective mode of synthesising aspirin, defining the purity and %yield of product formation calculated by theoretical and actual ways to improve them.

Discussion on Aspirin Synthesis and Analysis

Esterification is the process of reacting the carboxylic acid with acetic anhydride, acidic medium acting as a catalyst to enhance the reaction mechanism leads to product formation. According to Le Chatelier’s principle based on equilibrium effect one to determine the temperature, pressure and concentration (3).

Salicylic acid contains two acidic groups located at the rings leads to more acidic, essentially stomach irritation is happened for humans, to reduce the acidity, phenolic (-OH) replaced by esoteric compound(acetic anhydride) to form acetylsalicylic acid with less acidity than parent which is easier to digest in the human body.

Aspirin has formed the process of reacting salicylic acid, acetic anhydride and phosphoric acid, H+ ion attacks the C=O bond of acetic anhydride to form positive species (electrophile) C+- O formed, thus possible to attack nucleophile to balance the bond formation. The phenolic -OH group of nucleophiles capable to attack the carbocation of (C+) acetic anhydride to form the esoteric formation of acetylsalicylic acid is formed. Phosphoric acid acting as a catalyst to reduce the reaction rate is slow.

Possible to use acetic anhydride instead of acetic acid because anhydride easily reacts with water to form acetic acid, we can calculate the %yield for acetylsalicylic acid by theoretical calculation.

%yield = [actual product yield/theoretical product yield] x 100%

From our data, the theoretical yield of this experiment is 2.50g but the actual yield of our product which is come around 1.50g, the %yield would be around 60% approximately there is a large %error due to various factors such as not having constant temperature, pressure and cutting filter paper error and so on.

We confirmed the whole reaction is over by looking at the amount of salicylic acid is completely transformed into a product form by testing the reaction on ferric chloride with salicylic acid unless aspirin does not react with ferric chloride, thus will make sure about the whole reaction is done or not.

Analysis with A Melting Point

From our findings, the melting point of pure aspirin which is around 135-137C but our product of acetylsalicylic acid is 130-132C this indicates the impurity of our crude sample because if any compound impurity is present then the melting gets lower than the pure product, thus our reaction has made a proper mode of controlled factors such as temperature, pressure and the amount of substance we handled.

Analysis with IR Spectroscopy

 The infrared radiation spectroscopy tells us to the confirmation of functional group which are present in our experiment such as -OH presents in alcohol and C=O presents in carboxylic acid, from this observation we have got exactly same peaks observed from the reported data [O-H = 3600-3200, C-H = 3300-2800, C=O = 1800-1630]

Analysis with 1H NMR Spectroscopy

Though is a complicated observation of position at the peak possible to judge the what type of proton which is present in the group by observing the chemical shift in ppm, easily found the environmental group which is present in nearby. We observed the value which is not exact matches but it is well correlated with standard protocols.

Analysis of Mass spectra

The mass spectroscopy which is confirmed by the peak which is around at 140 is the mass/charge ratio comparatively relies on the original mass spectra of aspirin.

Conclusion on Aspirin Synthesis and Analysis

Aspirin was successfully synthesised and analysed by various methods and found the percentage yield is to around 60%, but the actual yield as we have got 1.5g though theoretically found 2.5g, has confirmed with less impurity from the various characterization techniques such as melting point and TLC along with well correlated spectroscopic data was found. 

References for Aspirin Synthesis and Analysis

C (2017). Uses, benefits, and risks of aspirin. MedicalNewsToday. Retrieved from https://www.medicalnewstoday.com/articles/161255.php.

Esobel, Synthesis of Aspirin (accessed Feb 23, 2014)


Williamson, K.; Masters, K. (2011) Macroscale and Microscale Organic Experiments, 6th ed.

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