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Synthesis of Aspirin

and its viability at the Santa Fe College laboratories

The most common pain killer used today is Acetylsalicylic acid, commonly called aspirin. It is possible to synthesize aspirin using Salicylic acid and Acetic anhydride. There are two main objectives for this experiment: synthesize a sample of Acetylsalicylic acid, and analyse the sample obtained by measuring its melting temperature, and conduct a spectrophotometric analysis of it. This two objectives allow us to determine if the protocol used can be validated.

 

I did three trials following the same protocol. The second and third trials were made a week after the first one.

Procedure:

  • Add 2.0g of salicylic acid into a 50 mL Erlenmeyer flask.

  • Add 5.0 mL of acetic anhydride and 5 drops of 85% phosphoric acid solution.

  • Swirl the mixture.

  • Partially submerge the Erlenmeyer flask into a 70-80ªC hot-water bath for 15 minutes.

  • After the reaction has reached completion, remove from hot bath and add 20 mL of distilled water.

  • Wash the product using vacuum suction.

  • Transfer the obtained poudre into the watch glass to dry.

After I obtained three samples of aspirin, I calculated the percentage of yield of each one of them.

These were the results:

The expected yield of the synthesis of aspirin is 60-70%

From this table, we can see that the first trial has a lower yield than the two others. This can be easily explained. Indeed, since it was the first trial, I had some losses during the different steps of the procedure, from mistakes due to the ignorance of the procedure. I didn't commit these mistakes again in the following trials.

Analysis:

After obtaining my three samples, I had to analyse them to know if they were, in fact, aspirin. To do so, I did a melting point analysis. The melting point of the aspirin is 136 ºC. Therefore, I wanted my melting temperatures to be as close to that number as possible.

 

I did the melting point analysis using a melt station, and my samples.

In the pictures above, we see the samples at room temperature (left), then almost completely melted (right) in the melt station.I did three trials for each sample. The melting temperatures I kept for each sample were the average of three melting temperatures.

 

I recorded the following melting temperatures: 

By analyzing the samples’ melting points, we can see that the first sample’s melting temperature is really close to the aspirin’s expected melting temperature, that is 136ºC. On the other hand, the other two samples’ melting temperatures are closer to the salicylic acid’s melting temperature, that is 159.0ºC. 

This could mean that the Acetic Anhydride, opened for the first trial, was vaporized into Acetic Acid during the week before the two last trials. Thus, the two last samples are mostly reactant, and almost no aspirin.

Conclusion:

While the first trial had a lower percentage yield, the product was closer to aspirin than the two other samples.

Therefore, we can say that this protocol is viable in the Santa Fe College laboratories only if the Acetic Anhydride is stored more carefully in order to avoid its vaporization.

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