Synthesis of Aspirin

The conversion of salicylic acid to acetylsalicylic acid, the main component of the drug commonly known as aspirin, involves an esterification reaction with ethanoic anhydride and is often used in laboratories for academic purposes.

When performed, the synthetic procedure involves a recrystallization process that has the purpose of separating impurities from the product. This separation is achieved by choosing a solvent that dissolves the solid when hot, but not when cold. Different steps are involved in the process:

Weigh 1 gram of salicylic acid on a tared scale and put it into a dry pear shaped flask.
Move the flask into the fume hood and secure the flask to the support over a warm bath, to secure the reagents. Then add 2 milliliters of ethanoic anhydride, followed by 8 drops of concentrated phosphoric acid. Insert the condenser to avoid the dispersion of fumes. In specific synthesis, the addition of activated charcoal can adsorb the impurities in the solution.
Add the magnetic stirrer and warm the solution in the hot water bath until the solid has dissolved, then for another 5 minutes.
Remove the flask from the warm bath and carefully add 5 milliliters of cold water. The solution needs to be cooled down in two steps. First, the conical flask needs to be cooled down to room temperature, and once it is at room temperature, it can be placed in an ice bath for further cooling. If the solution is placed straight into an ice bath, the fast cooling and formation of crystals could trap some of the impurities again in the crystals, making the purification less efficient.

The workflow to recrystallize involves different steps: heating up the solvent until the solid has dissolved, adding activated charcoal, when needed, to filter out the solution, and then cooling down the solution to room temperature. Once the solution has cooled, place it in an ice bath for further cooling. Now you can enjoy your newly formed crystals

Figure 1: Aspirin synthesis steps.