Question

Why do you think the Iron (III) Chloride Test was able to determine if there was still unreacted salicylic acid? Think about organic chemistry, and why this may be the case?

Answer 1

Synthesis of acetylsalicylic acid (or aspirin):

Salicylic acid contains a phenol group, and phenols are replaced the phenol group with an ester group. This esterified compound (acetylsalicylic acid, also known as aspirin). Aspirin is synthesized by reacting salicylic acid with acetic acid in the presence of an acid catalyst. The phenol group on the salicylic acid forms an ester with the carboxyl group on the acetic acid. However, this reaction is slow and has a relatively low yield. If acetic anhydride is used instead of acetic acid, the reaction is much faster and has a higher yield (since acetic anhydride is much more reactive than acetic acid).

The collected aspirin will be tested for its purity using FeCl₃ (aq). Iron (III) ion reacts with phenols to form a purple complex. Salicylic acid contains a phenol group, but acetylsalicylic acid does not. Therefore, if you add FeCl₃ to an aspirin sample and you see a purple color, it means that there is still some salicylic acid present and the sample is impure.