Chapter 14 – From Organic Molecules to Medicines

Aspirin Pharmaceutical products are often developed from substance found in a plant that has been used for traditional medicine. Aspirin is one of these products. Salicylic acid is the active substance that helps helps to reduce pain and fever. However, salicylic acid irritates the lining of the stomach. So when salicylic acid is reacted with ethanoic acid,it produces acetylsalicylic acid (aspirin). This reaction, however, is slow and the yield is low as the water formed tends to drive the reaction backwards. An alternative reaction pathway, that is faster and produces higerh yields, is one between ethanoic anhydride (acetic anhydride) and salicylic acid. Once the acid has passed through the stomach it reacts with water in the small intestine and returns to the more effective salicylic acid.

Reaction Pathways for Aspirin

Soluble Aspirin Pure acetylsalicylic acid is not very soluble in water, despite have a –COOH functional group. Though converting the carboxylic acid functional group into the sodium salt changes the molecule into an ion and makes it much more soluble. Buffered aspirin provides further protection against stomach irritation.

Identifying Aspirin - IR The analytical techniques IR, NMR and Mass Spectrometry can be used to analyse aspirin. The molecular structure of acetylsalicylic acid is shown below: There are two C=O groups present. One is part of an ester and the other is part of the carboxyl group. These correspond to the two absorptions at 1760 and 1770cm -1 in the IR spectrum.

Identifying Aspirin - NMR There are four hydrogen around the benzene ring, three Hs in the CH 3 group and one in the OH group. The four hydrogen attached to the benzene ring give four separate peaks. The three hydrogen atoms of CH 3 all experience the same environment so they are the large peak. The single hydrogen atom in the COOH group would give a small peak, it is not shown in the spectrum over.

Identifying Aspirin cont…

Identifying Aspirin – Mass Spec The peak at 180 is the molecular formula ion peak, this is the Mr of Aspirin. The masses of the various fragments are used to confirm the structural formula.