Chapter 14 – From Organic
Molecules to Medicines
• 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
• 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
• Once the acid has passed through the stomach it reacts with water
in the small intestine and returns to the more effective salicylic
Reaction Pathways for Aspirin
• Pure acetylsalicylic acid is not very soluble in
water, despite have a –COOH functional
• Though converting the carboxylic acid
functional group into the sodium salt changes
the molecule into an ion and makes it much
• Buffered aspirin provides further protection
against stomach irritation.
Identifying Aspirin - IR
• The analytical techniques IR, NMR and Mass Spectrometry can be used to
• 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
Identifying Aspirin - NMR
• There are four hydrogen around the benzene ring,
three Hs in the CH3 group and one in the OH group.
• The four hydrogen attached to the benzene ring give
four separate peaks.
• The three hydrogen atoms of CH3 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
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.