Identifying the Drug in Willow Bark

• Infrared Spectroscopy and Mass Spectrometry can be used to identify the chemical structure of compounds such as salicylic acid.
• Below is the mass spectrum of salicylic acid; chemists can use this to find the structures of compounds.
• The Infrared spectrum can also be used to provide important clues about the groups of atoms contained in salicylic acid.

Working out the structure

1. Chemical tests show the presence of a phenolic OH group (test with FeCl₃ and look for purple colour) and a COOH group (CO₂ gas produced when mixed with carbonate).
2. Mass spectrometry shows that salicylic acid is the same compound as 2-hydroxybenzoic acid:
3. The mass spectrum fragmentation pattern shows that the structure cant be either of the two below, as a H₂O has been lost from the spectrum, which cannot be lost from either of these molecules, as there must be hydrogen bonding occuring between the -COOH and -OH groups in order for a water fragment to be lost (above right):
4. Infrared spectroscopy shows the presence of an OH group and a C=O group.

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