Chapter 14

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Chapter 14 From Organic Molecules to Medicines
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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.

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Reaction Pathways for Aspirin
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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.

5
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 CO 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.

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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 over.

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Identifying Aspirin cont
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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.