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Aspirin
Chapter 14
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 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 higher 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.
Preparation of ethanoic
anhydride
Reaction Pathways for Aspirin
Aspirin converts back to
salicylic acid in the small
intestine
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.
New products: Soluble
“Aspro”
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 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.
NMR Aspirin
MS Aspirin
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.
Designing Drugs
Percentage Yield Calculations