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Pain and Aspirin

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What causes pain and how do we stop it? This free course, Pain and Aspirin, looks at how the human body responds to the release of certain chemicals and as a result feels pain. Pain can be reduced by inhibiting the formation of such chemicals and you will learn how the molecular structure of aspirin has been formulated to help in this process.

After studying this course, you should be able to:

  • demonstrate general knowledge and understanding of some of the basic facts, concepts and principles relating to the development of medicines
  • demonstrate knowledge and understanding of the science behind the development of some drugs to achieve particular tasks
  • demonstrate knowledge and understanding of how chemical bonding determines the properties of compounds and provides an explanation for the mode of action of drugs
  • apply this knowledge and understanding to address familiar and unfamiliar situations
  • express unit concepts in an objective and factually correct way.

By: The Open University

  • Duration 9 hours
  • Updated Thursday 24th March 2016
  • Introductory level
  • Posted under Biology
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1.4.3 Aspirin

Question 1

Compare the structure of aspirin, 2.8, with that of salicylic acid, 2.7. What similarities and differences can you see?


The structures look quite similar. They both have a benzene ring carrying two groups, on adjacent carbon atoms. In both of them one of the groups is a carboxylic acid group. But, salicylic acid carries a phenol group whilst aspirin does not.

Question 2

Can you identify the group that is carried by aspirin in the corresponding place to the phenol group in the molecule of salicylic acid? Have a look at options in Table 1.


You should have concluded that this is an ester group. If you did not identify this group correctly, try making a model. Remember there is free rotation about the single bonds and this should enable you to make it look like the ester group in Table 1.

You are going to study an important reaction between functional groups on molecules in this section. If you are new to chemistry you may not have seen chemical equations before, so before moving on work through Box 1 which provides you with a brief introduction to this topic.

Box 1 An introduction to chemical equations

When chemists want to refer to a chemical reaction in which bonds are broken and new bonds are formed to produce new molecules (the products) from other molecules (the reactants), they often do so by means of a chemical equation. The reactants are shown on the left and the products are on the right,

reactants = products

As atoms cannot be created or destroyed in chemical reactions, the total number of atoms of each element involved must be the same on each side of the equation, if the equation is to ‘balance’ correctly. The two sides are then linked by an equals (=) sign and the reaction is referred to as a ‘balanced reaction’.

For example, methane – the first molecule you made a model of and a fossil fuel gas – burns in the oxygen of the air to form carbon dioxide (CO2) and water (H2O). The reaction can be represented by the equation:

CH4 + 2O2 = CO2 + 2H2O

Count the atoms of each element on the left of the reaction and compare with the numbers on the right. They should be the same! Remember 2O2 means 2 × 2 = 4 atoms of oxygen (O).

Note the need for two molecules of oxygen and two molecules of water to balance the equation. The information that this equation contains is that one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.

Sometimes the numbers of the molecules are unimportant and we just want to focus on the formulae of the reactants and products, not how much of each is involved. Chemists often show this type of relationship with a reaction having an arrow (→) instead of an equals sign. The following reaction is in this style:

CH4 + O2 H2O

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