Ibuprofen is a common painkiller. It contains a chiral centre in its molecular structure. Only one of its optical isomers is pharmacologically active, meaning only one isomer relieves pain. Explain what is meant by a chiral centre, and explain why the two optical isomers of ibuprofen can have different effects in the human body despite having the same molecular formula and the same structural formula.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ibuprofen is a common painkiller sold in pharmacies worldwide. Its molecule contains a chiral centre, and it is sold commercially as a racemic mixture, meaning equal amounts of both optical isomers are present.
Model answer (5 marks)
A chiral centre is a carbon atom bonded to four different atoms or groups.
The two optical isomers are non‑superimposable mirror images of each other.
They rotate plane‑polarised light in opposite directions.
Biological receptor sites have a specific three‑dimensional shape.
Only one isomer has the correct three‑dimensional arrangement to fit the receptor site, so only that isomer is pharmacologically active.
The two optical isomers are non‑superimposable mirror images of each other.
They rotate plane‑polarised light in opposite directions.
Biological receptor sites have a specific three‑dimensional shape.
Only one isomer has the correct three‑dimensional arrangement to fit the receptor site, so only that isomer is pharmacologically active.
Examiner tips
- Use the exact definition of a chiral centre. Show the mirror‑image relationship. Mention optical rotation. Explain the receptor‑binding rationale. Keep each point short and to the point.
Common mistakes
- Confusing chirality with stereochemistry in general. Saying the isomers have different chemical formulas. Forgetting to link the 3D shape to receptor binding.
Mark scheme (5 marks)
- A chiral centre is a carbon atom bonded to four different atoms/groups
- The two optical isomers are non-superimposable mirror images of each other
- The optical isomers rotate plane-polarised light in opposite directions
- Biological receptor sites/molecules have a specific three-dimensional shape
- Only one isomer has the correct three-dimensional arrangement to fit the receptor site, so only one isomer is active
Key terms in this question
chiral centre · optical isomers
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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