Explain what is meant by optical isomerism and describe the conditions that must be met for a molecule to display optical isomerism. In your answer, state how the two optical isomers of a compound are related to each other.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Thalidomide was a drug used in the 1950s and 1960s. It exists as two optical isomers: one isomer relieved morning sickness, while the other caused serious birth defects. Both isomers have the same molecular formula and the same structural formula.
Model answer (5 marks)
Optical isomerism refers to a situation where two molecules are non‑superimposable mirror images of each other.
For a molecule to show optical isomerism it must contain at least one chiral centre – a carbon atom bonded to four different atoms or groups.
The two molecules that are mirror images are called enantiomers. Enantiomers rotate plane‑polarised light in equal but opposite directions.
For a molecule to show optical isomerism it must contain at least one chiral centre – a carbon atom bonded to four different atoms or groups.
The two molecules that are mirror images are called enantiomers. Enantiomers rotate plane‑polarised light in equal but opposite directions.
Examiner tips
- Use the phrase "non‑superimposable mirror images" to hit the key point. Mention a chiral centre with four different substituents. State that the pair are enantiomers and that they rotate light in opposite directions. Keep the answer concise – 5 marks can be earned with 4–5 short sentences.
Common mistakes
- Confusing optical isomerism with geometric isomerism. Failing to mention that the chiral carbon must have four different groups. Not stating that the two isomers are enantiomers or that they rotate light in opposite directions.
Mark scheme (5 marks)
- Optical isomers are non-superimposable mirror images of each other
- Optical isomers arise when a molecule contains a chiral centre (chiral carbon)
- The chiral carbon must have four different atoms or groups attached to it
- The two optical isomers are called enantiomers
- The two enantiomers rotate plane-polarised light in opposite directions
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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