A student is studying two organic molecules, molecule A and molecule B. Molecule A is butan-2-ol. Molecule B is butan-1-ol. The student claims that butan-2-ol displays optical isomerism but butan-1-ol does not. Explain why the student's claim is correct, including what is meant by optical isomerism and the structural feature responsible for it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Optical isomerism is a form of stereoisomerism. Two molecules that are optical isomers are non-superimposable mirror images of each other. A molecule displays optical isomerism when it contains a chiral centre.
Model answer (5 marks)
Optical isomerism is a type of stereoisomerism where two molecules are non‑superimposable mirror images.
1. It occurs when a molecule has a chiral centre (an asymmetric carbon).
2. A chiral centre is a carbon bonded to four different atoms or groups.
3. The two optical isomers are therefore non‑superimposable mirror images.
4. In butan‑2‑ol the carbon‑2 atom is bonded to H, OH, CH₃ and C₂H₅ – four different groups – so it is a chiral centre.
5. In butan‑1‑ol no carbon is bonded to four different groups, so there is no chiral centre and no optical isomerism.
1. It occurs when a molecule has a chiral centre (an asymmetric carbon).
2. A chiral centre is a carbon bonded to four different atoms or groups.
3. The two optical isomers are therefore non‑superimposable mirror images.
4. In butan‑2‑ol the carbon‑2 atom is bonded to H, OH, CH₃ and C₂H₅ – four different groups – so it is a chiral centre.
5. In butan‑1‑ol no carbon is bonded to four different groups, so there is no chiral centre and no optical isomerism.
Examiner tips
- Define optical isomerism and chiral centre first; then give the structural reason for each molecule; use the exact terminology "non‑superimposable mirror images" and "four different groups".
- Show the chain and label the carbon atoms to make the argument clear.
- Keep the answer concise – 5 points can be covered in 5 short sentences.
Common mistakes
- Claiming butan‑1‑ol is chiral because it has an –OH group; the –OH alone does not create chirality.
- Using the phrase "optical activity" instead of "optical isomerism"; the question asks for the type of isomerism, not the property.
Mark scheme (5 marks)
- Optical isomerism occurs when a molecule has a chiral centre / asymmetric carbon atom
- 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
- In butan-2-ol, carbon-2 is bonded to four different groups (H, OH, CH3, and C2H5), so it is a chiral centre
- In butan-1-ol, no carbon is bonded to four different groups, so there is no chiral centre and no optical isomerism
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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