Thalidomide is a drug that was prescribed in the late 1950s. It exists as a pair of optical isomers. One isomer relieved morning sickness in pregnant women, while the other caused severe birth defects. Explain why thalidomide exists as optical isomers and describe the relationship between the two isomers, including how they behave differently when plane-polarised light is passed through their solutions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Thalidomide has the molecular formula C₁₃H₁₀N₂O₄ and contains a carbon atom bonded to four different groups.
Model answer (5 marks)
Thalidomide contains a chiral carbon, i.e. an asymmetric centre bonded to four different groups.
The two molecules are non‑superimposable mirror images of each other; they are therefore enantiomers.
Each enantiomer rotates plane‑polarised light in opposite directions – one is dextrorotatory (+) and the other levorotatory (‑).
If an equal mixture of the two enantiomers (a racemate) is prepared, the rotations cancel and the solution shows no net rotation of plane‑polarised light.
The two molecules are non‑superimposable mirror images of each other; they are therefore enantiomers.
Each enantiomer rotates plane‑polarised light in opposite directions – one is dextrorotatory (+) and the other levorotatory (‑).
If an equal mixture of the two enantiomers (a racemate) is prepared, the rotations cancel and the solution shows no net rotation of plane‑polarised light.
Examiner tips
- Identify the chiral centre and use the term ‘asymmetric carbon’.
- State that the isomers are mirror images and are called enantiomers.
- Explain the opposite optical rotation of each enantiomer.
- Mention that a racemic mixture shows no overall rotation.
Common mistakes
- Calling the isomers diastereomers instead of enantiomers.
- Forgetting that the rotation directions are opposite.
- Assuming a racemate still rotates light instead of cancelling out.
Mark scheme (5 marks)
- Thalidomide contains a chiral carbon (chiral centre / asymmetric carbon)
- The two optical isomers are non-superimposable mirror images of each other
- The two optical isomers are called enantiomers
- Each enantiomer rotates plane-polarised light in opposite directions
- An equal mixture (racemic mixture / racemate) of the two enantiomers would show no overall rotation of plane-polarised light
Key terms in this question
optical isomers · plane-polarised light
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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