Lactic acid (2-hydroxypropanoic acid) is produced in muscle cells during intense exercise. It exists as two optical isomers. Explain why lactic acid displays optical isomerism and describe the relationship between its two optical isomers, including how they interact with plane-polarised light.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lactic acid has the molecular formula C₃H₆O₃. Its central carbon atom is bonded to four different groups: a hydroxyl group (–OH), a carboxyl group (–COOH), a methyl group (–CH₃), and a hydrogen atom.
Model answer (5 marks)
1. Lactic acid has a central carbon atom bonded to four different groups – a hydroxyl group, a carboxyl group, a methyl group and a hydrogen atom.
2. This gives the carbon a chiral centre, so the molecule is chiral.
3. The two possible configurations (R and S) are non‑superimposable mirror images.
4. These mirror‑image forms are called enantiomers.
5. Each enantiomer rotates plane‑polarised light in opposite directions; a 1:1 mixture (racemate) shows no net rotation and is optically inactive.
2. This gives the carbon a chiral centre, so the molecule is chiral.
3. The two possible configurations (R and S) are non‑superimposable mirror images.
4. These mirror‑image forms are called enantiomers.
5. Each enantiomer rotates plane‑polarised light in opposite directions; a 1:1 mixture (racemate) shows no net rotation and is optically inactive.
Examiner tips
- Use the word "chiral centre" and list the four different groups. Show the R/S distinction and that the forms are mirror images. Mention the opposite rotation of each enantiomer. Explain that a racemic mixture gives zero rotation.
Common mistakes
- Confusing a stereocentre with a double bond. Saying the molecule is achiral. Forgetting that the rotation directions are opposite or that a racemate is inactive.
Mark scheme (5 marks)
- The central carbon atom is bonded to four different groups/substituents, making it a chiral centre (or 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.
- A racemic mixture (equal amounts of both enantiomers) shows no overall rotation of plane-polarised light / is optically inactive.
Key terms in this question
optical isomerism · 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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