---
title: "Optical isomerism; chiral centre and optical activity"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:optical-isomerism-chiral"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/optical-isomerism-chiral
site: BCT Engineering
---

# Optical isomerism; chiral centre and optical activity

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2067 Chaitra, 1+4 marks
- Video: [Optical Isomerism and Chirality | A-level Chemistry | OCR, AQA, Edexcel](https://www.youtube.com/watch?v=cmG9CWpfFOY) — Launchpad Learning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/optical-isomerism-chiral
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

What is optical isomerism? Comment why presence of chiral centre is not sufficient for the molecule to be optically active.

## Short answer

Optical isomerism involves isomers that rotate plane-polarised light differently due to chirality. A chiral centre alone does not guarantee optical activity; a molecule may have a plane of symmetry (meso, internal compensation) or be a racemic mixture where rotations cancel externally.

## Answer

#### Optical isomerism
It is a type of stereoisomerism in which isomers have the same structural formula but differ in the direction in which they rotate plane-polarised light; they are mirror images that cannot be superimposed (enantiomers). It arises from chirality, as in lactic acid CH₃CH(OH)COOH, which has a carbon bonded to four different groups.

#### A chiral centre is not sufficient
The real condition for optical activity is that the molecule as a whole is **chiral**, i.e. it has no plane or centre of symmetry. A chiral centre alone does not guarantee this:
1. **Meso compounds:** meso-tartaric acid has two chiral centres, but an internal plane of symmetry makes it achiral and inactive (internal compensation).
2. **Racemic mixtures:** each molecule is chiral, but the equal and opposite rotations of the d and l forms cancel (external compensation), so the mixture is inactive.
3. **Centre of symmetry:** a molecule with chiral centres that also has a centre of symmetry (as in some meso compounds) is achiral and therefore inactive.

So optical activity requires a chiral molecule, not just a chiral centre.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is optical isomerism?
   A. Isomers with different structural formulae but the same molecular formula
   B. Stereoisomers with the same structural formula that rotate plane-polarised light differently and are non-superimposable mirror images
   C. Isomers differing only in the position of a double bond
   D. Isomers that interconvert freely by rotation about a single bond
   Answer: B. Enantiomers are non-superimposable mirror images that rotate plane-polarised light in opposite directions. (Key point: Optical isomerism)

2. What is the true requirement for a molecule to be optically active?
   A. It contains at least one chiral centre
   B. It contains a carbon bonded to four identical groups
   C. The whole molecule is chiral, with no plane or centre of symmetry
   D. It contains a double bond
   Answer: C. Chirality of the molecule as a whole, not just the presence of a chiral centre, is needed. (Key point: Real condition for optical activity)

3. Why is meso-tartaric acid optically inactive despite two chiral centres?
   A. An internal plane of symmetry makes it achiral (internal compensation)
   B. Its two chiral centres cancel because they are racemised by heat
   C. It is a racemic mixture of d and l forms
   D. Its chiral centres are not bonded to four different groups
   Answer: A. The internal mirror plane makes the molecule achiral; rotations of the two halves cancel. (Key point: Meso compounds)

4. Why is a racemic mixture optically inactive?
   A. Each molecule in it is achiral
   B. It has an internal plane of symmetry
   C. The molecules lose their chiral centres on mixing
   D. Equal and opposite rotations of d and l forms cancel (external compensation)
   Answer: D. Each molecule is chiral, but the equal and opposite rotations cancel between molecules. (Key point: Racemic mixture)

## Related in Engineering Chemistry

- [Optical activity; tartaric acid stereoisomers](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aoptical-activity-tartaric) — Engineering Chemistry, chapter 9
- [Optical isomerism; tartaric acid; maleic/fumaric Z,E](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aoptical-isomerism-tartaric-maleic) — Engineering Chemistry, chapter 9
- [Geometrical isomerism; trans stability; propene](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ageometrical-isomerism) — Engineering Chemistry, chapter 9

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