---
title: "Enantiomers, racemic mixture, meso compound"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:enantiomers-racemic-meso"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/enantiomers-racemic-meso
site: BCT Engineering
---

# Enantiomers, racemic mixture, meso compound

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2067 Mangsir, 4 marks
- Video: [Stereoisomers, Enantiomers, Meso Compounds, Diastereomers, Constitutional Isomers, Cis & Trans](https://www.youtube.com/watch?v=XftIUcLWTEE) — The Organic Chemistry Tutor
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/enantiomers-racemic-meso
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

Define enantiomers, racemic mixture and meso compound giving one example of each. Also comment on their optical activity.

## Short answer

Enantiomers are non-superimposable mirror images, like (+)- and (−)-lactic acid, each optically active with opposite rotation. A racemic mixture is an equimolar mixture of enantiomers, e.g. (±)-lactic acid, optically inactive by external compensation. A meso compound like meso-tartaric acid has chiral centres but an internal symmetry plane, rendering it inactive.

## Answer

| Term | Definition | Example | Optical activity |
|---|---|---|---|
| Enantiomers | Pair of stereoisomers that are non-superimposable mirror images | (+)- and (−)-lactic acid | Active; equal in magnitude and opposite in direction |
| Racemic mixture | Equimolar mixture of two enantiomers (written ±) | (±)-lactic acid, (±)-tartaric acid | Inactive, because rotations cancel (external compensation) |
| Meso compound | Compound with two or more chiral centres but with an internal plane of symmetry (achiral) | meso-tartaric acid | Inactive, because the rotation of one half cancels the other (internal compensation) |

Racemic mixtures can be resolved into the active enantiomers; meso compounds cannot, as they are a single compound.

## Check yourself

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

1. What are enantiomers?
   A. Non-superimposable mirror-image stereoisomers
   B. Stereoisomers that are not mirror images
   C. Equimolar mixtures
   D. Achiral compounds
   Answer: A. They rotate light equally in opposite directions. (Key point: Enantiomers)

2. What is the optical activity of a racemic mixture and why?
   A. Active, due to internal compensation
   B. Inactive, due to internal compensation
   C. Inactive, due to external compensation
   D. Active, as it is a single enantiomer
   Answer: C. Rotations of the two enantiomers cancel. (Key point: Racemic)

3. Why is a meso compound inactive despite chiral centres?
   A. An internal plane of symmetry makes it achiral
   B. It is a mixture
   C. It has no chiral centres
   D. It is a racemic mixture
   Answer: A. Internal compensation. (Key point: Meso)

4. Which can be resolved into active forms?
   A. Meso compound only
   B. Both
   C. Racemic mixture only
   D. Neither
   Answer: C. Meso is a single compound. (Key point: Resolution)

## Related in Engineering Chemistry

- [Racemic mixture vs meso compound](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aracemic-meso) — Engineering Chemistry, chapter 9
- [Diastereomers and resolution of racemic mixture](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Adiastereomers-resolution-racemic) — Engineering Chemistry, chapter 9
- [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; chiral centre and optical activity](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aoptical-isomerism-chiral) — Engineering Chemistry, chapter 9