---
title: "Optical activity; tartaric acid stereoisomers"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:optical-activity-tartaric"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/optical-activity-tartaric
site: BCT Engineering
---

# Optical activity; tartaric acid stereoisomers

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2069 Bhadra, 1+3+1 marks
- Video: [Optical isomerism in tartaric acid](https://www.youtube.com/watch?v=iQRyhtxO5x4) — Ejaz Saqi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/optical-activity-tartaric
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

What is optical activity? Give the stereoisomers of tartaric acid. Would you expect the following compound to be optically active? Explain.
[Figure: HOOC–CHBr–CHBr–COOH, with H and Br on each of the two central carbons]

## Short answer

Optical activity is the ability to rotate plane-polarised light. Tartaric acid has (2R,3R) and (2S,3S) enantiomers (active) and a meso form (inactive) with a plane of symmetry. The compound HOOC–CHBr–CHBr–COOH with both Br on the same side is the meso form, optically inactive due to internal compensation.

## Answer

#### Optical activity
Optical activity is the property of a substance of rotating the plane of plane-polarised light. A substance that rotates it clockwise is dextrorotatory (+), and one that rotates it anticlockwise is laevorotatory (−). It is caused by molecular chirality, e.g. a carbon bonded to four different groups.

#### Stereoisomers of tartaric acid
```text
   COOH         COOH         COOH
H—C—OH       HO—C—H       H—C—OH
HO—C—H       H—C—OH       H—C—OH
   COOH         COOH         COOH
 (2R,3R)      (2S,3S)      meso (2R,3S)
 L-(+)        D-(−)        inactive
```
- (2R,3R) and (2S,3S) are enantiomers (optically active, equal and opposite rotation).
- The meso form has a plane of symmetry (internal mirror) between C2 and C3, so it is optically inactive by internal compensation. It is a diastereomer of both active forms.
- An equimolar mixture of the (+) and (−) forms is the racemic (±) acid.

#### Is HOOC–CHBr–CHBr–COOH optically active?
**Assumption (figure not shown):** the two Br atoms are on the same side of the Fischer projection, with H and Br on each central carbon. The compound is 2,3-dibromosuccinic acid, which has two identical chiral carbons.
```text
   COOH
H—C—Br
H—C—Br
   COOH
```
With both Br atoms on the same side, the molecule has an internal plane of symmetry, so it is the **meso form** and is **optically inactive** (internal compensation), like meso-tartaric acid. If the two Br atoms were on opposite sides, the molecule would be chiral and optically active (d or l).

## Check yourself

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

1. What is optical activity?
   A. Ability to absorb light
   B. Ability to rotate the plane of plane-polarised light
   C. Ability to emit light
   D. Ability to refract light only
   Answer: B. It is caused by molecular chirality. (Key point: Definition)

2. What is a dextrorotatory substance?
   A. Anticlockwise
   B. No rotation
   C. Rotating both ways
   D. One that rotates plane-polarised light clockwise
   Answer: D. Laevorotatory is anticlockwise. (Key point: Dextro)

3. Why is HOOC-CHBr-CHBr-COOH (both Br same side) optically inactive?
   A. It has no chiral carbons
   B. It is a meso form with internal compensation
   C. It is racemic
   D. Br atoms cancel the light
   Answer: B. Internal plane of symmetry. (Key point: Meso)

4. What if the two Br were on opposite sides?
   A. It would still be meso
   B. It would have no isomers
   C. It would be racemic
   D. The molecule would be chiral and optically active
   Answer: D. No plane of symmetry. (Key point: Opposite sides)

## Related in Engineering Chemistry

- [Stereoisomers of tartaric acid; meso form](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Atartaric-acid) — 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
- [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