---
title: "Enantiomers vs diastereomers"
subject: "Engineering Chemistry"
chapter: 9
key: "semester_2/engineering_chemistry:enantiomers-diastereomers"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/enantiomers-diastereomers
site: BCT Engineering
---

# Enantiomers vs diastereomers

Engineering Chemistry · Chapter 9: Stereochemistry

- Asked in: 2073 Bhadra, 2.5 marks; 2073 Magh, 2 marks; 2072 Magh, 3 marks; 2070 Magh, 3 marks; 2069 Poush, 3 marks; 2068 Bhadra, 3 marks (asked 6 times)
- Video: [Enantiomers, Diastereomers and Meso Compounds: Multiple Chiral Centers](https://www.youtube.com/watch?v=Q3CULuxnUis) — Leah4sci
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/enantiomers-diastereomers
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-9

## Question

Distinguish between enantiomers and diastereomers. Give an example to support your answer.

## Short answer

Enantiomers are non-superimposable mirror images with identical physical properties and opposite optical rotation. Diastereomers are stereoisomers that are not mirror images, have different physical properties. Example: tartaric acid's (R,R) and (S,S) forms are enantiomers; meso-tartaric acid is a diastereomer of each.

## Answer

#### Enantiomers vs diastereomers
| Basis | Enantiomers | Diastereomers |
|---|---|---|
| Relationship | Stereoisomers that are non-superimposable mirror images | Stereoisomers that are not mirror images |
| Number of chiral centres | One or more (all centres inverted) | Usually two or more (only some inverted); cis–trans isomers are also diastereomers |
| Physical properties | Identical (m.p., b.p., solubility) | Different |
| Chemical properties | Same with achiral reagents | Different |
| Optical rotation | Equal in magnitude, opposite in sign | Different, unrelated values |
| Separation | Difficult (needs resolution) | Easy (crystallisation, distillation) |
| Example | (+)- and (−)-lactic acid; (2R,3R)- and (2S,3S)-tartaric acid | (2R,3R)-tartaric acid and meso-tartaric acid |

Example: tartaric acid has three forms: (2R,3R), (2S,3S) and meso (2R,3S). The (R,R) and (S,S) forms are enantiomers (mirror images). The meso form is a diastereomer of each, because it is not their mirror image, and it has a different m.p. (140 °C vs 170 °C for the active forms) and solubility.

## Check yourself

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

1. How do enantiomers differ from diastereomers in relation?
   A. Enantiomers are not mirror images; diastereomers are
   B. Both are mirror images
   C. Enantiomers are non-superimposable mirror images; diastereomers are not mirror images
   D. Enantiomers are structural isomers
   Answer: C. Enantiomers are mirror images, diastereomers are stereoisomers that are not. (Key point: Relationship)

2. Which statement about physical properties is correct?
   A. Enantiomers have identical properties; diastereomers differ
   B. Enantiomers differ in m.p.; diastereomers are identical
   C. Both differ
   D. Both are identical
   Answer: A. That is why diastereomers are easy to separate. (Key point: Physical properties)

3. Which pair are diastereomers?
   A. (2R,3R)- and (2S,3S)-tartaric acid
   B. (+)- and (-)-lactic acid
   C. (2R,3R)-tartaric acid and meso-tartaric acid
   D. Two molecules of the same meso acid
   Answer: C. Meso is not the mirror image of either active form. (Key point: Example)

4. How do optical rotations of enantiomers compare?
   A. Equal in magnitude, opposite in sign
   B. Equal in both magnitude and sign
   C. Unrelated values
   D. Both zero
   Answer: A. Diastereomers have different unrelated values. (Key point: Optical rotation)

## Related in Engineering Chemistry

- [Enantiomers and diastereomers of 3-bromo-2-butanol](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aenantiomers-diastereomers-bromobutanol) — Engineering Chemistry, chapter 9