# Enantiomers, diastereomers and meso compounds

Enantiomers are non-superimposable mirror images. Diastereomers are stereoisomers that are not mirror images. A meso compound has stereocentres but is achiral because of an internal mirror plane, so it has fewer isomers than 2ⁿ.

Chemistry (university) · Part 09: Organic structure and stereochemistry · Card 9 of 9 · Last updated 2026-10-19

## Analogy

Enantiomers are left and right gloves. Diastereomers are cousins: related but not mirror images.

## Example

**Tartaric acid**

Two stereocentres: expect 4
(R,R) and (S,S): enantiomers
(R,S) is meso (internal mirror plane)
Total: 3 stereoisomers

## In practice

Tartaric acid in grapes has three stereoisomers: a pair of enantiomers and one meso form.

## Practice

### Flashcards

- **What are enantiomers?** Non-superimposable mirror images.
- **What is a meso compound?** One with stereocentres but an internal mirror plane, so it is achiral.

## Questions people ask

### Why does a meso compound not rotate plane-polarised light?

A meso compound has an internal mirror plane, so one half of the molecule cancels the other half's effect on light. It is achiral and optically inactive, even though it contains stereocentres. A racemic mixture is also inactive, but for a different reason: equal amounts of enantiomers cancel.

### Do diastereomers have the same physical properties?

No. Unlike enantiomers, diastereomers have different melting points, boiling points and solubilities. This is useful: chemists separate a mixture of enantiomers by reacting it with a single pure chiral reagent to make diastereomers, which can then be separated by ordinary methods.

## Related concepts

- [Chirality and stereocentres](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-7)
- [R/S configuration](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-8)

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