# Chirality and stereocentres

A molecule is chiral if it cannot be placed on top of its mirror image. This usually happens at a carbon with four different groups, called a stereocentre (chiral centre). A molecule with n stereocentres has up to 2ⁿ stereoisomers.

Chemistry (university) · Part 09: Organic structure and stereochemistry · Card 7 of 9 · Last updated 2026-10-19

## Analogy

Your left and right hands: mirror images that never fit into the same glove.

## Example

**2-butanol**

C2 holds: OH, CH₃, C₂H₅, H
All four different, so chiral
Two mirror forms exist
2¹ = 2 stereoisomers

## In practice

One mirror form of a drug can treat illness while the other does nothing or causes side effects, so drug makers control which form they make.

## Practice

### Flashcards

- **What makes a carbon a stereocentre?** Four different groups attached.

### Interview questions

**Assign R or S to one form of 2-butanol, give the priority order, and explain what enantiomers are.**

- Priorities: OH > ethyl > methyl > H
- Lowest priority at the back
- Clockwise is R, anticlockwise is S
- Enantiomers are mirror images with opposite labels

## Questions people ask

### How do I spot a stereocentre?

Look for a carbon with four different groups attached, as in 2-butanol or lactic acid. A carbon with two identical groups, like CH₂ or a carbon in a double bond, is not a stereocentre. Check each group by following the chain until a difference appears.

### Do chiral molecules have different physical properties?

Mirror-image molecules (enantiomers) have identical melting points, boiling points and solubility. They differ in how they rotate plane-polarised light, in opposite directions, and in how they interact with other chiral things, like enzymes and receptors in the body. That is why one mirror form of a drug may work differently from the other.

## Related concepts

- [R/S configuration](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-8)
- [Enantiomers, diastereomers and meso compounds](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-9)

Web page: https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-7
Study this card in the app: https://studyvisually.com/app/learn?category=chemistry-university&card=chemistry-university-part-9-concept-7