# R/S configuration

The Cahn-Ingold-Prelog (CIP) rules label a stereocentre R or S. Rank the four groups by atomic number at the first point of difference. Point the lowest priority away from you, then trace 1 to 2 to 3: clockwise is R, anticlockwise is S.

Chemistry (university) · Part 09: Organic structure and stereochemistry · Card 8 of 9 · Last updated 2026-10-19

## Analogy

Steering a car with the horn pointing away: turn right for R (rectus), left for S (sinister).

## Example

**2-butanol priorities**

1: OH   2: CH₂CH₃
3: CH₃   4: H
Look with H at the back.
Clockwise 1→2→3 = (R)

## In practice

Naming R or S lets chemists say exactly which mirror form they mean, in writing, without a picture.

## Practice

### Flashcards

- **How do you assign R or S?** Rank groups by CIP priority, put the lowest away, and trace 1→2→3: clockwise R, anticlockwise S.

### 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

### What if the lowest-priority group points toward me?

If the lowest-priority group faces you, trace 1 to 2 to 3 as normal and then reverse the answer. Clockwise would normally mean R, but with the group toward you it is S. It is easy to forget this step, so check the position of the lowest group.

### How do I rank groups when the first atoms are the same?

Move outward to the next atoms and compare them in order, comparing the highest first. For CH₂CH₃ versus CH₃, the first atom is carbon in both, but the ethyl carbon has (C,H,H) and methyl has (H,H,H), so ethyl ranks higher.

## Related concepts

- [Chirality and stereocentres](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-7)
- [Enantiomers, diastereomers and meso compounds](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-9/chemistry-university-part-9-concept-9)

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