How to assign R and S configuration

Step 1: find the stereocentre

A stereocentre (chiral centre) is usually a carbon with four different groups attached. Butan-2-ol has one: C2 carries OH, CH₃, CH₂CH₃ and H.

Step 2: rank the four groups (CIP rules)

1. Higher atomic number of the atom attached directly to the stereocentre = higher priority. So Br > Cl > O > N > C > H.
2. If two attached atoms are the same, list the three atoms attached to each, highest first, and compare them one by one. The first difference decides.
3. Double and triple bonds count as duplicated atoms: –CH=O counts as C attached to (O, O, H).

For butan-2-ol: OH (1) > CH₂CH₃ (2) > CH₃ (3) > H (4). The ethyl beats methyl because its CH₂ carries (C,H,H) against methyl's (H,H,H).

Step 3: point the lowest priority away from you

In a wedge-and-dash drawing, the lowest-priority group should be on a dashed (hashed) bond, going into the page.

Step 4: trace 1 → 2 → 3

Clockwise is R (rectus, right). Anticlockwise is S (sinister, left).

Two shortcuts

• Lowest priority on a wedge (towards you)? Work out the direction anyway, then reverse it.
• Swap trick: swapping any two groups inverts the centre. Swap the lowest-priority group into the back, assign R/S, then reverse the answer.

Examples

R/S and reactions

An SN2 reaction inverts the stereocentre. That often turns R into S, but not always: R/S depends on priorities, which can change when the nucleophile replaces the leaving group. An SN1 reaction at a stereocentre gives a racemic mixture of R and S.

The same ranking rules give E and Z for alkenes: see E/Z vs cis/trans. To find the stereocentres in the first place, see how to identify chiral centres. For molecules drawn as crosses, see Fischer projections.