The Cahn-Ingold-Prelog (CIP) system is a method used to assign the absolute configuration of chiral centers in molecules as R or S. Here’s a simple step-by-step explanation:
Steps to assign R or S:
- Identify the chiral center
- A carbon atom with four different groups attached.
- Assign priorities (1–4) to the groups
- Higher atomic number → higher priority.
- If two atoms are the same, move outward along the chain until a difference is found.
- Orient the molecule
- Place the lowest priority group (4) pointing away from you.
- Determine the order of the other three groups
- Trace a path from priority 1 → 2 → 3.
- Assign configuration:
- Clockwise → R (rectus, right)
- Counterclockwise → S (sinister, left)
Example:
For a carbon with groups: -OH, -CH₃, -H, -Cl
- Priority order: Cl (1) > OH (2) > CH₃ (3) > H (4)
- Orient H away from you
- Trace Cl → OH → CH₃
- If clockwise → R, if counterclockwise → S
Simple memory tip:
- R = Right, clockwise
- S = Left, counterclockwise