How is the Cahn-Ingold-Prelog system used for naming chiral centers?

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:

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Steps to assign R or S:

1. Identify the chiral center
   • A carbon atom with four different groups attached.
2. 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.
3. Orient the molecule
   • Place the lowest priority group (4) pointing away from you.
4. Determine the order of the other three groups
   • Trace a path from priority 1 → 2 → 3.
5. Assign configuration:
   • Clockwise → R (rectus, right)
   • Counterclockwise → S (sinister, left)

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Example:

For a carbon with groups: -OH, -CH₃, -H, -Cl

1. Priority order: Cl (1) > OH (2) > CH₃ (3) > H (4)
2. Orient H away from you
3. Trace Cl → OH → CH₃
4. If clockwise → R, if counterclockwise → S

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Simple memory tip:

• R = Right, clockwise
• S = Left, counterclockwise

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