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CIP Priority Rules
• 1. Higher atomic number = higher priority
• 2. For isotopes: higher mass = higher priority
• 3. Double bond = phantom atom (count twice)
• 4. Point lowest priority AWAY, then read 1→2→3:
Clockwise = R, Anti-clockwise = S
R/S Configuration (CIP Rules)
CIP (Cahn-Ingold-Prelog) rules assign absolute configuration to each chiral centre. R = Rectus (right, clockwise). S = Sinister (left, anticlockwise).
(R)-2-Bromobutane
Priority Groups
1. Br (1st)
2. OH
3. CH₂CH₃
4. CH₃ (4th)
Priority: Br > OH > CH₂CH₃ > CH₃ (atomic number order). Rotate molecule so lowest priority (CH₃) points away. Remaining 3 groups go clockwise → R.
JEE Key Points
• If priority 4 is in front: reverse R/S assignment
• Switching any TWO groups changes R↔S
• Switching any TWO groups TWICE restores configuration
• Enantiomers: ALL stereocentres inverted (R→S and S→R)
• Switching any TWO groups changes R↔S
• Switching any TWO groups TWICE restores configuration
• Enantiomers: ALL stereocentres inverted (R→S and S→R)