Overview
This lecture explains how to assign absolute configurations (R and S) to chiral centers using the Cahn-Ingold-Prelog system, including naming conventions and handling tricky cases.
Absolute Configuration & Chiral Centers
- Absolute configuration refers to the spatial arrangement of atoms at a chiral center.
- A chiral center (usually a carbon) is bonded to four different groups, resulting in non-superimposable mirror images (enantiomers).
- R and S are used to distinguish between the two forms of a chiral center.
Cahn-Ingold-Prelog (CIP) Priority Rules
- Assign priorities (1-4) to each group attached to the chiral center based on atomic number; highest gets 1, lowest gets 4.
- If there is a tie (e.g., two carbons), move outwards to the next atoms until the tie is broken, always comparing the highest atomic number first.
- For double/triple bonds, count the bonded atom multiple times (e.g., a double bond to O counts as two O's).
Assigning R and S Configurations
- Orient the molecule so the lowest priority group (4) is pointing away (on the dashed bond).
- Trace a path from 1 → 2 → 3:
- Right-handed (clockwise) turn: R (from Latin 'rectus' for right).
- Left-handed (counter-clockwise) turn: S (from Latin 'sinister' for left).
- If the lowest priority group is not on a dashed bond:
- On a wedge (towards you): Reverse the result (e.g., a right turn means S).
- In the plane: Swap the lowest priority group with one on a dash or wedge, assign configuration, then invert the result.
Advanced Examples & Tips
- For complicated cases with multiple identical atoms, continue along the chains, comparing atoms at each branch until a difference is found.
- When multiple chiral centers exist, assign R or S to each individually.
Naming Chiral Molecules
- Name the basic structure as usual.
- For one chiral center: Add (R) or (S) in parentheses before the name (e.g., (R)-2-chlorohexane).
- For multiple chiral centers: Specify positions and configurations in parentheses, separated by commas (e.g., (2R,3S)-2-chloro-3-methylhexane).
Key Terms & Definitions
- Chiral Center — A carbon atom bonded to four different groups.
- Absolute Configuration — The precise 3D spatial arrangement of groups around a chiral center, denoted as R or S.
- Cahn-Ingold-Prelog Rules — A system for assigning priorities to groups attached to a chiral center.
- Enantiomers — Stereoisomers that are non-superimposable mirror images.
- Wedge/Dash — Wedge indicates bond coming out of the plane; dash indicates bond going behind the plane.
Action Items / Next Steps
- Practice assigning R and S configurations using the CIP system.
- Complete homework or practice problems on chiral centers and naming conventions.