Overview
This lecture explains how to assign absolute configurations (R and S) to chiral centers using the Cahn-Ingold-Prelog system, and how to name chiral molecules accordingly.
Chiral Centers and Absolute Configuration
- Chiral centers are carbons bonded to four different groups, existing as non-superimposable mirror images (enantiomers).
- The absolute configuration describes the spatial arrangement of these groups as R (rectus, right) or S (sinister, left).
Cahn-Ingold-Prelog Priority Rules
- Assign priorities (1–4) to groups attached to the chiral center based on descending atomic number.
- If two groups have the same first atom, compare the next set of atoms attached.
- For double bonds, treat them as if the atom is bonded multiple times to the same atom.
Assigning R and S Configurations
- Orient the molecule so the lowest priority group (4) is pointing away (dashed bond).
- Trace a path from group 1 → 2 → 3:
- Right-handed (clockwise) = R
- Left-handed (counterclockwise) = S
- If group 4 is toward you (wedge), reverse the result.
- If group 4 is in the plane, swap it with another group, assign configuration, and invert the answer.
Complicated Scenarios and Tricks
- For complex molecules, repeatedly compare atoms outward from the chiral center until the first difference is found.
- Swapping any two groups on the chiral center gives the opposite configuration.
Naming Chiral Molecules
- Name the molecule normally, then add the absolute configuration in parentheses at the start.
- For one chiral center: (R)- or (S)- before the name, e.g., (R)-2-chlorohexane.
- For multiple chiral centers: include location and configuration, e.g., (2R,3S)-2-chloro-3-methylhexane.
Key Terms & Definitions
- Chiral Center — a carbon atom attached to four different groups.
- Absolute Configuration — R or S label describing the 3D arrangement of groups on a chiral center.
- Cahn-Ingold-Prelog Rules — a system for assigning priorities to substituents.
- Enantiomers — non-superimposable mirror image molecules.
- Stereocenter — any atom at which the interchange of two groups produces a stereoisomer.
Action Items / Next Steps
- Practice assigning R and S configurations using the rules.
- Try naming molecules with one or more chiral centers including the proper configuration labels.