R-S System of Nomenclature (Absolute Configuration)

R-S System of Nomenclature (Absolute Configuration) designates chiral centers as R or S based on substituent priority and spatial arrangement.

R-S System of Nomenclature (Absolute Configuration)

The R/S system gives the absolute configuration of a chiral center using the sequence rules.

Steps to assign R or S:

1. Identify the chiral center.
2. Assign priority (1–4) to the four groups attached to the chiral center using the sequence rules.
3. Orient the molecule so that the lowest priority group (4) is pointing away from you (behind the plane).
4. Observe the sequence from 1 → 2 → 3:
   • If the sequence goes clockwise → the configuration is R (Rectus)
   • If the sequence goes counterclockwise → the configuration is S (Sinister)

Example: 2-butanol (CH₃–CH(OH)–CH₂–CH₃)

• Chiral carbon = second carbon.
• Groups: OH, CH₃, CH₂CH₃, H
  • OH > CH₂CH₃ > CH₃ > H
• Assign priorities.
• Make sure H is at the back.
• If sequence 1-2-3 is clockwise → R
• If counterclockwise → S

Note:

• Unlike the D-L system, the R/S system is universal, and can be applied to any type of compound (not just sugars or amino acids).
• R/S configuration does not correspond to D/L or +/–. They are different things:
  • R/S = absolute configuration
  • D/L = configuration based on glyceraldehyde
  • +/– = optical rotation

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