R-S-System of nomenclature

R-S-System of nomenclature

source : 2.chemistry.msu.edu


R,S-system of nomenclature was given by Cahn, ingold and Prelog in 1956. This is called CIP rule.

In this system of nomenclature, Chiral, Carbon atom has either R or S configuration . The configuration of the chiral carbon is decided as follow-

  1. The four atoms or groups attached to the chiral carbon are ranked in order of priority , priority is decided by following sequence rules.

(a)      Atom of higher atomic number gets priority. Atom of higher priority is assigned no.1 & atom of least priority is assigned no.4.


decreasing priority

(b)      If the first atom is the two groups have same atomic number then relative priority of the group is decided by a comparison of the atomic number of the next atom in the two groups.

Ex.1—-    In CH & –CH2–CH3


In both first atom is Carbon but next atom in –– CH 3     is H , while in ––CH2––CH3 , next atom is ‘C’. So ethyl group is given higher priority than methyl group.

Ex.2 —   –CHO is given higher priority than –CN because in group image003 , C is directly attached to oxygen while in –CN group C is directly attached to N. (at. no. of oxygen is greater than N).

(c)       Multiple bonds are treated as separate single bonds.

Ex.1—-   –CH=O is given priority over –CH2OH


  1. When priority of groups are decided by sequence rule then figure is viewed from the group of higher priority to lower priority (2,3,4) & move your eye. If eye moves in clock wise direction, the configuration is R (Rectus-Right)

If eye moves in anti clock wise direction, the configuration is S (sinister – left).



(Clock wise)                                        (Anti Clock wise)

R–                                                                   S–


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