R-S-SYSTEM OF NOMENCLATURE

R-S-System of nomenclature

source : 2.chemistry.msu.edu

R-S-SYSTEM OF NOMENCLATURE

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.

I>Br>Cl>O>N>C>H

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 CH₃  & –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  , 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

I>Br>Cl, OH>NO₂>NH₂>COOH>CHO>CH₂OH

2. 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).

Ex.

(Clock wise)                                        (Anti Clock wise)

R–                                                                   S–

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