R,S- Nomenclature:

The Cahn Ingold Prelog (CIP) System for

Naming Chiral Centres:

A solution to this quandary was proposed by

Robert Cahn, Chris Ingold, and Vladimir

Prelog in 1966. The resulting “CIP” protocol

works as follows:

Prioritize the four groups around a chiral

centre according to atomic number. The

highest atomic number is assigned priority

#1/a and the lowest atomic number is

assigned priority #4/d.

Orient the chiral centre such that the #4/d

priority substituent is pointing away from

the viewer. For our purposes, it’s enough for

it merely to be attached to a “dashed” bond.

Trace the path of priorities #1/a, #2/b, and

#3/c.

If the path traced from 1-2-3 is clockwise,

the chiral centre is assigned (R) (from Latin,

rectus)

If the path traced is counter clockwise, the

chiral centre is assigned (S) (from the Latin

sinister)

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