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)
2