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Basic IUPAC Organic Nomenclature

R- and S- nomenclature of chirality centers

The Cahn-Ingold-Prelog priority rules are used for naming chirality centers and geometric isomers (e.g. E- or Z-alkenes)

These rules are used to establish the priority of the groups attached to the chirality center and are based on atomic number, and the first point of difference.

In the simplest and most common case, a chirality center is characterised by an atom that has four different groups bonded to it in such a manner (e.g. tetrahedral) that it has a non-superimposable mirror image.  Terms such as an asymmetric, stereogenic or chiral center have been used in the past.

The most common chirality center in organic chemistry is a carbon atom with four different groups attached

In order to assign the configuration as R or S:
Then at each chirality center.... Subrules:
HINT:


Example:  chlorofluoroiodomethane

The chirality center should be easy to spot, and the four attached groups are in priority order, highest to to lowest:

I (purple), Cl (green), F(brown) and H (white)

Rotate the image on the left so the you are looking along the C-H bond and the H is away from you, then determine the direction of high to low priority. 
Since it decreases clockwise, this is the R configuration. 

identify chirality center identify chirality center
assign groups priority
low priority group away
direction for 1->3
starting point identify the chirality center(s assign the relative priorities then rotate the low priority group away
(to the back)
determine the sense of groups 1 - 3
clockwise = R

Example: cyclohex-2-enol

identify chirality center
assign groups priority
low priority group away
direction for 1->3
direction for 1->3
starting point

identify the chirality center(s) and show the implied H atom

assign the relative priorities....
O > C > C > H

Because we have two C groups we need to list the groups the C are attached to in atomic number order
(note how the multiple bond is handled)

the first point of difference is the C > H so the C group on the left is the higher priority
with the low priority group already at the back, determine the sense of groups 1 - 3
counterclockwise = S




organic chemistry Dr. Ian Hunt, Department of Chemistry University of Calgary