41 R-S notational system
Stereoisomers are properly named using the Cahn-Ingold-Prelog (CIP) priority rules to
decide which parts of the molecule to consider first.
The rules have evolved to cover many situations, but the basic rules are:
1.Consider the first atom of each part of the molecule. An atom with higher atomic
number has higher priority. (e.g. I > Cl > C > H)
2.If the first atom of two groups is the same, consider the second atom(s) in the same
way as the first. (e.g. -C(CH 3 ) 3 > -CH(CH 3 ) 2 > -CH 2 CH 3 > -CH 3 ). If this does not
assign priority, consider the next atoms until there is a difference.Realize that when you do this it will mean that sometimes groups with higher total weights
will have lower priority because of a lower weight of the atom that connects them.
41.1 R and S Notation
R- and S-notation use the CIP priority rules for the assignment of theabsolute configuration
around a stereocenter.
First, assign priorities as described above to each bonded group surrounding the stereocenter
(1, highest to 4, lowest).
Second, point the lowest priority (4) atom away from you. Follow the direction of the
remaining 3 priorities from highest to lowest priority (lowest to highest number, 1<2<3).
A counterclockwise direction is an S (Latin forsinister, left) configuration. A clockwise
direction is an R (Latin forrectus, right) configuration.
Figure 116 Direction of the travel 1-2-3 dictates configuration