Organic Chemistry/Chirality/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:
- 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)
- 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(CH3)3 > -CH(CH3)2 > -CH2CH3 > -CH3). 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.
R & S Notation
R- and S- notation use the CIP priority rules for the assignment of the absolute 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: sinister) configuration. A clockwise direction is an R (latin: rectus) configuration.
The R-/S- notation is valid only for the the absolute configuration of a center having single bonds only. In the case of a double bond, the traditional cis/trans nomenclature system is not sufficiently accurate and the E-/Z- is currently preferred.
The basis is again the CIP priority rules.
See main discussion: E-Z notation