Absolute Configuration: R – S Sequence Rules

R and S nomenclature

R and S nomenclature:The exact three-dimensional spatial arrangement of substituents at a chirality center is its absolute configuration. Neither the sign nor the magnitude of rotation by itself can tell us the absolute configuration of a substance.

To name enantiomers unambiguously, we need a system that allows us to indicate the handedness in the molecule, a sort of “left-hand” versus “right-hand” nomenclature. Such a system was developed by three chemists, R. S. Cahn, C. Ingold, and V. Prelog.

The first step is to rank all four substituents in the order of decreasing priority, the rules of which will be described shortly. Substituent a has the highest priority, b the second highest, c the third, and d the lowest.
Next, we position the molecule so that the lowest-priority substituent is placed as far away from us as possible.
This process results in two (and only two) possible arrangements of the remaining substituents. If the progression from a to b to c is counterclockwise, the configuration at the stereocenter is named S (sinister, Latin, left). Conversely, if the progression is clockwise, the center is R (rectus, Latin, right).

Sequence rules assign priorities to substituents

Before applying the R,S nomenclature to a stereocenter, we must first assign priorities by using sequence rules.
Rule 1. We look first at the atoms attached directly to the stereocenter. A substituent atom of higher atomic number takes precedence over one of lower atomic number. Consequently, the substituent of lowest priority is hydrogen. In regard to isotopes, the atom of higher atomic mass receives higher priority.

Rule 2. What if two substituents have the same rank when we consider the atoms directly attached to the stereocenter? In such a case, we proceed along the two respective substituent chains until we reach a point of difference.

Rule 3. Double and triple bonds are treated as if they were single, and the atoms in them are duplicated or triplicated at each end by the respective atoms at the other end of the multiple bond.


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