Rules for Naming Enantiomers
To assign R/S notation to a chiral center, priorities must be assigned to the substituents using the following rules:
- Rule 1: The priorities of the atoms attached to the chiral center are determined by atomic numbers. In the case of isotopes, the one with the higher atomic mass takes precedence.
Priorities are assigned by the atomic number of the atoms directly bonded to the asymmetric carbon (drawn in red)
- Rule 2: When two or more substituents attached to the chiral center have the same priority, the chains are compared atom by atom until a point of difference is found.
- Rule 3: Double and triple bonds are considered as if they were single bonds for the purpose of assigning priority.
To assign R/S notation, we follow the priority order a, b, c of the substituents. If this sequence is clockwise, the center is R (rectus, Latin for right). If it is counterclockwise when following the priorities a, b, c, it is S
(sinister, Latin for left). This rule is only valid when the group d is towards the back of the plane (dashed bond), if d comes towards us (wedge) the notation is the opposite (R for a left turn, S for a right turn).
