R or S Nomenclature

How do you determine R or S nomenclature of enantiomers?

1. Identify a stereocenter.
2. Assign priority # 1, # 2, # 3, and # 4 to each of the four groups attached to the stereocenter.
   • Priority is assigned on the basis of atomic weight: high atomic weight, high priority, i.e. # 1.
   • If two groups have the same atomic number, move outward for each group and then compare the atomic numbers, see Figure 40.6 below:

     Figure 40.6: Nomenclature of enantiomers: Assigning priority.

     

3. Rotate the formula - or model - so that the lowest priority group, # 4, is pointing away from you:
   

   Figure 40.7: Nomenclature of enantiomers: Directing the lowest priority away.

   

4. Draw an arrow in a circular fashion from highest priority to lowest, i.e. # 1, # 2 and # 3. If the arrow orientation is clockwise the designation is R. Conversely, if the arrow orientation is counterclockwise then the designation is S:
   

   Figure 40.8: Nomenclature of enantiomers: Determining R (left) or S (right) designation.