The Cahn-Ingold-Prelog R/S rules are used for naming enantiomers.

NB: The term stereogenic has superceded the term chiral.

1. Identify the stereogenic centers (most commonly an sp³ C with 4 different groups attached)
2. Assign the priority to each group (high = 1, low = 4) based on atomic number of the atom attached to the stereogenic center
3. Position the lowest priority group away as if you were looking along the C-(4) sigma bond. If you are using a model, grasp the group in your fist
4. For the other 3 groups, determine the direction of high to low priority (1 to 3)
5. If this is clockwise, then the center is R (Latin: rectus = right)
6. If this is counter clockwise, then it is S (Latin: sinister = left)

Example: chlorofluoroiodomethane

The stereogenic center is easy to spot, and the four attached groups are I (purple), Cl (green), F(brown) and H (white), listed in priority order, highest to to lowest. Rotate the CHIME image below so the you are looking along the C-H bond and the H is away from you, then determine the direction of high to low priority. It decreases clockwise, so this is the R enantiomer.

Subrules:

• Isotopes: H vs D ? use atomic mass number so D > H
• Same atom attached ? By moving out one unit at a time, locate the first point of difference and apply rules there.

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