Rules for Assigning Absolute Configuration
- 1.
- Examine the four atoms directly attached to the chiral center in question. Assign priorities in order of decreasing atomic number; the atom with the highest atomic number is #1, the next is #2, etc.
- 2.
- If a decision regarding priority cannot be reached using Rule #1, compare the atomic numbers of the second atoms in each substituent, then the third, etc., until a difference is found.
- 3.
- Multiple bonds count twice (or three times) when examining substituents.
- 4.
- Once the priorities have been assigned, rotate the molecule in space so that the lowest prioritiy group is pointing back. Connect the three remaining groups in order of decreasing priority and examine the direction of the resulting rotation. Rotation which is clockwise is termed R (rectus; right) and rotation which is counterclockwise is termed S (sinister; left).
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