How to Determine R or S

Cahn-Ingold-Prelog (CIP) priority is assigned based on the priority of groups attached to the chiral atom.

Step 1: Identify chiral atom/stereocenter, which has four different groups attached.
Step 2: Assign priority to the atoms directly attached to the stereocenter (highest atomic mass = 1, lower = 4). For each tie, continue to the next atom in the chain until the first point of difference.
Step 3: Orient molecule with priority 4 either hashed or wedged.

Helpful Tips

If only one bond on an atom is shown as a wedged or a hashed bond, only the adjacent bond can be assigned the opposite "direction".
Rotating a molecule 180 degrees looks like swapping the in-plane groups, and the wedged/hashed groups.
When priority #4 is in-plane, rotate the wedged/hashed groups to be in-plane and in-plane groups to be wedged/hashed.
Multiple bonds have priority in most cases. More accurately, we "break" pi bonds to "make" single bonds with phantom atoms.

Practice Assigning R and S Configurations

Assign the chiral atom (stereocenter) in the molecules below as being either R or S configuration. Filter by molecular weight for simpler or more complex molecules.

What is the absolute configuration of the stereocenter?

Max Molecular Weight Value:

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