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?
Common Questions
What do R and S mean in organic chemistry?
R (rectus, "right") and S (sinister, "left") describe the 3D arrangement of groups around a chiral center using the Cahn-Ingold-Prelog rules. They are absolute configuration labels, not related to the direction a molecule rotates light.
How do you assign R or S configuration?
Rank the four substituents by CIP priority (highest atomic number = #1). Orient the molecule with #4 pointing away. If #1 → #2 → #3 traces clockwise, it's R; counterclockwise is S.
What are the CIP priority rules?
Compare atoms directly attached to the stereocenter by atomic number. Higher atomic number = higher priority. Break ties by moving outward along each chain until a difference is found. Double/triple bonds are treated as two/three single bonds to phantom atoms.
Does R always mean clockwise?
Only when the lowest-priority group (#4) points away from you. If #4 faces toward you (e.g., on a wedge bond), the apparent clockwise/counterclockwise is reversed.
Is there a relationship between R/S and +/-?
No. R/S is based on the 3D arrangement of atoms, while +/− (dextrorotatory/levorotatory) describes how a compound rotates plane-polarized light. You cannot predict one from the other.