Cahn-Ingold-Prelog priority rules

Meaning: The Cahn-Ingold-Prelog rules rank the four groups on a stereocenter from 1 to 4 so its configuration can be named R or S. Compare the atoms directly attached to the center first; higher atomic number wins. If they tie, move outward and compare the sets of atoms at the first point of difference. A double bond duplicates the far atom, so C=O counts as a carbon bonded to (O, O). If atomic numbers are equal, higher mass number wins, so D outranks H.

On the MCAT: Point group 4 away from you and trace 1 → 2 → 3: clockwise is R, counterclockwise is S. If group 4 points toward you, reverse the result. The same ranking gives E/Z on alkenes: higher priority groups on the same side is Z, on opposite sides is E. Swapping any two groups on a stereocenter inverts R to S.

Common confusion: Priority depends on atomic number at the first point of difference, not on group size. CH₂OH (O, H, H) outranks CH(CH₃)₂ (C, C, H) because O beats C at the first difference, even though the isopropyl group has more carbons. Also, R or S does not tell you whether a compound rotates plane-polarized light (+) or (−); that must be measured.