R/S configuration

Meaning: R and S label the absolute configuration at a stereocenter, usually a carbon bonded to four different groups. The Cahn–Ingold–Prelog rules rank the groups 1 to 4. The higher atomic number of the directly attached atom wins. A tie is broken by comparing the next atoms outward, and a double or triple bond counts as duplicated atoms. Orient the molecule so group 4 points away from you, then trace 1 → 2 → 3. Clockwise is R (rectus); counterclockwise is S (sinister). If group 4 points toward you, read the direction and then reverse it.

On the MCAT: You assign R or S from a wedge-dash drawing or a Fischer projection. You also decide whether two structures are enantiomers (every stereocenter opposite) or diastereomers (some centers the same and some different). Swapping any two groups on a center inverts its label. A molecule with n stereocenters has at most 2ⁿ stereoisomers.

Common confusion: R and S do not tell you the sign of optical rotation, (+) or (−), which must be measured experimentally. Also, changing a substituent can change the letter even though the spatial arrangement stays the same, because the priorities shift.