absolute configuration

Meaning: Absolute configuration is the actual spatial arrangement of the four groups on a stereocenter, written as R or S using Cahn-Ingold-Prelog rules. Rank the groups by atomic number at the atom attached to the center (higher number, higher priority). If there is a tie, move outward along each branch to the first difference. A double bond counts as two single bonds to duplicate atoms. Point the lowest-priority group away from you and trace 1 → 2 → 3: clockwise is R, counterclockwise is S.

On the MCAT: Expect a wedge-dash drawing or Fischer projection to be labeled R or S, or a name such as (2S,3R) to be matched to a structure. If the lowest-priority group points toward you, reverse your answer. In a Fischer projection, a lowest-priority group on a horizontal bond also reverses the answer. Swapping any two groups on a stereocenter inverts it. Enantiomers have opposite configuration at every stereocenter; diastereomers differ at some but not all.

Common confusion: R and S are not the same as (+) and (−) or d and l. Optical rotation is measured experimentally and cannot be predicted from the label. Also, changing a substituent can change the R/S label without changing the spatial arrangement, because the priorities may shift.