relative configuration

Meaning: Relative configuration describes how the groups on a stereocenter are arranged compared with a reference molecule, without stating the true 3D arrangement. Absolute configuration is the actual arrangement, named R or S from Cahn-Ingold-Prelog priorities. The classic reference is glyceraldehyde. In a Fischer projection with the carbonyl carbon at the top, D-glyceraldehyde has the OH on the right and L-glyceraldehyde has it on the left. Sugars and amino acids are labeled D or L by relating their stereocenter to this standard. The phrase can also describe how stereocenters within one molecule relate to each other, as in cis/trans.

On the MCAT: A passage shows a reaction at or near a stereocenter and asks whether the product has the same relative configuration as the reactant. If no bond to the stereocenter is broken, the spatial arrangement is retained. You then reassign R or S from the new priorities. Other questions ask you to read a Fischer projection and call it D or L, or to decide whether two structures are enantiomers.

Common confusion: Same relative configuration does not guarantee the same R/S label, and it says nothing about the sign of optical rotation. (+) and (−) are measured experimentally and cannot be predicted from D/L or R/S. Most L-amino acids are S, but L-cysteine is R because sulfur in its side chain changes the priorities.