R/S configuration
MCAT glossary · MCAT
Definition
CIP priority assignment; lowest-priority group pointing away, trace 1→2→3: clockwise = R, counterclockwise = S.
R/S configuration in plain English
Some molecules come in two versions that are mirror images of each other, like your left and right hands — same parts, but not superimposable. R/S is just a naming system that tells you WHICH hand you're looking at, so chemists don't mix them up (this matters because your body often only 'fits' one version, like a glove). To find it, you rank the four groups on a carbon by priority (heavier atoms win), point the lowest-priority group away from you, then trace a path from priority 1 to 2 to 3: clockwise means R (Latin *rectus*, right), counterclockwise means S (*sinister*, left).
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