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The Cahn–Ingold–Prelog priority rules, abbreviated CIP, are the standard method in stereochemistry for assigning absolute configuration to stereogenic centers and for describing geometric isomerism in alkenes. Developed by Robert S. Cahn, Christopher Ingold, and Vladimir Prelog in the mid-20th century, the system provides a consistent way to label chiral centers as R or S and alkenes as E or Z.
The assignment begins by ranking substituents attached to a stereogenic center according to atomic number. The
To determine R or S, arrange the molecule so that the lowest-priority substituent is directed away from
For alkenes, the same prioritization is applied to the substituents on each carbon of the double bond.
The CIP rules are widely used in chemistry literature and nomenclature, with extensions for rings, fused systems,