CahnIngoldPrelog

CahnIngoldPrelog refers to the set of stereochemical priority rules used to assign absolute configuration at stereogenic centers and to describe geometric isomerism around double bonds. The rules, commonly called the Cahn–Ingold–Prelog (CIP) system, were developed in the 1950s by Robert S. Cahn, Christopher K. Ingold, and Vladimir Prelog and have become the standard method for describing stereochemistry in chemistry texts and databases.

The core idea of the CIP system is to rank substituents around a stereocenter by priority. Priority is determined primarily by the atomic number of the atoms directly attached to the stereocenter; higher atomic numbers receive higher priority. If two substituents begin with the same atom, the comparison proceeds along the substituents in a defined order, comparing the atoms bonded to those atoms step by step until a difference is found. Isotopes with greater mass numbers outrank lighter ones. For bonds represented as multiple bonds, the bonded atom is duplicated in the comparison (phantom atoms) to reflect bond order, ensuring accurate priority. The rules are extended to handle rings and complex configurations.

To assign R or S, one identifies the stereocenter, assigns priorities 1 through 4 to the substituents, and orients the molecule so that the lowest-priority substituent is directed away from the observer. If the sequence from 1 to 3 proceeds clockwise, the configuration is R; if counterclockwise, it is S. For double bonds, E (entgegen) indicates that the highest-priority substituents on each carbon appear on opposite sides, while Z (zusammen) indicates same-side placement. The CIP framework also expands to pseudoasymmetric centers and conformational considerations in more complex structures.