CahnIngoldPrelogsääntöjen

The Cahn-Ingold-Prelog (CIP) priority rules are a set of conventions used in organic chemistry to uniquely assign stereochemical descriptors to atoms and functional groups. These descriptors, commonly R and S for chiral centers and E and Z for double bonds, are crucial for unambiguously naming and differentiating stereoisomers. The CIP system was developed by Robert Cahn, Christopher Ingold, and Vladimir Prelog, who published their influential paper on the subject in 1966.

The core principle of the CIP rules is to establish a hierarchy of substituents around a stereogenic element. For chiral centers, substituents are ranked based on their atomic number. The atom directly bonded to the stereogenic center with the highest atomic number receives the highest priority. If there is a tie, the atomic numbers of the atoms attached to those tied atoms are compared, and so on, moving outward until a difference is found. In cases of multiple bonds, they are treated as if they were single bonds with phantom atoms, effectively increasing the priority of the atom involved in the multiple bond.

Once priorities are assigned, the molecule is oriented such that the lowest priority group points away from the viewer. The direction of the curve traced by the remaining three groups from highest priority to lowest priority determines the configuration. A clockwise direction indicates an R configuration (from the Latin rectum, meaning right), while a counterclockwise direction indicates an S configuration (from the Latin sinister, meaning left). For double bonds, the priority rules are applied to each atom of the double bond independently, and the designations E (entgegen, meaning opposite) and Z (zusammen, meaning together) are assigned based on the relative positions of the higher priority groups on each atom of the double bond. The CIP rules provide a systematic and universal method for describing the three-dimensional structure of molecules, which is fundamental to understanding their chemical and biological properties.