Cahn-Ingold-Prelog priority rule

The Cahn–Ingold–Prelog (CIP) sequence rules, named for organic chemists R.S. Cahn, C.K. Ingold, and V. Prelog—alternatively termed the CIP priority rules, system, or conventions—are a standard process used in organic chemistry to completely and unequivocally name a stereoisomer of a molecule. The purpose of the CIP system is to assign an R or S descriptor to each stereocenter and an E or Z descriptor to each double bond so that the configuration of the entire molecule can be specified uniquely by including the descriptors in its systematic name. A molecule may contain any number of stereocenters and any number of double bonds, and each usually gives rise to two possible isomers. A molecule with an integer ${\displaystyle n}$ describing the number of its stereogenic centers will usually have ${\displaystyle 2^{n}}$ stereoisomers, ${\displaystyle 2^{n-1}}$ diastereomers each having an associated pair of enantiomers. The CIP sequence rules contribute to the precise naming of every stereoisomer of every organic and organometallic molecule with all atoms of ligancy of fewer than 4 (but including ligancy of 6 as well, this term referring to the "number of neighboring atoms" bonded to a center).

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