Cahn–Ingold–Prelog priority rules

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The Cahn–Ingold–Prelog priority rules, CIP system or CIP conventions (after Robert Sidney Cahn, Christopher Kelk Ingold and Vladimir Prelog) are a set of rules used in organic chemistry to name the stereoisomers of a molecule [1][2]. A molecule may contain any number of stereocenters and any number of double bonds, and each gives rise to two possible configurations. 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.

The key article by the three authors setting out the CIP rules was published in 1966 [3][4][5]

The Cahn–Ingold–Prelog rules are distinctly different from those of other naming conventions, such as general IUPAC nomenclature, since they are designed for the specific task of naming stereoisomers rather than the general classification and description of compounds.

Contents

Steps for naming

The steps for naming molecules using the CIP system are often presented as:

Assignment of priorities

R/S and E/Z descriptors are assigned by using a system for ranking priority of the groups attached to each stereocenter. This procedure, often known as the sequence rules, is the heart of the CIP system.

1. Compare the atomic number (Z) of the atoms directly attached to the stereocenter; the group having the atom of higher atomic number receives higher priority.

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