Cahn–Ingold–Prelog priority rules

In <a href="/facts/Organic_chemistry/djxhxaNg">organic chemistry</a>, the Cahn–Ingold–Prelog (CIP) sequence rules (also the CIP priority convention; named after <a href="/facts/Robert_Sidney_Cahn/I1D5hxGo">Robert Sidney Cahn</a>, <a href="/facts/Christopher_Kelk_Ingold/cT2t2zGu">Christopher Kelk Ingold</a>, and <a href="/facts/Vladimir_Prelog/pWcL8M26">Vladimir Prelog</a>) are a standard process to completely and unequivocally <a href="/facts/Chemical_nomenclature/scv3SqmM">name</a> a <a href="/facts/Stereoisomer/I3kzlDE8">stereoisomer</a> of a molecule.: 26  The purpose of the CIP system is to assign an <a href="/facts/Absolute_configuration/QFr0Pre3">R or S descriptor</a> to each <a href="/facts/Stereocenter/t3XGwk1Q">stereocenter</a> and an <a href="/facts/E-Z_notation/z9CVaJwH">E or Z descriptor</a> to each <a href="/facts/Double_bond/qDemMn0c">double bond</a> 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 n describing the number of stereocenters will usually have 2n <a href="/facts/Stereoisomer/I3kzlDE8">stereoisomers</a>, and 2n−1 <a href="/facts/Diastereomer/ReBkFoCP">diastereomers</a> each having an associated pair of <a href="/facts/Enantiomer/wjPv5J3y">enantiomers</a>. The CIP sequence rules contribute to the precise naming of every stereoisomer of every <a href="/facts/Organic_compound/gqCNaN45">organic</a> molecule with all atoms of <a href="/facts/Coordination_number/mf48hsoX">ligancy</a> of fewer than 4 (but including ligancy of 6 as well, this term referring to the "number of neighboring atoms" bonded to a center).: 26f 
The key article setting out the CIP sequence rules was published in 1966, and was followed by further refinements, before it was incorporated into the rules of the <a href="/facts/IUPAC/2AV6myCK">International Union of Pure and Applied Chemistry</a> (IUPAC), the official body that defines <a href="/facts/IUPAC_nomenclature/pepvzNun">organic nomenclature</a>, in 1974.: 26ff  The rules have since been revised, most recently in 2013, as part of the IUPAC book <a href="/facts/Nomenclature_of_Organic_Chemistry/wnk0nsvC">Nomenclature of Organic Chemistry</a>. The IUPAC presentation of the rules constitute the official, formal standard for their use, and it notes that "the method has been developed to cover all compounds with ligancy up to 4... and... [extended to the case of] ligancy 6... [as well as] for all configurations and conformations of such compounds.": 26ff  Nevertheless, though the IUPAC documentation presents a thorough introduction, it includes the caution that "it is essential to study the original papers, especially the 1966 paper, before using the sequence rule for other than fairly simple cases.": 26f 
A recent paper argues for changes to some of the rules (sequence rules 1b and 2) to address certain molecules for which the correct descriptors were unclear. However, a different problem remains: in rare cases, two different stereoisomers of the same molecule can have the same CIP descriptors, so the CIP system may not be able to unambiguously name a stereoisomer, and other systems may be preferable.: 27

Cahn–Ingold–Prelog priority rules open-in-new

Cahn–Ingold–Prelog priority rules