Editing Cahn–Ingold–Prelog priority rules (section)

===Assignment of priorities===
[[Absolute_configuration#By_absolute_configuration:_R-_and_S-|''R''/''S'']] and [[E–Z notation|''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. The overview in this section omits some rules that are needed only in rare cases.
#Compare the [[atomic number]] (''Z'') of the atoms directly attached to the stereocenter; the group having the atom of higher atomic number Z receives higher priority (i.e. number 1).
#If there is a tie, the atoms at distance 2 from the stereocenter have to be considered: a list is made for each group of further atoms bonded to the one directly attached to the stereocenter. Each list is arranged in order of decreasing atomic number Z. Then the lists are compared atom by atom; at the earliest difference, the group containing the atom of higher atomic number Z receives higher priority.
#If there is still a tie, each atom in each of the two lists is replaced with a sublist of the other atoms bonded to it (at distance 3 from the stereocenter), the sublists are arranged in decreasing order of atomic number Z, and the entire structure is again compared atom by atom. This process is repeated recursively, each time with atoms one bond farther from the stereocenter, until the tie is broken.

====Isotopes====
If two groups differ only in [[isotope]]s, then the larger [[atomic mass]] is used to set the priority.<ref>{{Cite web |last=Ashenhurst |first=James |date=2017-01-17 |title=Assigning Cahn-Ingold-Prelog (CIP) Priorities (2) - The Method of Dots |url=https://www.masterorganicchemistry.com/2017/01/17/determining-rs-2-the-method-of-dots/ |access-date=2022-11-18 |website=Master Organic Chemistry |language=en-US}}</ref>

====Double and triple bonds====
[[File:Cahn-Ingold-Prelog Double Bond.png|class=skin-invert-image|thumb|This example showcases the "divide and duplicate rule" for double bonds. The vinyl group (C=C) or alkene portion has a higher priority over the alkane (C−C) portion.]]
If an atom, A, is double-bonded to another atom, then atom A should be treated as though it is "connected to the same atom twice".<ref name=":0">{{Cite web |date=2014-08-05 |title=3.6 Cahn-Ingold Prelog Rules |url=https://chem.libretexts.org/Courses/Purdue/Purdue%3A_Chem_26505%3A_Organic_Chemistry_I_(Lipton)/Chapter_3._Stereochemistry/3.6_Cahn-Ingold_Prelog_Rules |access-date=2022-11-18 |website=Chemistry LibreTexts |language=en}}</ref> An atom that is double-bonded has a higher priority than an atom that is single bonded.<ref name=":0" /> When dealing with double bonded priority groups, one is allowed to visit the same atom twice as one creates an arc.<ref name=":1">{{Cite book |last=Patrick |first=Graham |title=Instant Notes Organic Chemistry |publisher=Garland Science |year=2004 |isbn=0203427610 |edition=2nd |pages=52–61}}</ref>

When B is replaced with a list of attached atoms, A itself, but not its "phantom", is excluded in accordance with the general principle of not doubling back along a bond that has just been followed. A triple bond is handled the same way except that A and B are each connected to two phantom atoms of the other.<ref name="IUPAC" />{{Rp|28}}

==== Geometrical isomers ====
If two substituents on an atom are [[Cis–trans isomerism|geometric isomers]] of each other, the ''Z''-isomer has higher priority than the ''E''-isomer. A stereoisomer that contains two higher priority groups on the same face of the double bond (''cis'') is classified as "Z." The stereoisomer with two higher priority groups on opposite sides of a carbon-carbon double bond (''trans'') is classified as "E."<ref>{{Cite book |last1=Okuyama |first1=Tadashi |title=Organic Chemistry, A Mechanistic Approach |last2=Maskill |first2=Howard |publisher=Oxford University Press |year=2014 |isbn=9780199693276 |pages=38–39}}</ref>

====Cyclic molecules====
To handle a molecule containing one or more [[Cyclic molecule|cycles]], one must first expand it into a [[Tree (graph theory)|tree]] (called a '''hierarchical digraph''') by traversing bonds in all possible paths starting at the stereocenter. When the traversal encounters an atom through which the current path has already passed, a phantom atom is generated in order to keep the tree finite. A single atom of the original molecule may appear in many places (some as phantoms, some not) in the tree.<ref name="ciprev1982">{{cite journal |last1=Prelog |first1=Vladlmir |last2=Helmchen |first2=Guenter |title=Basic Principles of the CIP-System and Proposals for a Revision |journal=Angewandte Chemie International Edition in English |date=August 1982 |volume=21 |issue=8 |pages=567–583 |doi=10.1002/anie.198205671 }}</ref>{{rp|at=572}}