Editing Cahn–Ingold–Prelog priority rules (section)

=== Assigning descriptors ===
==== Stereocenters: ''R''/''S'' ====
[[File:CIP system example.svg|class=skin-invert-image|thumb|Two examples of stereocenters. The lowest substituent (number 4) is shown only by a wavy line, and is assumed to be behind the rest of the molecule. Both centers shown are ''S'' isomers.]]

A [[Chirality (chemistry)|chiral]] sp<sup>3</sup> hybridized isomer contains four different substituents. All four substituents are assigned prorites based on its atomic numbers. After the [[substituent]]s of a [[stereocenter]] have been assigned their priorities, the molecule is oriented in space so that the group with the lowest priority is pointed away from the observer. If the substituents are numbered from 1 (highest priority) to 4 (lowest priority), then the sense of rotation of a curve passing through 1, 2 and 3 distinguishes the [[stereoisomer]]s. In a [[Stereoisomerism|configurational isomer]], the lowest priority group (most times hydrogen) is positioned behind the plane or the hatched bond going away from the reader. The highest priority group will have an arc drawn connecting to the rest of the groups, finishing at the group of third priority. An arc drawn clockwise, has the ''rectus'' (''R'') assignment. An arc drawn counterclockwise, has the ''sinister'' (''S'') assignment. The names are derived from the [[Latin]] for 'right' and 'left', respectively.<ref>{{Cite book |last=Klein |first=David R. |title=Organic Chemistry |date=2013-12-31 |publisher=Wiley |isbn=978-1118454312 |edition=2nd |page=203 |language=en}}</ref><ref>{{cite journal |last1=Cahn |first1=R. S. |date=March 1964 |title=An introduction to the sequence rule: A system for the specification of absolute configuration |journal=Journal of Chemical Education |volume=41 |issue=3 |pages=116 |bibcode=1964JChEd..41..116C |doi=10.1021/ed041p116}}</ref> When naming an organic isomer, the abbreviation for either rectus or sinister assignment is placed in front of the name in parentheses. For example, 3-methyl-1-pentene with a rectus assignment is formatted as (''R'')-3-methyl-1-pentene.<ref name=":1" /> 
[[File:(1R,2s,3S)-1,2,3-trichlorocyclopentane.svg|class=skin-invert-image|thumb|left|An example of a (''s'') descriptor: (1''R'',2''s'',3''S'')-1,2,3-trichlorocyclopentane]]
A practical method of determining whether an enantiomer is ''R'' or ''S'' is by using the [[right-hand rule]]: one wraps the molecule with the fingers in the direction {{nowrap|1 → 2 → 3}}. If the thumb points in the direction of the fourth substituent, the enantiomer is ''R''; otherwise, it is ''S''.

It is possible in rare cases that two substituents on an atom differ only in their absolute configuration (''R'' or ''S''). If the relative priorities of these substituents need to be established, ''R'' takes priority over ''S''. When this happens, the descriptor of the stereocenter is a lowercase letter (''r'' or ''s'') instead of the uppercase letter normally used.<ref>{{GoldBookRef|title=pseudo-asymmetric carbon atom|file=P04921}}</ref>

==== Double bonds: ''E''/''Z'' ====
{{Main article|E-Z notation}}

For double bonded molecules, Cahn–Ingold–Prelog priority rules (CIP rules) are followed to determine the priority of substituents of the double bond. If both of the high priority groups are on the same side of the double bond ([[Cis-trans isomerism|''cis'' configuration]]), then the [[stereoisomer]] is assigned the configuration ''Z'' (''zusammen,'' [[German language|German]] word meaning "together").  If the high priority groups are on opposite sides of the double bond ([[Cis-trans isomerism|''trans'' configuration]]), then the stereoisomer is assigned the configuration ''E'' (''entgegen'', German word meaning "opposed")<ref>{{Cite book |last=International Union of Pure and Applied Chemistry. Commission on the Nomenclature of Organic Chemistry |url=https://www.worldcat.org/oclc/27431284 |title=A guide to IUPAC nomenclature of organic compounds : recommendations 1993 |date=1993 |publisher=Blackwell Scientific Publications |others=Robert Panico, Warren H. Powell, Jean-Claude Richer |isbn=0-632-03702-4 |location=Oxford |oclc=27431284}}</ref>

==== Coordination compounds ====
In some cases where [[stereogenic]] centers are formed, the configuration must be specified. Without the presence of a [[Non-covalent interaction|non-covalent]] interaction, a compound is [[achiral]]. Some professionals have proposed a new rule to account for this. This rule states that "non-covalent interactions have a fictitious number between 0 and 1" when assigning priority.<ref>{{Cite journal |last=Elguero |first=José |date=2016-12-01 |title=Is it possible to extend the Cahn-Ingold-Prelog priority rules to supramolecular structures and coordination compounds using lone pairs? |journal=Chemistry International |language=de |volume=38 |issue=6 |pages=30–31 |doi=10.1515/ci-2016-0633 |s2cid=99300397 |issn=1365-2192|doi-access=free }}</ref> Compounds in which this occurs are referred to as [[coordination compounds]].

==== Spiro compounds ====
Some spiro compounds, for example the SDP ligands ((''R'')- and (''S'')-7,7'-bis(diphenylphosphaneyl)-2,2',3,3'-tetrahydro-1,1'-spirobi[indene]), represent chiral, C<sub>2</sub>-symmetrical molecules where the rings lie approximately at right angles to each other and each molecule cannot be superposed on its mirror image.<ref name=":1" /> The spiro carbon, C, is a stereogenic centre, and priority can be assigned a>a′>b>b′, in which one ring (both give the same answer) contains atoms a and b adjacent to the spiro carbon, and the other contains a′ and b′. The configuration at C may then be assigned as for any other stereocentre.