Editing Cahn–Ingold–Prelog priority rules (section)

==== 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>