Editing Cahn–Ingold–Prelog priority rules (section)

====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}}