Editing Stereoisomerism (section)

==Cis–trans and E–Z isomerism==
{{Main|Cis–trans isomerism|E–Z notation}}

Stereoisomerism about double bonds arises because rotation about the double bond is restricted, keeping the substituents fixed relative to each other.<ref>{{Cite web |title=Geometric Isomers Definition And Examples {{!}} Chemistry Dictionary |date=30 October 2017 |url=https://chemdictionary.org/geometric-isomers/ |access-date=2022-06-20 |language=en-US}}</ref> If the two substituents on at least one end of a double bond are the same, then there is no stereoisomer and the double bond is not a stereocenter, e.g. propene, CH<sub>3</sub>CH=CH<sub>2</sub> where the two substituents at one end are both H.<ref>{{Cite web |last=Clark |first=Jim |title=geometric (cis / trans) isomerism |url=https://www.chemguide.uk/basicorg/isomerism/geometric.html |access-date=2022-08-09 |website=Chemguide.uk |date=February 2020}}</ref>

Traditionally, double bond stereochemistry was described as either ''cis'' (Latin, on this side) or ''trans'' (Latin, across), in reference to the relative position of substituents on either side of a double bond. A simple example of ''cis''–''trans'' isomerism is the 1,2-disubstituted ethenes, like the dichloroethene (C<sub>2</sub>H<sub>2</sub>Cl<sub>2</sub>) isomers shown below.<ref>{{Cite web |last=Helmenstine |first=Anne Marie |title=Geometric Isomer Definition (Cis–Trans Isomers) |url=https://www.thoughtco.com/definition-of-geometric-isomer-cis-trans-604481 |website=ThoughtCo}}</ref>

[[File:Dichloroethene.png|center|200px|Dichloroethene isomers]]

Molecule I is ''cis''-1,2-dichloroethene and molecule II is ''trans''-1,2-dichloroethene. Due to occasional ambiguity, IUPAC adopted a more rigorous system wherein the substituents at each end of the double bond are assigned priority based on their [[atomic number]]. If the high-priority substituents are on the same side of the bond, it is assigned Z (Ger. ''zusammen'', together). If they are on opposite sides, it is E (Ger. ''entgegen'', opposite).<ref>{{Cite book |last1=Roberts |first1=John D. |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles_of_Organic_Chemistry_(Roberts_and_Caserio)/19%3A_More_on_Stereochemistry/19.07%3A_EZ_Notation |title=Basic Principles of Organic Chemistry |last2=Caserio |first2=Marjorie C. |publisher=W. A. Benjamin, Inc. |year=1977 |isbn=0805383298 |edition=second |pages=19.7}}</ref> Since chlorine has a larger atomic number than hydrogen, it is the highest-priority group.<ref>{{Cite book |last1=Roberts |first1=John D. |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles_of_Organic_Chemistry_(Roberts_and_Caserio)/19%3A_More_on_Stereochemistry/19.06%3A_The_RS_Convention_for_Designating_Stereochemical_Configurations |title=Basic Principles of Organic Chemistry |last2=Caserio |first2=Marjorie C. |publisher=W. A. Benjamin, Inc. |year=1977 |isbn=0805383298 |edition=second |pages=19.6}}</ref> Using this notation to name the above pictured molecules, molecule I is (''Z'')-1,2-dichloroethene and molecule II is (''E'')-1,2-dichloroethene. It is not the case that Z and ''cis'', or E and ''trans'', are always interchangeable. Consider the following fluoromethylpentene:

[[File:Fluoromethylpentene.png|center|200px|Fluoromethylpentene]]

The proper name for this molecule is either ''trans''-2-fluoro-3-methylpent-2-ene because the alkyl groups that form the backbone chain (i.e., methyl and ethyl) reside across the double bond from each other, or (''Z'')-2-fluoro-3-methylpent-2-ene because the highest-priority groups on each side of the double bond are on the same side of the double bond. Fluoro is the highest-priority group on the left side of the double bond, and ethyl is the highest-priority group on the right side of the molecule.

The terms ''cis'' and ''trans'' are also used to describe the relative position of two substituents on a ring; ''cis'' if on the same side, otherwise ''trans''.<ref>{{cite web | url=https://agclass.nal.usda.gov/vocabularies/nalt/concept?uri=https://lod.nal.usda.gov/nalt/309551 | title=Cis–trans isomerism &#124; NAL Agricultural Thesaurus }}</ref><ref>{{cite web | url=https://www.chemguide.co.uk/basicorg/isomerism/ez.html | title=E–Z notation for geometric isomerism |last=Clark |first=Jim |date=November 2012 |access-date=2022-08-09 |website=chemguide.co.uk }}</ref>