Editing Cis–trans isomerism (section)

=== ''E''–''Z'' notation ===
{{Main article|E–Z notation}}
[[File:(Z)-1-Bromo-1,2-dichloroethene.svg|class=skin-invert-image|thumb|Bromine has a higher [[Cahn–Ingold–Prelog priority rules|CIP priority]] than chlorine, so this alkene is the ''Z'' isomer]]

In principle, ''cis''–''trans'' notation should not be used for alkenes with two or more different substituents. Instead the ''E''–''Z'' notation is used based on the priority of the substituents using the [[Cahn–Ingold–Prelog priority rules|Cahn–Ingold–Prelog (CIP) priority rules]] for absolute configuration. The IUPAC standard designations ''E'' and ''Z'' are unambiguous in all cases, and therefore are especially useful for tri- and tetrasubstituted alkenes to avoid any confusion about which groups are being identified as ''cis'' or ''trans'' to each other.

''Z'' (from the German {{lang|de|zusammen}}) means "together". ''E'' (from the German {{lang|de|entgegen}}) means "opposed" in the sense of "opposite". That is, ''Z'' has the higher-priority groups ''cis'' to each other and ''E'' has the higher-priority groups ''trans'' to each other. Whether a molecular configuration is designated ''E'' or ''Z'' is determined by the CIP rules; higher atomic numbers are given higher priority. For each of the two atoms in the double bond, it is necessary to determine the priority of each substituent. If both the higher-priority substituents are on the same side, the arrangement is ''Z''; if on opposite sides, the arrangement is ''E''.

Because the ''cis''–''trans'' and ''E''–''Z'' systems compare different groups on the alkene, it is not strictly true that ''Z'' corresponds to ''cis'' and ''E'' corresponds to ''trans''. For example, ''trans''-2-chlorobut-2-ene (the two methyl groups, C1 and C4, on the [[2-Butene|but-2-ene]] backbone are ''trans'' to each other) is (''Z'')-2-chlorobut-2-ene (the chlorine and C4 are together because C1 and C4 are opposite).

==== Undefined alkene stereochemistry ====
Wavy single bonds are the standard way to represent unknown or unspecified stereochemistry or a mixture of isomers (as with tetrahedral stereocenters). A crossed double-bond has been used sometimes; it is no longer considered an acceptable style for general use by [[IUPAC]] but may still be required by computer software.<ref name="IUPAC-2006">{{cite journal |first=Jonathan |last=Brecher |title=Graphical representation of stereochemical configuration (IUPAC Recommendations 2006) |journal=[[Pure and Applied Chemistry]] |year=2006 |volume=78 |issue=10 |pages=1897–1970 |url=http://www.iupac.org/publications/pac/pdf/2006/pdf/7810x1897.pdf |doi=10.1351/pac200678101897 |s2cid=97528124 }}</ref>

[[File:E-Z notation in alkenes.svg|class=skin-invert-image|thumb|400px|center|Alkene stereochemistry]]