Editing Alkene (section)

=== Isomerism ===
{{main|Cis–trans isomerism|E–Z notation}}

In [[organic chemistry]],the [[prefix]]es [[cis-trans isomerism|cis- and trans-]]  are used to describe the positions of functional groups attached to [[carbon]] atoms joined by a double bond. In Latin, ''cis'' and ''trans'' mean "on this side of" and "on the other side of" respectively. Therefore, if the functional groups are both on the same side of the carbon chain, the bond is said to have '''cis-''' configuration, otherwise (i.e. the functional groups are on the opposite side of the carbon chain), the bond is said to have '''trans-''' configuration.

<gallery>
Cis-2-Buten.svg|structure of cis-2-butene
Trans-2-Buten.svg|structure of trans-2-butene
Trans-2-butene.svg|(''E'')-But-2-ene
Cis-2-butene.svg|(''Z'')-But-2-ene
</gallery>

For there to be cis- and trans- configurations, there must be a carbon chain, or at least one [[functional group]] attached to each carbon is the same for both. [[E–Z notation|E- and Z- configuration]] can be used instead in a more general case where all four functional groups attached to carbon atoms in a double bond are different. E- and Z- are abbreviations of German words ''zusammen'' (together) and ''entgegen'' (opposite). In E- and Z-isomerism, each functional group is assigned a priority based on the [[Cahn–Ingold–Prelog priority rules]]. If the two groups with higher priority are on the same side of the double bond, the bond is assigned '''Z-''' configuration, otherwise (i.e. the two groups with higher priority are on the opposite side of the double bond), the bond is assigned '''E-''' configuration. Cis- and trans- configurations do not have a fixed relationship between '''E'''- and '''Z'''-configurations.