Editing Alkene (section)

==IUPAC Nomenclature==
Although the nomenclature is not followed widely, according to IUPAC, an alkene is an acyclic hydrocarbon with just one double bond between carbon atoms.<ref name=PAC1995.alkenes>{{GoldBookRef |title=alkenes |file=A00224 }}</ref> Olefins comprise a larger collection of cyclic and acyclic alkenes as well as dienes and polyenes.<ref name=PAC1995.olefins>{{GoldBookRef |title=olefins |file=O04281 }}</ref>

To form the root of the [[IUPAC nomenclature|IUPAC names]] for straight-chain alkenes, change the ''-an-'' infix of the parent to ''-en-''. For example, '''CH<sub>3</sub>-CH<sub>3</sub>''' is the [[alkane]] ''ethANe''. The name of '''CH<sub>2</sub>=CH<sub>2</sub>''' is therefore ''ethENe''.

For straight-chain alkenes with 4 or more carbon atoms, that name does not completely identify the compound.  For those cases, and for branched acyclic alkenes, the following rules apply:
# Find the longest carbon chain in the molecule. If that chain does not contain the double bond, name the compound according to the alkane naming rules.  Otherwise:
# Number the carbons in that chain starting from the end that is closest to the double bond.
# Define the location ''k'' of the double bond as being the number of its first carbon.
# Name the side groups (other than hydrogen) according to the appropriate rules.
# Define the position of each side group as the number of the chain carbon it is attached to.
# Write the position and name of each side group.
# Write the names of the alkane with the same chain, replacing the "-ane" suffix by "''k''-ene".

The position of the double bond is often inserted before the name of the chain (e.g. "2-pentene"), rather than before the suffix ("pent-2-ene").

The positions need not be indicated if they are unique.  Note that the double bond may imply a different chain numbering than that used for the corresponding alkane: {{chem|(H|3|C|)|3}}C–{{chem|CH|2}}–{{chem|CH|3}} is "2,2-dimethyl pentane", whereas {{chem|(H|3|C|)|3}}C–{{chem|CH}}={{chem|CH|2}} is "3,3-dimethyl 1-pentene".

More complex rules apply for polyenes and [[cycloalkene]]s.<ref name=PAC1995/>

[[Image:Alkene nomenclature.svg|550px|center|thumb|Naming substituted hex-1-enes]]

===''Cis''–''trans'' isomerism===
If the double bond of an acyclic mono-ene is not the first bond of the chain, the name as constructed above still does not completely identify the compound, because of [[cis–trans isomerism|''cis''–''trans'' isomerism]].  Then one must specify whether the two single C–C bonds adjacent to the double bond are on the same side of its plane, or on opposite sides. For monoalkenes, the configuration is often indicated by the prefixes ''cis''- (from [[Latin]] "on this side of") or ''trans''- ("across", "on the other side of") before the name, respectively; as in ''cis''-2-pentene or ''trans''-2-butene.
[[Image:Cis-trans example.svg|thumb|300px|center|The difference between ''cis-'' and ''trans-'' isomers]]

More generally, ''cis''–''trans'' isomerism will exist if each of the two carbons of in the double bond has two different atoms or groups attached to it.  Accounting for these cases, the IUPAC recommends the more general [[E–Z notation]], instead of the ''cis'' and ''trans'' prefixes.  This notation considers the group with highest [[Cahn-Ingold-Prelog priority rule|CIP priority]] in each of the two carbons. If these two groups are on opposite sides of the double bond's plane, the configuration is labeled ''E'' (from the [[German language|German]] ''entgegen'' meaning "opposite"); if they are on the same side, it is labeled ''Z'' (from German ''zusammen'', "together").  This labeling may be taught with mnemonic  "''Z'' means 'on ze zame zide'".<ref name=murr2014>{{cite book |first=John E. |last=McMurry |date=2014 |page=[https://books.google.com/books?id=KDIeCgAAQBAJ&pg=PA189 189] |title=Organic Chemistry with Biological Applications |publisher=Cengage Learning |edition=3rd |isbn=978-1-285-84291-2}}</ref>

[[Image:EZalkenes2.png|400px|center|thumb|The difference between ''E'' and ''Z'' isomers]]

===Groups containing C=C double bonds===
IUPAC recognizes two names for hydrocarbon groups containing carbon–carbon double bonds, the [[vinyl group]] and the [[allyl]] group.<ref name="PAC1995"/>
[[Image:AlkenylGroups.png|200px|center]]