Nomenclature

As a general rule, the main chain of an alkene is that containing the largest possible number of double bonds with the lowest possible numbering.

The main chain of (4Z)-4-propyl-1,4-hexadiene contains both double bonds, even though it is not the longest

The chain must be numbered in such a way that the carbons of the double bond should end up with the lowest possible locants, exception made when there is an additional dominant function.

If substituents are present, they must bear the lowest possible locants.

In the case that one ends up with two equivalent double-bond numberings, that with the lowest one for the substituents must be taken.

In cycloalkenes, the numbering starts by the double bond and it is continued in order to assign the lowest possible numbers for the substituents, if any.

In disubstituted alkenes, the cis-trans or Z/E stereochemistry must be specified at the beginning of the name.

Nomenclature examples: 4-methylhex-2-ene, 2-methylhex-3-ene, 3-bromocyclohexene and prop-2-en-1-ol (allyl alcohol) — The main chain is numbered so that the double bond gets the lowest locant; alcohols take priority over the double bond.

Cis/trans nomenclature examples: cis-1,2-dibromoethene, trans-3-cyclohexyl-1-(2-methylcyclopentyl)propene, trans-cyclodecene and cis-cyclodecene — The prefixes *cis* and *trans* show whether the substituents are on the same side or on opposite sides of the double bond, also in large rings.

In disubstituted alkenes, in addition to the "classical" cis-trans stereochemical nomenclature, the Z/E prefixes can be used, from the German entgegen (opposite, E) and zusammen (together, Z).

There is no other choice in the case of tri- or tetrasubstituted alkenes. The substituent priority rules used at both ends of the olefin, in order to define whether the most important radicals are far away (E) or together (Z), are the same as those applied to R/S nomenclature.

The following "joke" may be useful to remember when an olefin is E or Z.

In Spanish there is a saying that the moon is a liar: when it is D-shaped it is waxing (creciente), and when it is C-shaped it is waning (decreciente), the opposite of what the letters suggest.

Alkenes are as much liars as the moon because when the most important substituents at both ends of the double bond are in Z shape, the olefin is E and vice versa.

(Z)-1-Bromo-2-chloro-1-fluoroethene looks like an E but is Z: the highest-priority groups (Br and Cl) are on the same side — (Z): the highest-priority groups on each carbon (Br and Cl) are on the same side, even though the shape suggests an E.

(E)-1-Chloro-3-ethyl-4-methyl-3-octene looks like a Z but is E: the highest-priority groups are on opposite sides — (E): the highest-priority groups on each carbon are on opposite sides, even though the shape suggests a Z.

When a double bond acts as a substituent, the group is named alkenyl or alkylidene:

Double bonds as substituents: vinylbenzene (styrene), (1-methylprop-2-en-1-yl)cyclopentane, 2-ethylidenecyclopentanol, cis-1-propenylcyclohexene, cyclopentylidenecyclopentane and methylidenecyclohexane