IUPAC Nomenclature

Nomenclature of alkenes is very similar to that of alkanes, except the -ane ending is replaced by -ene. A four carbon alkene would be butene, for example, and a number would be used to show the place of the double bond.

1-butene	2-butene

Common nomenclature

A few alkenes are often referred to by their common names. The most common ones are the following:

ethylene	propylene	styrene

Cis/Trans Nomenclature

Because double bonds are rigid and cannot rotate, it is possible for stereoisomers, compounds that differ only in their positioning of their atoms in space, to be isolated from each other.

cis-2-pentene	trans-2-pentene

The above compounds have identical structure except for the placement of the atoms on the double bond. One, with the two hydrogens on the same side, is called the cis isomer. The other, which has the two hydrogens on opposite sides of the double bond, is called the trans isomer. When two identical constituents come off the double bond (in the above example, hydrogen) on the same side they are cis. When two identical constituents come off the double bond on opposite sides to each other, they are trans. Naming the compound works the same as normal, only put the word cis- or trans- in front of the name like in the above example.

E/Z Nomenclature

E/Z nomenclature is similar to cis-trans nomenclature. This time, if instead of two identical groups there are four unique substituents, the E/Z assignment identifies between the two possible structures.

Z-isomer	E-isomer

To determine whether the compound is the E or Z isomer, first number all of the substituents using the Cahn-Ingold-Prelog Rules that are used to determine R/S stereochemistry. Then, if the two highest substituents are on the same side it is the Z isomer, and if the two highest substituents are on opposite sides it is the E isomer. An easy way to differentiate E/Z is to remember that the Z isomer has the highest priority constituents on Ze Zame Zide (the same side; why aren't you laughing). Just as with the cis-trans nomenclature, simply put the E or Z in front of the compound name.