Naming the Alkenes : The Basic Rules

For straight chain alkenes, it is the same basic rules as nomenclature of alkanes except change the suffix to "-ene."

1. Find the Longest Carbon Chain that Contains the Carbon Carbon double bond. If you have two ties for longest Carbon chain, and both chains contain a Carbon Carbon double bond, then identify the most substituted chain.
2. Give the lowest possible number to the Carbon Carbon double bond.
   • Do not need to number cycloalkenes because it is understood that the double bond is in the one position.
   • Alkenes that have the same molecular formula but the location of the doble bonds are different means they are constitutional isomers.
   • Functional Groups with higher priority:
3. Add substituents and their position to the alkene as prefixes. Of course remember to give the lowest numbers possible. Remember to name them in alphabetical order when writting them.
4. Next is identifying stereoisomers. when there are only two non hydrogen attachments to the alkene then use cis and trans to name the molecule.

In this diagram this is a cis conformation. It has both the substituents going upward. This molecule would be called (cis) 5-chloro-3-heptene.)

Trans would look like this

v. On the other hand if there are 3 or 4 non-hydrogen different atoms attached to the alkene then use the E, Z system.

E (entgegen) means the higher priority groups are opposite one another relative to the double bond.

Z (zusammen) means the higher priority groups are on the same side relative to the double bond.

(You could think of Z as Zame Zide to help memorize it.)

In this example it is E-4-chloro-3-heptene. It is E because the Chlorine and the CH₂CH₃ are the two higher priorities and they are on opposite sides.

vi. A hydroxyl group gets precedence over th double bond. Therefore alkenes containing alchol groups are called alkenols. And the prefix becomes --enol. And this means that now the alcohol gets lowest priority over the alkene.

vii. Lastly remember that alkene substituents are called alkenyl. Suffix --enyl.

Here is a chart containing the systemic name for the first twenty straight chain alkenes.

Did you notice how there is no methene? Because it is impossible for a Carbon to have a double bond with nothing.