# How do you name alkenes using systematic names?

Feb 4, 2014

You name alkenes as you do alkanes, but with the ending ene and giving the double bond the highest numbering priority.

#### Explanation:

1. Find the longest continuous chain of carbon atoms that contains both carbons of the double bond.

2. Give the lowest possible number to the $\text{C=C}$ double bond.

3. Add substituents and their positions to the name of the alkene as prefixes.

4. Identify stereoisomers. It is always safest to use the Cahn-Ingold-Prélog E/Z notation.

EXAMPLE

Give a name for

We have a six-carbon chain with a double bond between $\text{C-2}$and $\text{C-3}$, so this is a hexene. The base name becomes hex-2-ene.

The methyl group at $\text{C-5}$ changes the name to 5-methylhex-2-ene.

Now we indicate stereochemistry.

The two higher-priority groups are the methyl group on $\text{C-2}$ and the rest of the chain on $\text{C-3}$.

Since they are on opposite sides of the double bond, the configuration is E.

The full name for the compound is (E)-5-methylhex-2-ene.
(E)-5-methyl-2-hexene is an acceptable name, but the preferred name puts the numbers closest to the groups they locate.