Question

How do you name alkenes using systematic names?

Answer 1

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 `"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 `"C-2"`and `"C-3"`, so this is a hexene. The base name becomes hex-2-ene.

The methyl group at `"C-5"` changes the name to 5-methylhex-2-ene.

Now we indicate stereochemistry.

The two higher-priority groups are the methyl group on `"C-2"` and the rest of the chain on `"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.