Question

How do you name alkenes with double bonds?

Answer 1

The basic rules for naming polyenes (alkenes with more than one double bond) are the same as those for the nomenclature of alkenes, except that we use a multiplying prefix to indicate the number of double bonds.

1. Find the longest continuous chain of carbon atoms that contains the maximum number of C=C double bonds (this may not necessarily be the longest continuous chain of carbon atoms). If you have two ties for longest continuous chain and both chains contain a C=C double bond, then choose the chain with more substituents.

2. Give the lowest possible number to one of the C=C double bonds. If the double bonds are the same distance from the end of the chain, pick the chain that gives the first substituent from either end the lower number.

3. Add substituents and their position to the name of the alkene as prefixes. Of course, remember to give them the lowest numbers possible and remember to write them in alphabetical order.

4. Indicate the number of double bonds by inserting the multiplying prefixes di, tri, tetra … prefixes before the -ene ending.

5. Identify stereoisomers. You can use the cis/trans notation when there is a hydrogen atom on each of the alkene carbons, but it is always safer to use the Cahn-Ingold-Prélog E/Z notation.
   E (entgegen) means the higher priority groups are on opposite sides of the double bond. Z (zusammen) means the higher priority groups are on the same side of the double bond.

Example 1

Give a name for

We have a four-carbon chain with two double bonds, so this is a butadiene. The double bonds start at C1 and C3, so the name is buta-1,3-diene (It is acceptable to call this 1,3-butadiene, but it is now the preferred practice to put the locating numbers as close as possible to the groups they locate).

Example 2

Give a name for

We have a seven-carbon chain with three double bonds, so this is a heptatriene. The double bonds start at C1, C3, and C6, so the name without stereochemistry is hepta-1,3,6-triene.

Now we indicate stereochemistry. The C3=C4 double bond is trans, so we could name the compound as trans-hepta-1,3,6-triene.

However, it is preferable to indicate the stereochemistry by the Cahn-Ingold-Prélog system. The two higher-priority groups are the part of the chain to the left of C3 and the rest of the chain on C4. Since they are on opposite sides of the double bond, the configuration is E.

The full name for the compound is (3E)-hepta-1,3,6-triene.