Naming Alkenes

Alkenes are named using a series of rules that are similar to those for alkanes, except with the suffix –ene instead of –ane to identify the functional group.

Step 1: Name the parent hydrocarbon.  Find the longest carbon chain containing a double bond. 

Name the compound based off the number of carbons, using the suffix –ene.

Step 2: Number the carbon atoms in the chain.  Begin at the end closest to the double bond, or if

the double bond is exactly in the center of the chain, begin at the end closer to the first branch point. 

This assures that the double bond receives the lowest number possible.

Step 3: Write the Full Name.  Number the substituents according to their position on the parent

chain, and list alphabetically.  Show the position of the double bond by giving the number of the first

alkene carbon directly before the parent name.  If there is more than one double bond, show the

position of each double bond and use the suffix  –diene, -triene and so forth.

In 1993 IUPAC changed their naming recommendations to place the position of the double bond immediately before the –ene suffix rather than before the parent name.  This change is not widely accepted in the United States, so the old system is used.

Cycloalkenes are named in a similar fashion, but because there is no end to the chain the cycloalkene is named so the double bond is between C1 and C2.  The first substituent has the lowest possible just like in a regular chain.  The position of the double bond is not necessary to mention in the name, because the double bond is always assumed to be located between C1 and C2.  In the new naming system the locant in position right before the suffix in a diene.

Some alkenes names have been used for so long that they are accepted despite the fact that they do not follow the IUPAC naming system.  For example, ethene is the alkene derived from ethane, but the name ethylene has been used for so long that it has been accepted by IUPAC.

Here are a few molecules with more accepted common names:

Naming alkenes are very similar to naming alkanes.  Identifying where the double bond is, is the key to successfully naming the molecule.