John McMurry, Professor Emeritus

7.3 • Naming Alkenes

Alkenes are named using a series of rules similar to those for alkanes (Section 3.4), with the suffix -ene used instead of -ane to identify the functional group. There are three steps to this process.

STEP 1
Name the parent hydrocarbon. Find the longest carbon chain containing the double bond, and name the compound accordingly, using the suffix -ene:

Two structures of 2-ethylpent-1-ene highlighted differently to justify that the molecule is named as a pentene, not a hexane, since the double bond is not contained in the six-carbon chain.

STEP 2
Number the carbon atoms in the chain. Begin at the end nearer the double bond or, if the double bond is equidistant from the two ends, begin at the end nearer the first branch point. This rule ensures that the double-bond carbons receive the lowest possible numbers.

The first structure has a 6-carbon chain with a double bond between C2-C3. The second compound has a 6-carbon chain with methyl group at C2 and double bond between C3-C4.

STEP 3
Write the full name. Number the substituents according to their positions in the chain, and list them alphabetically. Indicate the position of the double bond by giving the number of the first alkene carbon and placing that number directly before the parent name. If more than one double bond is present, indicate the position of each and use one of the suffixes -diene, -triene, and so on.

The structures of 2-hexene, 2-methyl-3-hexene, 2-ethyl-1-pentene, and 2-methyl-1 3-butadiene. The carbon atoms are numbered in all structures.

We should also note that IUPAC changed their naming recommendations in 1993 to place the locant indicating the position of the double bond immediately before the -ene suffix rather than before the parent name: but-2-ene rather than 2-butene, for instance. This change has not been widely accepted by the chemical community in the United States, however, so we’ll stay with the older but more commonly used names. Be aware, though, that you may occasionally encounter the newer system.

The condensed structures of two compounds. First compound: Newer naming system: 2,5-dimethylhept-3-ene and older naming system: 2,5-dimethyl-3-heptene. Second compound: Newer naming system: 3-propylhexa-1,4-diene and older naming system: 3 propyl-1,4-hexadiene.

Cycloalkenes are named similarly, but because there is no chain end to begin from, we number the cycloalkene so that the double bond is between C1 and C2 and the first substituent has as low a number as possible. It’s not necessary to indicate the position of the double bond in the name because it’s always between C1 and C2. As with open-chain alkenes, the newer but not yet widely accepted naming rules place the locant immediately before the suffix in a cyclic alkene.

The structures of 1-methylcyclohexene, 1,4-cyclohexadiene (new name: cyclohexa-1,4-diene) and 1,5-dimethylcyclopentene. The carbon atoms are numbered in all structures.

For historical reasons, there are a few alkenes whose names are firmly entrenched in common usage but don’t conform to the rules. For example, the alkene derived from ethane should be called ethene, but the name ethylene has been used for so long that it is accepted by IUPAC. Table 7.1 lists several other common names that are often used and are recognized by IUPAC. Note also that a =CH₂ substituent is called a methylene group, a $H_{2} C═CH–$ substituent is called a vinyl group, and a $H_{2} C═ {CHCH}_{2} –$ substituent is called an allyl group.