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7.3: Naming Alkenes

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    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 \(\PageIndex{1}\) lists several other common names that are often used and are recognized by IUPAC. Note also that a \(\ce{=CH2}\) substituent is called a methylene group, a \(\ce{H2C═CH–H2C═CH–}\) substituent is called a vinyl group, and a \(\ce{H2C═CHCH2–H2C═CHCH2–}\) substituent is called an allyl group.

    The structures of methylene (double bond to C H 2), vinyl (C H C H 2), and allyl (C H 2 C H C H 2) groups.
    Table \(\PageIndex{1}\): Common Names of Some Alkenes
    Compound Systematic name Common name
    \(\ce{H2C═CH2}\) Ethene Ethylene
    \(\ce{CH3CH═CH2}\) Propene Propylene
    A condensed structural formula that has a 3-carbon chain with a double bond between C1-C2. C2 is bonded to a methyl group. 2-Methylpropene Isobutylene
    A condensed structural formula has a 4-carbon chain with double bonds between C1-C2 and C3-C4. C2 is also bonded to a methyl group. 2-Methyl-1,3-butadiene Isoprene
    Exercise \(\PageIndex{1}\)

    Give IUPAC names for the following compounds:

    1. a=
    2. A condensed structural formula for a six-carbon chain that (counting from left) has a double bond at third carbon and methyl group at fourth carbon.
    3. A condensed structural formula for an eight-carbon chain that (counting from left) has double bonds at second and fifth carbon and methyl groups at third and sixth carbons.
    4. A condensed structural formula for an eight-carbon chain that (counting from left) has a double bond at fourth carbon and a sec butyl group at sixth carbon.
    Answer
    1. 3,4,4-Trimethyl-1-pentene
    2. 3-Methyl-3-hexene
    3. 4,7-Dimethyl-2,5-octadiene
    4. 6-Ethyl-7-methyl-4-nonene
    Exercise \(\PageIndex{2}\)

    Draw structures corresponding to the following IUPAC names:

    1. 2-Methyl-1,5-hexadiene
    2. 3-Ethyl-2,2-dimethyl-3-heptene
    3. 2,3,3-Trimethyl-1,4,6-octatriene
    4. 3,4-Diisopropyl-2,5-dimethyl-3-hexene
    Answer
    1. A condensed structural formula has a 6-carbon chain with double bonds between C1-C2 and C5-C6. C2 is bonded to a methyl group.
    2. A condensed structural formula has a 7-carbon chain with a double bond between C3-C4. C1 and C3 are bonded to three methyl and one ethyl group, respectively.
    3. A structure of octane with double bonds between C1-C2, C4-C5, and C6-C7. C2 and C3 are bonded to one methyl and two methyl groups, respectively.
    4. A 2-carbon chain with a double bond in between. C1 and C2 are each bonded to two isopropyl groups.
    Exercise \(\PageIndex{3}\)

    Name the following cycloalkenes:

    1. a=
    2. A seven-membered ring with a double bond. There are two methyl substituents on the same carbon, two carbons away from the double bond.
    3. A five-membered ring with a double bond. There is an isopropyl group one carbon away from the double bond
    Answer
    1. 1,2-Dimethylcyclohexene
    2. 4,4-Dimethylcycloheptene
    3. 3-Isopropylcyclopentene

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