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1.4: Alkenes and Alkynes

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  • Learning Objective

    • How to name alkenes and alkynes.

    An alkane is a saturated hydrocarbon, meaning that the molecule contains all the possible hydrogen atoms because all the carbon-carbon bonds are single bonds. If one of those carbon-carbon bonds is a double bond, the resulting hydrocarbon is unsaturated and called an alkene.

    This alkene is named propene.

    Figure \(\PageIndex{1}\).svg
    Figure \(\PageIndex{1}\): propene

    If one of the carbon-carbon bonds is a triple bond, the resulting hydrocarbon is called an alkyne.

    Practice Question

    1. This alkyne is named ethyne.

    Figure \(\PageIndex{2}\).svg
    Figure \(\PageIndex{2}\): ethyne

    What is the name of Molecule A?

    Figure \(\PageIndex{3}\).svg
    Figure \(\PageIndex{3}\): Molecule A

    The double or triple bond is called a functional group, and is often the site where chemical reactions occur. Like a substituent, it is specified in the molecular name. When naming molecules according to the IUPAC system of nomenclature, remember prefix-parent-suffix (like un-believe-able).

    prefix: what are the substituents?

    parent: how many carbons? If there is a double or triple carbon-carbon bond in the molecule, both carbons in that bond must belong to the parent carbon chain, even if that chain does not have the greatest number of carbons.

    suffix: what is the family of compounds?

    Practice Questions

    1. This molecule is named 2-pentene.

    Figure \(\PageIndex{4}\).svg
    Figure \(\PageIndex{4}\): 2-pentene

    What is the name of Molecule B?

    Figure \(\PageIndex{5}\).svg
    Figure \(\PageIndex{5}\): Molecule B

    What is the name of Molecule C?

    Figure \(\PageIndex{6}\).svg
    Figure \(\PageIndex{6}\): Molecule C

    2. This molecule is named 4-methyl-2-pentene.

    Figure \(\PageIndex{7}\).svg
    Figure \(\PageIndex{7}\): 4-methyl-2-pentene

    What is the name of Molecule D?

    Figure \(\PageIndex{8}\).svg
    Figure \(\PageIndex{8}\): Molecule D

    3. This molecule is named 3-isobutyl-1-octyne.

    Figure \(\PageIndex{9}\).svg
    Figure \(\PageIndex{9}\): 3-isobutyl-1-octyne

    What is the name of Molecule E?

    Figure \(\PageIndex{10}\).svg
    Figure \(\PageIndex{10}\): Molecule E

    4. This molecule is named cyclohexene.

    Figure \(\PageIndex{11}\).svg
    Figure \(\PageIndex{11}\): cyclohexene

    What is the name of Molecule F?

    Figure \(\PageIndex{12}\).svg
    Figure \(\PageIndex{12}\): Molecule F

    5. This molecule is named 4-methylcyclohexene. Number the carbons.

    Figure \(\PageIndex{13}\).svg
    Figure \(\PageIndex{13}\): 4-methylcyclohexene

    What is the name of Molecule G?

    Figure \(\PageIndex{14}\).svg
    Figure \(\PageIndex{14}\): Molecule G

    6. This molecule is named 1,3-pentadiene.

    Figure \(\PageIndex{15}\).svg
    Figure \(\PageIndex{15}\): 1,3-pentadiene

    What is the name of Molecule H?

    Figure \(\PageIndex{16}\).svg
    Figure \(\PageIndex{16}\): Molecule H

    7. The location of substituents relative to the double bonds can lead to a type of constitutional isomer known as a positional isomer.

    The name of this molecule is 5-methyl-1,3-cyclohexadiene.

    Figure \(\PageIndex{17}\).svg
    Figure \(\PageIndex{17}\): 5-methyl-1,3-cyclohexadiene

    What is the name of Molecule I?

    Figure \(\PageIndex{18}\).svg
    Figure \(\PageIndex{18}\): Molecule I

    Double or triple carbon-carbon bonds are rigid and planar. Since the carbons cannot rotate freely around the bond, cis/trans isomers are common, and the orientation may be important for chemical reactions.

    Figure \(\PageIndex{19}\).svg
    Figure \(\PageIndex{19}\): cis-2-butene and trans-2-butene

    Practice Questions

    1. Write the steps that you use to name an alkene and an alkyne, in order, as instructions for a student who doesn't know how to do it.
    2. Draw any alkene or alkyne and go through the steps in naming your molecule.
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