1.4: Alkenes and Alkynes

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Rebekah O'Donnell
Boston University via OpenStax CNX

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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

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.
Draw any alkene or alkyne and go through the steps in naming your molecule.