3.10: Alkenes and Alkynes
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- Sol Parajon Puenzo
- Cañada College
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.
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.
What is the name of 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.
What is the name of Molecule B?
What is the name of Molecule C?
2. This molecule is named 4-methyl-2-pentene.
What is the name of Molecule D?
3. This molecule is named 3-isobutyl-1-octyne.
What is the name of Molecule E?
4. This molecule is named cyclohexene.
What is the name of Molecule F?
5. This molecule is named 4-methylcyclohexene. Number the carbons.
What is the name of Molecule G?
6. This molecule is named 1,3-pentadiene.
What is the name of 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.
What is the name of 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.
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.