Unsaturated Hydrocarbons
Jean Brainard, Ph.D.
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Printed: October 14, 2014
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Unsaturated Hydrocarbons
1
Unsaturated Hydrocarbons
• Define unsaturated hydrocarbon.
• Describe alkenes, aromatic hydrocarbons, and alkynes.
Look at these colored lozenges. Notice how the unsaturated form of each color is dimmer and less colorful than
the saturated form. Compounds called hydrocarbons can also be saturated or unsaturated. Can you guess what an
unsaturated hydrocarbon is?
What Are Unsaturated Hydrocarbons?
Hydrocarbons are compounds that contain only carbon and hydrogen. The carbon atoms in hydrocarbons may share
single, double, or triple covalent bonds. Unsaturated hydrocarbons contain at least one double or triple bond
between carbon atoms. They are classified on the basis of their bonds as alkenes, aromatic hydrocarbons, or alkynes.
Q: Why do you suppose hydrocarbons with double or triple bonds are called unsaturated?
A: A carbon atom always forms four covalent bonds. Carbon atoms with double or triple bonds are unable to bond
with as many hydrogen atoms as they could if they were joined only by single bonds. This makes them unsaturated
with hydrogen atoms.
Alkenes
Unsaturated hydrocarbons that contain one or more double bonds are called alkenes. The name of a specific alkene
always ends in –ene and has a prefix indicating the number of carbon atoms. The structural formula in the Figure
1.1 represents the smallest alkene, named ethene, which has two carbon atoms and four hydrogen atoms (C2 H4 ).
Ethene is produced by most fruits and vegetables. It speeds up ripening. The Figure 1.1 show the effects of ethene
on bananas.
Alkenes can have different shapes. They can form straight chains, branched chains, or rings. Alkenes with the same
atoms but different shapes are called isomers. Smaller alkenes have relatively high boiling and melting points, so
they are gases at room temperature. Larger alkenes have lower boiling and melting points, so they are liquids or
waxy solids at room temperature.
Aromatic Hydrocarbons
Unsaturated hydrocarbons called aromatic hydrocarbons are cyclic hydrocarbons that have double bonds. These
compounds have six carbon atoms in a ring with alternating single and double bonds. The smallest aromatic
hydrocarbon is benzene, which has just one ring. Its structural formula is shown in the Figure 1.2. Larger aromatic
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FIGURE 1.1
The bananas on the left were stored in
a special bag that absorbs ethene. The
bananas on the right were stored without
a bag.
hydrocarbons consist of two or more rings, which are joined together by bonds between their carbon atoms. The
name of aromatic hydrocarbons comes from their strong aroma, or scent. That’s why they are used in air fresheners
and mothballs. You can learn more about these interesting hydrocarbons at this URL:
http://www.youtube.com/watch?v=8gW7H0ReN5g
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/5091
Q: How many bonds does each carbon atom in benzene form?
A: Each carbon atom forms four covalent bonds. Carbon atoms always form four covalent bonds, regardless of the
atoms to which it bonds.
Alkynes
Unsaturated hydrocarbons that contain one or more triple bonds are called alkynes. The names of specific alkynes
always end in –yne and have a prefix for the number of carbon atoms. The structural formula in the Figure 1.3
represents the smallest alkyne, named ethyne, which has two carbon atoms and two hydrogen atoms (C2 H2 ). Ethyne
is also called acetylene. It is burned in acetylene torches, like the one pictured in the Figure 1.4. The flame of an
acetylene torch is so hot that it can melt metal.
Alkynes may form straight or branched chains. They rarely occur in ring shapes. In fact, alkynes of all shapes are
relatively rare in nature.
Summary
• Unsaturated hydrocarbons are hydrocarbons with at least one double or triple bond between carbon atoms.
• Alkenes are unsaturated hydrocarbons with at least one double bond between carbon atoms.
• Aromatic hydrocarbons are unsaturated cyclic hydrocarbons with alternating single and double bonds between
carbon atoms.
• Alkynes are unsaturated hydrocarbons with at least one triple bond between carbon atoms.
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Chapter 1. Unsaturated Hydrocarbons
FIGURE 1.2
FIGURE 1.3
Vocabulary
• alkene: Unsaturated hydrocarbon that contains at least one double bond between carbon atoms.
• alkyne: Unsaturated hydrocarbon that contains at least one triple bond between carbon atoms.
• aromatic hydrocarbon: Unsaturated cyclic hydrocarbon that has a strong aroma and alternating single and
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FIGURE 1.4
Cutting metal with an acetylene (ethyne)
torch.
double bonds between carbon atoms.
• unsaturated hydrocarbon: Hydrocarbon that contains at least one double or triple bond between carbon
atoms.
Explore More
Draw a plan for a three-dimensional model of an unsaturated hydrocarbon molecule. Think of a way to represent
the different kinds of atoms and bonds in the molecule. For ideas on making three-dimensional molecular models,
go to the URL below.
http://www.ehow.com/how_7707580_make-3d-molecular-model.html
Review
1. What does it mean for a hydrocarbon to be unsaturated?
2. Which chemical formula could represent an unsaturated hydrocarbon?
a.
b.
c.
d.
CH4
C2 H6
C3 H 8
C2 H 2
3. Which of the following hydrocarbons has a double bond between carbon atoms?
a.
b.
c.
d.
propane
hectane
ethyne
hexene
4. Describe an aromatic hydrocarbon
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Chapter 1. Unsaturated Hydrocarbons
References
1. Ethene: Jodi So (CK-12 Foundation); Good bananas: Image copyright Sally Scott, 2013; Bad bananas: Image
copyright PeJo, 2013. Structure of ethene, as well as well it can be found. Ethene: CC BY-NC 3.0; Bananas:
Used under licenses from Shutterstock.com
2. Jodi So. Structure of benzene. CC BY-NC 3.0
3. Jodi So. Structure of acetylene(ethyne). CC BY-NC 3.0
4. Jason Kaechler. Application of acetylene(ethyne). CC BY 2.0
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