Unsaturated Hydrocarbons
Jean Brainard, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both in
the U.S. and worldwide. Using an open-source, collaborative, and
web-based compilation model, CK-12 pioneers and promotes the
creation and distribution of high-quality, adaptive online textbooks
that can be mixed, modified and printed (i.e., the FlexBook®
textbooks).
Copyright © 2016 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/about/
terms-of-use.
Printed: August 25, 2016
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 the difference between cooking oil and butter. Fats can be either liquid, like oil, or solid, like butter, at
room temperature depending on their chemical structure. Saturated fats are straight in structure and tightly packed
together to form a solid at room temperature. Double and triple bonds in unsaturated fats make them easier to break
down at lower temperatures.
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.
1
www.ck12.org
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.
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
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.
FIGURE 1.2
2
www.ck12.org
Chapter 1. Unsaturated Hydrocarbons
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.
FIGURE 1.3
FIGURE 1.4
Cutting metal with an acetylene (ethyne)
torch.
Alkynes may form straight or branched chains. They rarely occur in ring shapes. In fact, alkynes of all shapes are
relatively rare in nature.
3
www.ck12.org
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.
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
Resources
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/177406
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/186336
4
www.ck12.org
Chapter 1. Unsaturated Hydrocarbons
References
1. Bill Branson, National Cancer Institute. http://commons.wikimedia.org/wiki/File:Butter_and_Oil_-_NCI_Visuals_Online.jpg . Public Domain
2. 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
3. Jodi So. Structure of benzene . CC BY-NC 3.0
4. Jodi So. Structure of acetylene(ethyne) . CC BY-NC 3.0
5. Jason Kaechler. Application of acetylene(ethyne) . CC BY 2.0
5