Chapter 12 Unsaturated
Hydrocarbons
12.1
Alkenes and Alkynes
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Unsaturated Hydrocarbons
Unsaturated
hydrocarbons
• Have fewer hydrogen
atoms attached to the
carbon chain than
alkanes.
• Are alkenes with
double bonds.
• Are alkynes with triple
bonds.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Bond Angles in Alkenes and
Alkynes
According to VSEPR
theory:
• Three groups in a double
bond are bonded at 120°
angles.
• Alkenes are flat, because
the atoms in a double
bond lie in the same
plane.
• The groups attached to a
triple bond are at 180°
angles.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Naming Alkenes
The names of alkenes
• Use the corresponding alkane name.
• Change the ending to -ene.
Alkene
IUPAC
Common
H2C=CH2
ethene
ethylene
H2C=CH─CH3
propene
propylene
cyclohexene
Ethene (ethylene)
Ethene or ethylene
• Is an alkene C2H4.
• Has two carbon atoms
connected by a double bond.
• Has two H atoms bonded to
each C atom.
• Is flat with all the C and H
atoms in the same plane.
• Is used to accelerate the
ripening of fruits.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Naming Alkynes
The names of alkynes
• Use the corresponding alkane name.
• Change the ending to -yne.
Alkyne
IUPAC
Common
HC≡CH
ethyne
acetylene
HC≡C─CH3
propyne
Guide to Naming Alkenes and
Alkynes
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Naming Alkenes
Write the IUPAC name for CH2=CH─CH2─CH3
and
CH3─CH=CH─CH3
STEP 1 Name the longest carbon chain
butene
STEP 2 Number the chain from the double bond
CH2=CH─CH2─CH3 1-butene
1
2
3
4
CH3─CH=CH─CH3
2-butene
Comparing Names of Alkanes,
Alkenes, and Alkynes
TABLE 12.1
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Naming Alkenes with Substituents
CH3
│
Write the IUPAC name for CH3─CH─CH=CH─CH3
STEP 1 Name the longest carbon chain
pentene
STEP 2 Number the chain from the double bond
CH3
│
CH3─CH─CH=CH─CH3 2-pentene
5
4
3
2
1
STEP 3 Give the location of each substituent
4-methyl- 2-pentene
Naming Alkynes with
Substituents
CH
3
│
Write the IUPAC name for HC≡C─CH─CH3
1
2
3
4
STEP 1 Name the longest carbon chain
butyne
STEP 2 Number the chain from the double bond
1-butyne
STEP 3 Give the location of each substituent
3-methyl-1-butyne
Learning Check
Write the IUPAC name for each of the following:
1. CH2=CH─CH2─CH3
2. CH3─CH=CH─CH3
CH3
|
3. CH3─CH=C─CH3
4. CH3─C≡C─CH3
Solution
Write the IUPAC name for each of the following:
1. CH2=CH─CH2─CH3
1-butene
2. CH3─CH=CH─CH3
CH3
|
3. CH3─CH=C─CH3
2-butene
2-methyl-2-butene
4. CH3─C≡C─CH3
2-butyne
Learning Check
Write the structural formula for each of the
following:
A.
2-pentyne
B. 3-methyl-2-pentene
Solution
Write the structural formula for each of the
following:
A.
CH3─CH2─C≡C─CH3
CH3

B. CH3─CH2─C=CH─CH3
2-pentyne
3-methyl-2-pentene
Chapter 12 Unsaturated
Hydrocarbons
12.2
Cis-Trans Isomers
Copyright © 2007 by Pearson Education, Inc.
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Cis and Trans Isomers
In an alkene, cis and trans isomers are possible
because the double bond
• Is rigid.
• Cannot rotate.
• Has groups attached to the carbons of the double bond that
are fixed relative to each other.
CH3
CH = CH
cis
CH3
CH3
CH = CH
trans
CH3
Cis-Trans Isomers
Cis-trans isomers
• Can be modeled by
making a “double
bond” with your
fingers with both
thumbs on the same
side or opposite
from each other.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Cis-Trans Isomers
Cis-trans isomers occur when
different groups are attached to
the double bond.
• In a cis isomer, groups are
attached on the same side of
the double bond.
• In the trans isomer, the
groups are attached on
opposite sides.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Cis-Trans Isomerism
• Cis-trans isomers do not occur if a carbon atom in the
double bond is attached to identical groups.
Identical
HH
Identical
Br
H
Br
C C
HH
Br
H
C C
CH3
2-bromopropene
(not cis or trans)
H
Br
1,1-dibromoethene
(not cis or trans)
Naming Cis-Trans Isomers
• The prefixes cis or trans are placed in front of the
alkene name when there are cis-trans isomers.
cis
trans
Br
Br
C C
H
H
Br
C C
H
cis-1,2-dibromoethene
dibromoethene
H
Br
trans-1,2-
Pheromones
A pheromone
• Is a chemical messenger emitted by insects in tiny
quantities.
• Called bombykol emitted by the silkworm moth to
attract other moths has one cis and one trans
double bond.
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
Learning Check
Name each, using cis-trans prefixes when needed.
Br
Br
C C
A.
H
H
H
CH3
C C
B.
H
CH3
Cl
CH3
C.
C C
H
Cl
Solution
Br
Br
C C
A.
cis-1,2-dibromoethene
H
H
H
CH3
C C
B.
H
CH3
Cl
CH3
C.
trans-2-butene
C C
H
Cl
1,1-dichloropropene
Identical atoms on one C;
no cis or trans
Chapter 12 Unsaturated
Hydrocarbons
12.5
Aromatic Compounds
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Publishing as Benjamin Cummings
Benzene Structure
Benzene
• Has 6 electrons shared equally among the 6 C atoms.
• Is also represented as a hexagon with a circle drawn inside.
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
Aromatic Compounds in Nature and
Health
Vanillin
Aspirin
O
CH
O
COH O
C
O
CH 3
OCH 3
O
OH
NH C CH3
Ibuprofen
CH3
H3C CH CH2
Acetaminophen
CH3 O
CH COH
OH
Naming Aromatic Compounds
Aromatic compounds are named
• With benzene as the parent chain.
• With one side group named in front of benzene.
CH3
methylbenzene
Cl
chlorobenzene
Some Common Names
Some substituted benzene rings
• Have common names used for many years.
• With a single substituent use a common name or are
named as a benzene derivative.
CH 3
toluene
(methylbenzene)
NH 2
aniline
(benzenamine)
OH
phenol
(hydroxybenzene)
Learning Check
Identify the organic family for each:
A. CH3─CH2─CH=CH2
B.
C. CH3─C≡CH
D.
Solution
Identify the organic family for each:
A. CH3─CH2─CH=CH2
alkene
B.
cycloalkane (alkane)
C. CH3─C≡CH
alkyne
D.
aromatic