General, Organic, and
Biological Chemistry
Fourth Edition
Karen Timberlake
Chapter 12
Alkenes, Alkynes, and
Aromatic Compounds
12.1
Alkenes and Alkynes
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Lectures
Unsaturated Hydrocarbons
Unsaturated hydrocarbons
 do not contain the maximum
number of hydrogen atoms
that could be attached to each
carbon atom.
 are alkenes when they contain
double C=C bonds.
 are alkynes when they contain
triple C≡C bonds.
 react with hydrogen gas to
form saturated hydrocarbons.
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Chapter 12 Section 1
2
Alkenes
Alkenes
 contain a carbon–carbon
double bond that forms when
adjacent carbon atoms share
two pairs of valence electrons.
 are flat because the atoms in a
double bond lie in the same
plane.
 have a trigonal planar
arrangement around the double
bonded carbon atoms.
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Chapter 12 Section 1
3
Alkynes
Alkynes
 have a triple bond that forms
when two carbon atoms share
three pairs of valence electrons.
 have a 180˚ angle and are linear.
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Chapter 12 Section 1
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Guide to Naming Alkenes and
Alkynes
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Chapter 12 Section 1
5
Naming Alkenes and Alkynes
Write the IUPAC name for the following.
A.
B.
Step 1 Name the longest carbon chain that contains
the double or triple bond.
Analyze the Problem.
Functional
Group
IUPAC
Naming
IUPAC
Name
A. Double bond Alkene
Replace –ane of
alkane with –ene
Alkene
B. Triple bond
Replace –ane of
alkane with –yne
Alkyne
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Class
Alkyne
Chapter 12 Section 1
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Naming Alkenes and Alkynes
Write the IUPAC name for the following.
A.
B.
Step 1 Name the longest carbon chain that contains
the double or triple bond.
A.
B.
butene
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heptyne
Chapter 12 Section 1
7
Naming Alkenes and Alkynes
Write the IUPAC name for the following.
A.
B.
Step 2 Number the carbon chain from the end nearer
the double or triple bond.
A
B.
1
2
3
4
2−butene
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1
2
3
4
5 6 7
2−heptyne
Chapter 12 Section 1
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Naming Alkenes and Alkynes
Write the IUPAC name for
A.
B.
Step 3 Give the location and name of each
substituent (alphabetical order) as a prefix to
the alkene or alkyne name.
A.
B.
1
2
3
4
2−butene
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1
2
3
4
5 6 7
4-methyl−2−heptyne
Chapter 12 Section 1
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Naming Cycloalkenes
When naming a cycloalkene,
 if there are no substituents, the double bond does
not need a number.
 if there is a substituent, the carbons in the double
bond are numbered as 1 and 2, and the ring is
numbered from carbon 2 in the direction that will give
the lower number to the substituent.
3-methylcyclohexane
cyclopentene
3
1
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2
Chapter 12 Section 1
10
Cis and Trans Isomers
Cis and trans isomers
 have the same formula, but
different physical and chemical
properties.
 occur because double bonds in
alkenes are rigid and cannot
rotate.
We add the prefix cis or trans to
denote whether the atoms bonded
to the carbon atoms are on the
same side or the opposite sides of
the double bond.
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Chapter 12 Section 1
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Cis–Trans Isomers
Cis–trans isomers occur when different groups are
attached to the carbon atoms in a 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 of the double bond.
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Chapter 12 Section 1
12
Cis–Trans Isomerism
Cis–trans isomers do not occur if one of the carbon
atoms in the double bond has identical groups.
(neither cis nor trans)
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Chapter 12 Section 1
(neither cis nor trans)
13
Naming Cis–Trans Isomers
The prefixes cis- or trans- are placed in front of the
alkene name when there are cis–trans isomers.
cistrans-
Br
Br
C C
H
C C
H
H
cis-1,2-Dibromoethene
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H
Br
Chapter 12 Section 1
Br
trans-1,2-Dibromoethene
14
Alkenes: Drawing Cis–Trans
Isomers
Draw the cis and trans isomers of 2-hexene.
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Chapter 12 Section 1
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Learning Check
Name each, using cis–trans prefixes when needed.
Br
Br
C C
A.
A.
H
H
H
CH3
C C
B.
H
CH3
Cl
CH3
C.
C C
H
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Cl
Chapter 12 Section 1
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Solution
Name each, using cis–trans prefixes when needed.
Br
Br
C C
A.
A.
H
H
H
CH3
C C
B.
H
CH3
C C
H
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trans-2-Butene
Cl
CH3
C.
cis-1,2-Dibromoethene
1,1-Dichloropropene
Cl
Identical atoms; no cis or
trans
Chapter 12 Section 1
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Addition Reactions
There are different addition reactions as shown in
Table 12.2.
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Chapter 12 Section 1
18
Hydrogenation
In hydrogenation,
 hydrogen atoms add to the carbon atoms of a double
or triple bond.
 a catalyst, such as Pt, Pd or Ni, is used to speed up
the reaction.
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Chapter 12 Section 1
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Halogenation
Halogenation reactions of alkenes and alkynes occur
rapidly without a catalyst.
Br Br
H2C CH2 + Br2
H2C CH2
Cl Cl
HC C CH3 + 2Cl2
HC C CH3
Cl Cl
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Chapter 12 Section 1
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Hydrohalogenation
Hydrohalogenation reactions occur in two steps.
Step 1
Step 2
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Chapter 12 Section 1
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Markovnikov’s Rule
In assymetrical alkenes, the H in HX adds to the
carbon in the double bond that has the greater
number of H atoms.
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Chapter 12 Section 1
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Hydration
In hydration,
 a strong acid (H+) catalyst is required.
 water (HOH) adds to a double bond.
 an −H atom bonds to one C in the double C═C bond.
 an −OH bonds to the other C.
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Chapter 12 Section 1
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Hydration
When hydration occurs with a double bond that has an
unequal number of H atoms,
 the −H atom bonds to the C in the double C═C bond
that has more hydrogen atoms.
 the −OH bonds to the C in the double C═C bond that
has the fewer hydrogen atoms.
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Chapter 12 Section 1
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Polymers
Polymers are
 large, molecules that consist of small repeating units
called monomers.
 found in nature, including cellulose in plants,
starches in food, and proteins and DNA in the body.
 also made synthetically, such as polyethylene and
polystyrene, Teflon, and nylon.
 often made by reaction of small alkenes at high
temperature and pressure.
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Chapter 12 Section 1
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Polymerization
In polymerization, small repeating units called monomers
join to form a long chain polymer.
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Chapter 12 Section 1
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Common Synthetic Polymers
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Chapter 12 Section 1
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Polymers from Alkenes
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Chapter 12 Section 1
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Polymers from Alkenes
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Chapter 12 Section 1
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Recycling Plastics
Recycling is simplified by using codes found on plastic
items.
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Chapter 12 Section 1
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Benzene Structure
Benzene
 has 6 electrons shared equally among the 6 C atoms.
 behaves more like an alkane; it does not undergo
addition reactions.
 is also represented as a hexagon with a circle drawn
inside.
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Chapter 12 Section 1
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Aromatic Compounds
At the time of its discovery, many compounds containing
benzene had fragrant odors, so the family of benzene
compounds became known as aromatic compounds.
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Chapter 12 Section 1
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Common Aromatic Compounds
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Chapter 12 Section 1
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Naming Aromatic Compounds
Aromatic compounds are named
 with benzene as the parent chain.
 with one side group named in front of benzene.
CH3
Cl
Methylbenzene
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Chapter 12 Section 1
Chlorobenzene
34
Some Common Names
Some substituted benzene rings
 have common names that have been used for many
years.
 with a single substituent use a common name or are
named as a benzene derivative.
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Chapter 12 Section 1
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Aromatic Compounds with Two
Groups
When two groups are attached to a benzene ring,
 number the ring to give the lowest numbers to the
side groups (IUPAC),
or
 use prefixes to show the arrangement of the
substituents (common) on the ring, as shown
below.
ortho (o) for 1,2meta (m) for 1,3para (p) for 1,4-
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Chapter 12 Section 1
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Aromatic Compounds with Two
Substituents
CH3
Cl
OH
Cl
Cl
Cl
3-Chlorotoluene
(m-chlorotoluene)
1st and 3rd carbons
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1,4-Dichlorobenzene
(p-dichlorobenzene)
Substituents on
1st and 4th carbons
Chapter 12 Section 1
2-Chlorophenol
(o-chlorophenol)
1st and 2nd carbons
37
Xylene
For isomers of dimethylbenzene, the common name,
xylene may be used.
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Chapter 12 Section 1
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Aromatic Compounds with Three
Substituents
Cl
CH3
Cl
Cl
CH3
Br
Br
Cl
Br
Cl
1,3,5-Trichlorobenzene
2,6-Dibromo-4-chlorotoluene
4-Bromo-2-chlorotoluene
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Chapter 12 Section 1
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Properties of Aromatic
Compounds
Aromatic compounds
 have higher melting and boiling points due to
the flat symmetrical structure of the rings.
 are not soluble in water unless they contain
–OH or –COOH substituents.
 have a stable aromatic bonding system.
 are resistant to many reactions that break up
the aromatic system.
 are flammable like other hydrocarbons.
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Chapter 12 Section 1
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