Chapter 11 Unsaturated Hydrocarbons
1
Saturated Hydrocarbons
Saturated hydrocarbons
• have the maximum number of hydrogen atoms
attached to each carbon atom
• are alkanes and cycloalkanes with single C–C bonds
CH3–CH2–CH3
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
Ball-and-stick models of
ethene and ethyne show
functional groups of double or
triple bonds and bond angles.
3
Bond Angles in Alkenes and Alkynes
According to VSEPR theory:
• the three groups bonded to carbon atoms
in a double bond are at 120 angles
• alkenes are flat because the atoms in a
double bond all lie in the same plane
• the two groups bonded to each carbon in
a triple bond are at 180 angles
Ethene (Ethylene)
Ethene, or ethylene,
• is the simplest alkene, with the
formula 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 a plant hormone used to
accelerate the ripening of fruits
H
H
C
H
C
H
5
Fragrant Alkenes
The odors associated with the smell of lemons, oranges,
roses, and lavender are due to volatile compounds that are
synthesized by the plants.
6
Naming Alkenes and Alkynes
When the carbon chain of an alkene or alkyne has four or more
C atoms, number the chain to give the lowest number to the
first carbon in the double or triple bond.
CH2=CHCH2CH3
1-butene
1
2 3
4
7
CH3CH=CHCH3
1
2
3 4
2-butene
CH3CH2C=CCH3
5
4 3 2 1
2-pentyne
Guide to Naming Alkenes and Alkynes
8
Naming Alkenes
Using the IUPAC system, name the following compounds:
CH3
CH3 CH3
A. CH3CHH2C=CH2
B. H2C=CHCHCHCH3
Step 1 Name the longest carbon chain that contains the
double bond. Use the ending ene for alkenes.
CH3
CH3 CH3
A. CH3CHH2C=CH2 B. H2C=CHCHCHCH3
butene
pentene
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Naming Alkenes
Step 2 Number the longest carbon chain starting from the
end nearer the double bond.
CH3
CH3 CH3
A. CH3CHH2C=CH2
B. H2C=CHCHCHCH3
1-butene
1-pentene
Step 3 Give the location and name of each substituent (in
alphabetical order) as a prefix to the alkene name.
CH3
CH3 CH3
A. CH3CHH2C=CH2
3-methyl-1-butene
B. H2C=CHCHCHCH3
3,4-dimethyl-1-pentene
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Naming Alkynes
Using the IUPAC system, name the following compound:
CH3 CH3
HC
CCHCHCH3
Step 1 Name the longest carbon chain that contains the triple
bond. Use the ending yne for alkynes.
CH3 CH3
HC
CCHCHCH3
pentyne
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Naming Alkynes
Step 2 Number the longest carbon chain starting from the
end nearer the triple bond.
CH3 CH3
HC
CCHCHCH3
1-pentyne
Step 3 Give the location and name of each substituent (in
alphabetical order) as a prefix to the alkyne name.
CH3 CH3
HC
CCHCHCH3 3,4-dimethyl-1-pentyne
12
Learning Check
Write the IUPAC name for each of the following:
A. CH2=CHCH2CH3
B. CH3CH=CHCH3
CH3
C. CH3CH=CCH3
D. CH3C
CCH3
13
Solution
14
Write the IUPAC name for each of the following:
A. CH2=CHCH2CH3
1-butene
B. CH3CH=CHCH3
CH3
2-butene
C. CH3CH=CCH3
D. CH3C
CCH3
2-methyl-2-butene
2-butyne
Cis and Trans Isomers
In an alkene, the double bond
• is rigid
• holds attached groups in fixed positions
• makes cis–trans isomers possible
15
Cis–Trans Isomers
In cis–trans isomers,
• there is no rotation around the
double bond in alkenes
• groups attached to the double
bond are fixed relative to each
other
You can make a “double bond” with
your fingers with both thumbs on
the same side or opposite from
each other.
16
Cis–Trans Isomers
Two isomers are possible when
the
groups attached to each side of
the double bond are different.
• In a cis isomer, the alkyl groups
are attached on one side of the
double bond and H atoms are
on the other side.
• In the trans isomer, the groups
and H atoms are attached on
opposite sides.
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Ball-and-stick models of the cis and
trans isomers of 2-butene.
Cis–Trans Isomerism
• Alkenes cannot have cis–trans isomers if a carbon atom in
the double bond is attached to identical groups.
Identical
Identical
BrH
Br
H
C C
H
H
Br
H
C C
CH3
2-Bromopropene
(not cis or trans)
H
Br
1,1-Dibromoethene
(not cis or trans)
Cis–Trans Isomers in Nature
• Insects emit tiny quantities of pheromones, which are
chemicals that send messages.
• The silkworm moth attracts other moths by emitting
bombykol, which has one cis and one trans double bond.
Pheromones allow insects to attract mates
from a great distance.
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Naming Cis–Trans Isomers
The prefix of cis or trans is placed in front of the alkene name
when the compound is a cis or trans isomer.
cis
trans
Br
Br
C C
H
C C
H
cis-1,2-dibromoethene
20
H
Br
H
Br
trans-1,2-dibromoethene
Learning Check
Name each, using cis or trans prefixes when needed.
Br
Br
C C
A.
H
H
H
CH3
C C
B.
H
CH3
Cl
CH3
C.
C C
H
21
Cl
Solution
Name each, using cis or trans prefixes when needed.
Br
Br
H
H
H
CH3
C C
B.
H
C.
Cl
C C
H
trans-2-butene
CH3
CH3
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cis-1,2-dibromoethene
C C
A.
Cl
1,1-dichloropropene
Addition Reactions
In alkene and alkynes,
• the double or triple bond is easily broken, providing
electrons to form new bonds
• double and triple bonds are very reactive
• in addition reactions, reactants are added to the carbon
atoms in the double or triple bond
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Hydrogenation
In hydrogenation,
• hydrogen atoms add to the carbon atoms of a double bond
or triple bond
• a catalyst such as Pt or Ni is used to speed up the reaction
H H
H2C CH2 + H2
Pt
H2C CH2
H H
HC CH + 2H2
Ni
HC CH
H H
24
Trans Fats
The process of hydrogenation is used commercially to
convert the double bonds in the unsaturated fats in
vegetable oils to saturated fats such as margarine, which
are more solid.
Adjusting the amount of added hydrogen produces
partially hydrogenated fats such as soft margarine, solid
margarine in sticks, and shortenings, which are used in
cooking.
25
Learning Check
Write the product of each the following:
CH3CH=CHCH3 + H2
Pt
Pt
+ H2
26
Solution
Write the product of each the following:
CH3CH=CHCH3 + H2
Pt
CH3CH2CH2CH3
Pt
+ H2
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Halogenation Reactions
In halogenation reactions,
• halogen atoms such as chlorine or bromine add across a
double bond
• the reaction occurs without a catalyst
• a dihaloalkane product results
In the general equation for halogenation, X2 is used for
either Cl2 or Br2.
28
Halogenation Reactions
The addition reaction of Br2 across a double bond is used to
test for the presence of double bonds. A positive result turns
a clear solution red.
29
Hydration
In the addition reaction called hydration,
• an acid H+ catalyst is required
• water (H−OH) adds to a double bond
• an H atom bonds to one C in the double bond
• an OH bonds to the other C
• an alkene is converted to an alcohol
H OH
H+
CH3CH=CHCH3 + HOH
CH3CHCHCH3
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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 bond with the more H
• the OH bonds to the C in the double bond with the fewer
H atoms
Learning Check
Write the product for the hydration of each of the following.
H+
1. CH3CH2CH=CHCH2CH3 + HOH
CH3
2.
CH3C=CHCH2CH3 + HOH
H+
3.
+ HOH
32
H+
Solution
Write the product for the hydration of each of the following.
1.
CH3CH2CH=CHCH2CH3 + HOH
H+
CH3CH2CHCHCH2CH3
H
OH
Polymers
Polymers are
• large, long-chain molecules
• found in nature, including cellulose in plants,
starches in food, proteins, and DNA in the body
• also synthetic, such as polyethylene and polystyrene,
Teflon, and nylon
• composed of small repeating units called monomers
• made from reaction of small alkenes
34
Addition Reactions: Polymerization
In polymerization, small repeating units called monomers join to
form a long-chain polymer in an addition reaction.
H
H
+
C C
H
H
H
H
C C
H
H
+
H
H
monomer unit repeats
C C
H
H
Ethylene monomers
chain continues
H H H
H H H
C C C
C C C
H H H
H H H
n
Polyethylene
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chain continues
Common Synthetic Polymers: Polyethylene
Polyethylene is made using an addition
reaction with the monomer unit ethylene.
Common uses for polyethylene include
plastic bottles, film, and insulation
materials.
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Common Synthetic Polymers:
Polytetrafluoroethylene
Polytetrafluoroethylene, primarily used in nonstick coatings,
is made using an addition reaction with the monomer unit
of tetrafluoroethene.
F
F
F
C C F
F
+
F
tetrafluoroethene
F
C C F
F
F
C C
F
F
F
F
F
C C C
F
F
F
polytetrafluoroethylene
37
Common Synthetic Polymers: Polypropylene
Polypropylene is used for ski and hiking
clothing, carpets, and artificial joints.
38
Common Synthetic Polymers: Polystyrene
Polystyrene is used for plastic coffee cups,
cartons, and insulation.
39
Common Synthetic Polymers:
Polyvinylchloride
Polyvinylchloride, also known as PVC, is used
for plastic pipes and tubing, garden hoses, and
garbage bags.
40
Common Synthetic Polymers:
Polydichloroethylene
Polydichloroethylene is used in plastic film
and wraps.
41
Learning Check
Name the starting monomer and give its structure for each of the
following polymers:
A. polyethylene
B. polyvinylchloride
C. polystyrene
42
Solution
Name the starting monomer and give its structure for each of the
following polymers:
A. polyethylene
ethene
B. polyvinylchloride
chloroethene
C. polystyrene
phenylethene, styrene
43
Recycling Plastics
Recycling is simplified by using codes on plastic items.
1
2
3
4
5
6
7
44
PETE Polyethylene terephtalate
HDPE High-density polyethylene
PV Polyvinyl chloride
LDPE Low-density polyethylene
PP Polypropylene
PS Polystyrene
OTHER Other plastic: fiberglass, polycarbonate, etc.
Aromatic Compounds
Benzene is
• an aromatic compound
• a ring of six C atoms and six H atoms
• a flat ring structure drawn with three double bonds
• represented by two structures because the electrons are
shared among all the C atoms
Benzene Structure
Benzene
• has six electrons shared equally among the six C atoms
• is also represented as a hexagon with a circle drawn inside
46
Aromatic Compounds in Nature and Health
Vanillin
O
Aspirin O
CH
COH O
C O CH3
OCH3
O
OH
Ibuprofen
CH3
H3C CH CH2
Acetaminophen
NH C CH3
CH3 O
CH COH
OH
47
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 that have been in use for many years
• with a single substituent use a common name or are named
as a benzene derivative
CH3
toluene
(methylbenzene)
NH2
aniline
(benzenamine)
OH
phenol
(hydroxybenzene)
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Naming Aromatic Compounds
When two groups are attached to the benzene ring, the ring is
numbered to give the lowest numbers to the side groups.
CH3
Cl
3-chlorotoluene
OH
Cl
Cl
Cl
1,4-dichlorobenzene
2-chlorophenol
50
Learning Check
Write the structural formulas for each of the following:
1. 1,3-dichlorobenzene
2. 2-chlorotoluene
51
Solution
Write the structural formulas for each of the following:
1. 1,3-dichlorobenzene
Cl
Cl
2. 2-chlorotoluene
CH 3
Cl
52
Concept Map – Unsaturated Hydrocarbons
53