Chapter Eleven
Unsaturated
Hydrocarbons
Alkenes, Alkynes, and Aromatics
•
•
Alkanes are often referred to as
saturated because each carbon atom
bonds to the maximum number of
hydrogen atoms and no more
hydrogen atoms can be added.
Alkenes and alkynes are referred to
as unsaturated because they contain
carbon-carbon double and triple
bonds to which more hydrogen
atoms can be added.
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11–2
Aromatic hydrocarbons exhibit a special type of
“delocalized” bonding that usually involves a
six-membered carbon ring (Section 11.8).
A functional group is a specific part of a molecule,
a cluster of atoms with special properties. A
carbon-carbon multiple bond is a functional group
in an unsaturated hydrocarbon.
In an unsaturated hydrocarbon reactions can
occur at the multiple bonds: an example would
be the addition of hydrogen atoms at the bond
positions.
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Characteristics of Alkenes,
Cycloalkenes, and Alkynes
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•
•
Alkenes are hydrocarbons that contain
carbon-carbon double bonds.
Alkynes are hydrocarbons that contain
carbon-carbon triple bonds.
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11–5
Three-dimensional representations of the structures of
ethene and methane. In ethene, the atoms are in a flat
(planar) form rather than a tetrahedral arrangement. The
bond angles in ethene are 120º.
Note: The
common name
of ethene
is ethylene.
Source: James and Kara Birk
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11–6
The general formula for an alkene is CnH2n and
the general formula for an alkyne is CnH2n-2.
CH3-CH2-CH2-CH=CH-CH3
C6H12
CH3-CH2-CH2-C
C-CH3
C6H10
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Cycloalkenes are cyclic unsaturated
hydrocarbons that contain one or more carboncarbon double bonds within the ring system.
cyclohexene
A general formula for cycloalkenes containing
only one double bond is CnH2n-2.
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11–8
Naming Alkenes and Alkynes
• IUPAC nomenclature rules for alkenes and
alkynes are similar to alkanes.
• Step 1. Name the parent compound. Find
the longest chain containing the double or
triple bond, and name the parent compound
by adding the suffix –ene or –yne to the
name of the main chain.
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• Step 2: Number the carbon atoms in the main
chain, beginning at the end nearer to the double
or triple bond. If the multiple bond is an equal
distance from both ends, begin numbering at the
end nearer the first branch point.
• Step 3: Write the full name. Assign numbers to
the branching substituents, and list the
substituents alphabetically. Use commas to
separate numbers, and hyphens to separate
words from numbers. Indicate the position of
the double bond carbon.
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11–10
EXAMPLE
C-C-C-C-C-C-C=C-C-C
Step 1: The longest straight chain = 10 carbons,
i.e., decane. Since it is an alkene drop the ane
suffix and add ene = decene.
Step 2: Number to give the functional group (the
double bond) the lowest number. The correct name
Is 3-decene.
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If more than one multiple bond is present,
identify the position of each multiple bond and
use the appropriate ending diene, triene,
tetraene, and so forth.
1
3
1
4
1,3-cyclohexadiene
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1,4-cyclohexadiene
11–12
Note: The functional group must be present in the
parent chain.
C-C-C-C-C-C-C-C-C=C-C
2-undecene
How would you name the following molecule?
C=C
C-C-C-C-C-C-C-C-C-C
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11–13
The Structures of Alkenes
• Recall that methane is tetrahedral,
ethylene is planar, and acetylene is linear.
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11–14
CIS-TRANS ISOMERS
Unlike carbon-carbon single bonds, rotation
around a carbon-carbon double bond is very
difficult. As a result, a new kind of isomerism
is possible for alkenes. For example, there
are two different kinds of 2-butenes. These
are called cis-trans isomers.
C
C
C==C
cis-2-butene
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C
C==C
C
trans-2-butene
11–15
• In the cis isomer, the two methyl groups
are close together on the same side of
the double bond.
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• In the trans isomer, the two methyl groups are
far apart on opposite sides of the double bond.
• The cis and trans isomers have the same
formula and the same connections between the
atoms, but they have different threedimensional structures because of the way the
groups are attached to the carbons.
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• Cis-trans isomerism occurs in an alkene
whenever the double-bond carbons are bonded
to two different substituent groups. Cis-trans
isomerism is not present when one of the
double-bond carbons is attached to two
identical groups.
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11–18
Cis - trans isomers: Different representations of the
cis and trans isomers of 2-butene.
Source: James and Kara Birk
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11–19
Comparison of Structural Isomers for Four- and
Five-Carbon Alkane and Alkene Systems.
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11–20
Physical and Chemical Properties
of Alkenes
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Properties of Alkenes and Alkynes
• Nonpolar, insoluble in water, soluble in nonpolar
organic solvents (recall “like dissolves like”).
• Less dense than water: they float on water.
• Flammable
• Generally nontoxic (minor problems may occur)
• Alkenes display cis-trans isomerism whereas
alkynes do not.
• Both alkenes and alkynes are chemically
reactive—the multiple bonds are reaction centers
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11–22
Kinds of Organic Reactions
• Addition reaction: A substance adds to
the multiple bond of an unsaturated
reactant to yield a saturated product that
has only single bonds.
• Elimination reaction: In which a
saturated reactant yields an unsaturated
product by losing groups from two
adjacent carbons.
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• Symmetrical Addition Reactions: In
which identical groups are added to each
carbon of the double bond. For example,
hydrogenation or halogenation addition
reactions.
• Unsymmetrical Addition Reactions: In
which different groups are added to each
carbon of the double bond. For example,
hydration or hydrohalogenation.
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Reactions of Alkenes and Alkynes
• Addition of H2 to alkenes and alkynes: Alkenes
and alkynes react with hydrogen in the
presence of a metal catalyst such as palladium
to yield the corresponding saturated alkane
products. This is called hydrogenation. It is a
symmetrical addition reaction.
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• Addition of Cl2 and Br2 to alkenes:
halogenation. Alkenes react with the
halogens Br2 and Cl2 to give the 1,2dihaloalkanes. This is also a symmetrical
addition reaction.
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A solution of bromine in water is reddish brown (left).
When a small amount of unsaturated hydrocarbon is
added to such a solution, the resulting solution is
decolorized because the bromine adds to the
hydrocarbon to form colorless dibromo compounds
(right).
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• Addition of HCl and HBr to alkenes:
Alkenes react with hydrogen bromide and
hydrogen chloride to give alkyl bromide or
alkyl chloride products. This is an
unsymmetrical addition reaction.
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11–28
Markovnikov rule: In the addition of HX to an
alkene, the H becomes attached to the carbon that
already has more H’s, and X becomes attached to
the carbon that has fewer H’s.
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In an alkene addition reaction, the atoms provided by
an incoming molecule are attached to the carbon
atoms originally joined by a double bond. In the
process, the double bond becomes a single bond.
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Addition of water to alkenes: Hydration.
An alcohol is produced on treatment of an
alkene with water in the presence of a
strong acid catalyst (such as H2SO4).
(Markovnikov’s rule can be used to predict
the product when water adds to an
unsymmetrically substituted alkene.)
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Let’s try this: Let H2O attack propene:
H2O
CH3CH==CH2  ???
Where does the H go? Is the product …
or
CH3CH2CH2OH
(normal propyl alcohol)
CH3CH-CH3 ?
(isopropyl alcohol)
OH
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How Does an Alkene Addition
Reaction Occur?
Reaction mechanism = A description of
the individual steps by which old bonds are
broken and new bonds are formed.
Consider the following two-step
mechanism for addition of HBr to an
alkene.
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In the 1st step, the alkene reacts with H+ from the HBr
and produces a carbocation (positive ion with the +
charge on carbon).
In the 2nd step, this reactive carbocation quickly reacts
with Br- ion to form the product.
Br
CH3-CH2-CH2-CH=CH2
HBr
CH3-CH2-CH2-CH-CH3
The H attaches
here
Br
Carbocations have a positive charge on a carbon atom.
Secondary carbocations are formed in preference to
primary carbocations.
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11–34
Polymers of Alkenes
Polymer: A large molecule formed by the
repetitive bonding together of many smaller
molecules called monomers. Many simple
alkenes undergo polymerization reactions
when treated with the proper catalyst.
Z
excess H2C=CH
polymerization
catalyst
H H Z H
C C C C
H H H H
n
Z=H
polyethylene
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11–35
An addition polymer is a polymer in which the monomers
simply “add together” with no other products formed
besides the polymer.
An example of an addition polymer is polystyrene.
“n” molecules of styrene polymerize to form polystyrene.
polymerize
n
styrene =
vinyl benzene
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*
HC
CH2
*
n
Note: “vinyl” refers to an
ethylene (H2C=CH2) substituent
11–36
When styrene is heated with a catalyst (benzoyl
peroxide), it yields a viscous liquid. After some time,
this liquid sets to a hard plastic (sample shown at left).
Source: James Scherer
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11–37
Addition Polymers
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A co-polymer is one in which two different monomers
are used and a polymer is formed which contains both
monomers in the polymeric chain. An important
co-polymer is styrene-butadiene rubber. It contains
the monomers styrene and 1,3-butadiene in a 1:3 ratio.
This polymer is a major ingredient in automobile tires.
Saran wrap is a co-polymer of vinyl chloride
(chloroethene) and 1,1-dichloroethene.
H
H
+
Cl
H
Cl
Cl
H
polymerize
*
H
H
C
C
Cl
H
C
C
H
Cl
H
*
n
Cl
H
Saran Wrap
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11–39
Some Common
Polymers
Obtained from
Ethene - Based
Monomers.
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11–40
Chemistry at a Glance:
Chemical Reactions of Alkenes
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Alkynes
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An alkyne is an acyclic unsaturated hydrocarbon
in which one or more carbon-carbon triple bonds
are present.
CH3-CH2-CH2-C CH
1-pentyne
4-decyne
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Structural representations of ethyne (acetylene), the
simplest alkyne.
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Chemical Reactions
of Alkynes
Alkynes are reduced to alkenes in the presence of
hydrogen and a catalyst.
H
CH3
H2
catalyst
H
H
H
CH3
Alkynes are insoluble in water, but soluble in organic
solvents.
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11–45
Aromatic Hydrocarbons
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11–46
An historical note
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Aromatic Compounds and
the Structure of Benzene
• In the early days the word aromatic was
used to described many fragrant molecules
isolated from natural sources. Today the
term aromatic is used to describe benzenelike molecules.
• Benzene is a flat, symmetrical molecule with
the molecular formula C6H6.
• It can be drawn with alternating carboncarbon double and single bonds.
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• Unlike alkenes, benzene does not undergo
addition reactions.
• Benzene’s relative lack of chemical reactivity is
due to its structure.
• There are two possible structures with
alternating double and single bonds. These are
called “resonance structures.”
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• Experimental evidence shows that all six
carbon-carbon bonds in benzene are identical
(with the same bond length).
• The properties, including the above one, of
benzene can only be explained by assuming
that the actual structure of benzene is an
average of the two equivalent structures—
this is referred to as resonance.
• Simple aromatic compounds like benzene are
non-polar, insoluble in water, volatile, and
flammable.
• Unlike alkenes, several aromatic
hydrocarbons are toxic. Benzene itself is
implicated as a cancer-causing chemical.
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11–50
Names for Aromatic
Hydrocarbons
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Naming Aromatic Compounds
• Substituted benzenes are named using
benzene as the parent. No number is
needed for mono-substituted benzene since
all the ring positions are identical.
• Disubstituted aromatics are named using
one of the prefixes ortho, meta-, or para-.
• An ortho- or o-disubstituted aromatic has its
two substituents in the 1,2-relationship on
the ring.
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• A meta- or m-disubstituted aromatic has
its two substituents in the 1,3-relationship
on the ring.
• A para- or p-disubstituted aromatic has its
two substituents in the 1,4-relationship on
the ring.
X
X
X
X
X
X
ortho
meta
para
Para-dichlorobenzene is used as modern moth balls.
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11–53
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When more than two groups are present on
the benzene ring, their positions are indicated with
numbers. The ring is numbered so as to obtain the
lowest possible numbers for the carbon atoms that
have substituents.
The group that comes first alphabetically is
given the lowest number.
Note: di-, tri-, and tetra- do not count in determining
alphabetical order. Isopropyl is considered an “i” and
not a “p”.
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11–55
Many substituted aromatic compounds have
common names in addition to their
systematic names.
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Fused-ring aromatic hydrocarbons are aromatic
compounds with two or more rings fused together.
benzene
naphthalene
anthracene
benz[a]pyrene
Benz[a]pyrene is a carcinogen (cancer-causing
chemical) found in cigarette smoke and automobile
exhaust; naphthalene was formerly used as moth
balls.
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11–57
Chemical Portraits:
Single and Double-Ring Aromatic Hydrocarbons
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Properties and Reactions of
Aromatic Hydrocarbons
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Reactions of Aromatic Compounds
Unlike alkenes which undergo addition
reactions, aromatic compounds
undergo substitution reactions. That is,
a group Y substitutes for one of the
hydrogen atoms on the aromatic ring.
An example is given below:
H
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Y
11–60
Some Common Aromatic Reactions:
Nitration: Substitution of a nitro group (NO2) for a ring hydrogen.
Halogenation: Substitution of a halogen
(Cl or Br) for a ring hydrogen.
Sulfonation: Substitution of a sulfonic (SO3H) group for a ring hydrogen.
Alkylation: Substitution of an alkyl group
(-CH2CH3, etc) for a ring hydrogen.
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H
NO2
HNO3/H2SO4
Nitration
H
Halogenation
Br
Br2/FeBr3
H
SO3H
SO3/H2SO4
Sulfonation
H
Alkylation
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CH2CH3
CH3CH2Cl
AlCl3
11–62
Key Things to Understand
in Chapter 11
--The term “unsaturated” means that multiple
bonds are present in a compound.
--Alkenes contain carbon-carbon double bonds.
--Alkynes contain carbon-carbon triple bonds.
--Aromatic compounds contain six carbons in a
ring arrangement with three double and three
single bonds alternating between carbon atoms.
In reality, all the bonds are equivalent due to
“resonance.”
--Alkenes are named using the family ending –ene,
while the alkynes use the family ending –yne.
--Alkenes and alkynes generally undergo addition
reactions and aromatic compounds generally
undergo substitution reactions.
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Key Things (cont’d)
--A “reaction mechanism” is a description of the
individual steps by which old bonds are broken
and new bonds are formed.
--In unsaturated compounds the multiple bonds are
common targets for reaction.
--Ethene (a.k.a., ethylene, H2C==CH2) is a planar
(flat) molecule with 120˚ bond angles.
--Ethyne (a.k.a. acetylene, H-C≡C-H) is a linear
(straight-chain) molecule with 180˚ bond angles.
--(Recall that methane, CH4, has H-C-H bond angles
of 109.5˚, the “tetrahedral angle.”)
--The general formula for an alkene is CnH2n and the
general formula for an alkyne is CnH2n-2.
--Cycloalkenes are ring compounds containing one or
more C=C double bonds.
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Key Things (cont’d)
--Cycloalkynes are ring compounds containing one or
more C≡C bonds.
--Know how to name alkenes, alkynes, cycloalkenes,
and cycloalkynes.
--Understand, and be able to identify, cis and trans
isomers of alkenes.
--Understand that alkenes and alkynes are nonpolar,
don’t dissolve in water, float on water, and are
reactive (e.g., they are flammable).
--Understand that “addition reactions” involve the
addition of compounds (e.g., H2 or HBr) across the
double bond. They can be symmetrical (H2) or
unsymmetrical (HBr).
--Be able to use the Markovnikov rule to predict the
product of an addition reaction.
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Key Things (cont’d)
--Understand that “polymers” are large molecules
formed by linking small molecules called
“monomers.”
--Understand that addtion polymers are formed by
linking monomers by means of addition reactions
(e.g., polyethylene is formed from ethylene
monomers).
--Be prepared to identify “styrene” (benzene with an
ethene group stuck on it) and polystyrene.
--Understand that a copolymer involves linking two
different types of monomers into a polymer.
--Understand that “hydrogenation” reactions reduce
the bond order of a multiple bond.
--Understand that benzene is the prototype
“aromatic” compound.
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11–66
Key Things (cont’d)
--Appreciate that although we may sometimes draw
benzene as having alternating double and single
C-C bonds, all six C-C bonds are in fact
equivalent, halfway between a double and a
single C-C bond.
--Be prepared to name di-substituted benzenes
using the ortho-, meta-, para- description.
--Understand that several benzene derivatives
(phenol, aniline, toluene) are very important and
have their own special names.
--Be able to recognize the two most common fusedring aromatic compounds, naphthylene (two
rings) and anthracene (three rings).
--Understand that aromatic compounds typically
undergo “substitution” reactions, in which a new
functional group replaces an H atom.
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11–67
To Do List
• Read chapter 11!!
• Do additional problems
• Do practice test T/F
• Do practice test MC
• Review Lecture notes for
Chapter Eleven
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11–68