Chapter 13
Unsaturated
Hydrocarbons
Homework
16, 20, 36, 40, 50, 54, 58, 70, 78, 88, 92, 100
13.1 Unsaturated Hydrocarbons
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An unsaturated hydrocarbon is a hydrocarbon in which one or more
carbon-carbon multiple bonds are present
Unsaturated hydrocarbons have physical properties similar to those
of saturated hydrocarbons
Unsaturated hydrocarbons are chemically more reactive than their
saturated counterparts
Whenever a certain part of a molecule determines its chemical
properties it is called a functional group
A functional group is a part of an organic molecule where most of its
chemical reactions occur
There are 3 types of unsaturated hydrocarbons
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Alkenes
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Alkynes
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Aromatic Hydrocarbons
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C-C Double Bond
C-C Triple Bond
Delocalized bonding in a 6-membered ring
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13.2 Characteristics of Alkenes and
Cycloalkenes
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An Alkene is an acyclic unsaturated hydrocarbon
that contains one or more C-C double bonds
Functional Group C=C
General Formula CnH2n
End in –ene
No rotation around C=C
Ex.
13.2 Characteristics of Alkenes and
Cycloalkenes
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Unlike the carbon
found in methane
the shape of a
carbon double
bonded is trigonal
planar
Consistent with
VSEPR
Ethene
Methane
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13.2 Characteristics of Alkenes and
Cycloalkenes
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A cycloalkene is a cyclic unsaturated
hydrocarbon that contains one or more C=C
bonds within the ring system
General Formula CnH2n-2
Alkenes with 2 double bonds present are
common and are called dienes
Cycloalkenes with more than one double bond
are possible but not common
13.3 Names for Alkenes and
Cycloalkenes
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Rules similar to those for alkanes and
cycloalkanes with the following changes
  Replace
–ane with –ene, to indicate presence of
double bond
  The parent chain is the longest chain that contains
both C atoms of the double bond
  Number the parent chain so that the double bond is
nearest the end
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If the Double bond is equidistant from the ends number so
that the any other substituents have the lowest number
  Give
the position of the double bond as 1 number, the
number corresponding to the carbon in the double
bond with the lower number
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13.3 Names for Alkenes and
Cycloalkenes
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Rules (con’t)
  Use
the suffixes –diene, -triene, -tetrene, etc… when
more than one double bond is present. A separate
number must be used to locate each double bond
  Do not use a number to locate the double bond in an
unsubstituted cycloalkene with only one double bond
  In substituted cycloalkenes with only one double bond
the double bond carbon atoms are numbered 1 & 2 in the
direction which will give the substituent the lowest
number
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Still no number for the double bond
  In
cycloalkenes with more than one double bond within
the ring assign one double bond the numbers 1 & 2 and
the other double bonds the lowest numbers possible
13.3 Names for Alkenes and
Cycloalkenes
  Common
Names
 Simple
member of most families of organic
compounds, including, alkenes have common
names
 Some compounds known by common names
include
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13.3 Names for Alkenes and
Cycloalkenes
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Name the following molecules
13.3 Names for Alkenes and
Cycloalkenes
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Alkenes as Substituents
Alkenyl groups are noncyclic hydrocarbon
substituent in which a C=C is present
3 very common groups
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In Compounds
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13.5 Constitutional Isomerism in Alkenes
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Structural isomers are often positional isomers
where the position of the double bond is moved
- A comparison of
structural isomerism for 4 and
5 carbon alkane and alkene systems
- More isomers possible for alkenes than alkanes
13.6 Cis-Trans Isomerism in Alkenes
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Can also have cis-trans isomers due to rigidity
around double bond
Cis isomer is when similar substituents are on
the same side of the double bond
Trans isomer is when similar substituents are
on opposite sides of the double bond
Cis-trans isomerism is nor possible when one of
the double bond carbons has 2 identical groups
When there is more than one double bond is
present cis-trans isomerism can get much more
complicated
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2-butene
cis-cis, trans-trans, trans-cis, cis-trans
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13.7 Naturally Occurring Alkenes
  Alkenes
are abundant in nature
  Many important biological molecules are
characterized by C=C bond in their
structure
  2 such classes of molecules are
 Pheromones
 Terpenes
13.7 Naturally Occurring Alkenes
  Pheromones
are compounds used by
insects (and some animals) to transmit a
message to other members of the same
species
  Biological activity is depend upon cis/trans
isomerism
  If the cis/trans orientation is not correct the
pheromone is ineffective
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13.7 Naturally Occurring Alkenes
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A Terpene is an organic compound whose carbon
skeleton is composed of 2 or more 5-Carbon
isoprene (2-methyl-1,3-butadiene) structural units
Most widely distributed class of organic compounds
in the biological world
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>22,000 structures known
Responsible for odors of trees and plants
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Β-carotene (orange in carrots)
Menthol
13.8 Physical Properties of Alkenes
  General
physical properties of alkenes are
 Insoluble
in water
 Soluble in nonpolar solvents
 Densities lower than water
 Melting pts lower than alkane equivalent
  2-4
carbons are gases at room temp
  5-17 carbons are liquids at room temp (1 C=C)
  > 17 carbons are solids at room temp (1 C=C)
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13.9 Chemical Reactions of Alkenes
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Alkenes are very flammable and therefore
undergo combustion reactions like alkanes
  Form
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CO2 and H2O
Undergo addition reactions in which atoms or
groups of atoms are added to the carbons of the
C=C in a hydrocarbon or hydrocarbon derivative
  General
Equation:
13.9 Chemical Reactions of Alkenes
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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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13.9 Chemical Reactions of Alkenes
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Addition reactions can be classified as
symmetrical or unsymmetrical
A symmetrical addition reaction is a addition
reaction in which identical atoms (or groups of
atoms) are added to each carbon of a C=C
An unsymmetrical addition reaction is a addition
reaction in which different atoms (or groups of
atoms) are added to the carbons of a C=C
13.9 Chemical Reactions of Alkenes
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Symmetrical addition reactions
  A
Hydrogenation reaction is a reaction in which H2 is
incorporated into molecules of an organic
compound
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Requires a catalyst
  A
Halogenation reaction is a reaction in which a
halogen (Br2,F2, Cl2, or I2) is incorporated into
molecules of an organic compound
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13.9 Chemical Reactions of Alkenes
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Unsymmetrical addition reactions
  A
Hydrohalogenation reaction is a reaction in which a
hydrogen halide (HCl, HBr, HI, or HF) is incorporated
into molecules of an organic compound
  A
hydration reaction is a reaction in which a water
molecule is incorporated into molecules of an organic
compound
13.9 Chemical Reactions of Alkenes
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When the alkene involved in a hydrohalogenation or a hydration
reaction is unsymmetrical more than one type of product is possible
Ex
Markovnikov’s Rule states that when an unsymmetrical molecule of
the form HQ adds to an unsymmetrical alkene the H atom from the
HQ becomes attaches to the unsaturated Carbon that already has
the most hydrogen atoms
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Those that have get
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13.10 Polymerization of Alkenes
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A polymer is a large molecule formed by the repetitive
bonding together of many smaller molecules
A monomer is the small molecule that is the structural
repeating unit in a polymer
A polymerization reaction is a reaction in which the
repetitious combining of many monomers produces a very
large molecule (the polymer)
An Addition polymer is a polymer in which the monomers
simply add together with no other products formed
Exact formula cannot be written so instead –(CH2-CH2)n-
13.10 Polymerization of Alkenes
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Substituted ethene molecules can undergo
polymerization as well
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When the monomers are more complex the
polymers become more complex as well
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A copolymer is when 2 different monomers are
present
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13.11 Alkynes
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A second type of unsaturated hydrocarbons are
Alkynes
Alkynes are acyclic unsaturated hydrocarbons
that contain one or more Carbon-carbon triple
bonds
General Formula CnH2n-2
Use ending –yne, linear structure
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13.11 Alkynes
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IUPAC Nomenclature for Alkynes
  Same
as for Alkenes with the exception of –yne
instead of –ene
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Physical and Chemical Properties of Alkynes
  Physical
and chemical properties are similar to those
of alkenes
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Melting pts, densities and solubilities similar
Addition reactions possible
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Difference 2 equivalents can be added to an alkyne
13.12 Aromatic Hydrocarbons
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The third class of unsaturated hydrocarbons are
Aromatic Hydrocarbons
An aromatic hydrocarbon is a unsaturated cyclic
hydrocarbon that does not readily undergo
addition reactions
Simplest aromatic hydrocarbon is benzene
  Flat,
symmetrical molecule with 2 resonance structures
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13.15 Aromatic Hydrocarbons
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Since the double bonds ‘jump’ between the two
positions the electrons are said to be delocalized
  Delocalized
electron are shared between more than
two atoms
  An
aromatic ring system is a highly unsaturated
carbon ring system in which both localized and
delocalized bonds are present
13.13 Names for Aromatic
Hydrocarbons
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Benzene derivatives
with one substituent
are with benzene as
the parent chain. One
side group is named
in front of the name
benzene.
Some go by Common
names
When only one
substituent is present
normally placed at 12
o’clock but all
positions are
equivalent
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13.13 Names for Aromatic
Hydrocarbons
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When two groups are
attached to benzene
3 isomers are
possible
The ring is numbered
to give the lower
numbers to the side
groups
Or non-numerical
prefixes can be used
13.13 Names for Aromatic
Hydrocarbons
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Two methyl groups
attached to a benzene
created a new base
name
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When three groups are
attached to benzene
only numbers are used
to denote position
The ring is numbered to
give the lower numbers
to the side groups
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13.13 Names for Aromatic
Hydrocarbons
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When a common named
benzene has 1 additional
substituent the common name is
still used as the base name
When a common named
benzene has 2 additional
substituent the common name
NOT used as the base name
When a benzene group is a
substituent it in named as a
phenyl group
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Occurs when straight chain is
larger than benzene ring
13.13 Names for Aromatic
Hydrocarbons
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Name the following molecules
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13.14 Aromatic Hydrocarbons:
Physical Properties and Sources
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Physical and Chemical Properties of Aromatic
Hydrocarbons
Physical and chemical properties are similar to
those of other hydrocarbons
  Benzene
a liquid and room temp
  Very flammable
  Benzene was once widely used organic solvent not
used anymore b/c of short- and long-term toxic effects
13.15 Chemical Reactions of
Aromatic Hydrocarbons
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Benzene undergoes substitution reactions
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Alkylation- An alkyl group from an alkyl chloride substitutes for
a hydrogen on the benzene ring (AlCl3 catalyst needed)
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Halogenation-(Chlorination and Bromination only) benzene can
undergo substitution with Cl2 or Br2 in the presence of a catalyst
(FeX3)
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13.16 Fused-Ring Aromatic
Compounds
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Benzene and its derivative are only a small class of
aromatic hydrocarbons
Another class of aromatic hydrocarbons are fusedring aromatic hydrocarbons
A fused-ring aromatic hydrocarbon in an aromatic
hydrocarbon whose structure contains 2 or more
rings fused together
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