Transcript Chapter 4

Alkenes.
Chapter 4
Chapter 4
1
Contents of Chapter 3

General Formulae and Nomenclature of
Alkenes
Cis-Trans Isomerism

Reactivity Considerations

Thermodynamics and Kinetics

Chapter 4
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General Molecular Formula for
Alkenes

General molecular formula for acyclic
alkanes is CnH2n+2
CH3CH2CH2CH2CH3
an alkane
C5H12
CnH2n+2
Chapter 4
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General Molecular Formula for
Alkenes

Each  bond introduced, reduces the H
content by 2
CH3CH2CH2CH=CH2
an alkene
C5H10
CnH2n
Chapter 4
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General Molecular Formula for
Alkenes

Each ring also reduces the H content by 2
a cyclic
alkane
C5H10
CnH2n
Chapter 4
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General Molecular Formula for
Alkenes
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Generalization: The molecular formula for a
hydrocarbon is CnH2n+2 minus 2 hydrogens for
every  bond and/or ring present in the molecule
Each  bond or ring is considered a unit of
unsaturation.
C 5 H8
CnH2n-2
a cyclic alkene with 2 units of unsaturation
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Saturated and Unsaturated
Hydrocarbons

Alkanes or saturated hydrocarbons
contain the maximum number of
carbon-hydrogen bonds
CH3CH2CH2CH2CH3
a saturated hydrocarbon
Chapter 4
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Saturated and Unsaturated
Hydrocarbons

Alkenes contain fewer than the
maximum number of carbon-hydrogen
bonds and are therefore referred to as
unsaturated hydrocarbons
CH3CH2CH2CH=CH2
an unsaturated hydrocarbon
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Nomenclature of Alkenes
IUPAC names of alkenes are
based on the corresponding
alkane with “ane” replaced by
“ene”
Chapter 4
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IUPAC Rules for Alkene
Nomenclature
1. The longest chain containing the
functional group (the double bond) is
numbered such that the double bond is
the lowest possible number
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IUPAC Rules for Alkene
Nomenclature
2. If there are substituents, the chain is still
numbered in a direction that gives the
double bond the lowest number
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IUPAC Rules for Alkene
Nomenclature
3. If chain has more than one substituent, they
are cited in alphabetical (not numerical) order.
Rules for alphabetizing are the same as for
alkanes
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IUPAC Rules for Alkene
Nomenclature
4. If the same number for the double bond
is obtained in both directions, number in
the direction that gives lowest number to
a substituent.
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IUPAC Rules for Alkene
Nomenclature
5. In cyclic compounds, a number is not
needed to denote the position of the
functional group
The double bond is assumed to be
between carbons 1 and 2
Chapter 4
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IUPAC Rules for Alkene
Nomenclature
6. If both directions yield same low number for
a functional group and for one substituent,
number in the direction which yields the
lower number for one of the remaining
substituents
Chapter 4
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IUPAC Rules for Alkene
Nomenclature
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Two groups containing double bonds
that are used as names for substituents
are the vinyl group and the allyl group
Chapter 4
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IUPAC Rules for Alkene
Nomenclature
The sp2 carbons of an alkene are called
vinylic
An sp3 adjacent carbon
is called allylic
Chapter 4
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IUPAC Nomenclature of
Dienes
•
Find the longest chain containing both
double bonds
CH2
1
CH2CH2CH2CH3
CHCHCH CH2
2 3
4
5
3-butyl-1,4-pentadiene
Chapter 4
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IUPAC Nomenclature of
Dienes
•
Use corresponding alkane name but
replace the “ne” ending with “diene”
CH2
CH2CH2CH2CH3
CHCHCH CH2
3-butyl-1,4-pentadiene
“pentane” changed to “pentadiene”
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IUPAC Nomenclature of
Dienes
•
Number in the direction that gives the
lowest number to a double bond
CH2 CHCH2CH2CH
CHCH3
1,5-heptadiene
not 2,6-heptadiene
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IUPAC Nomenclature of
Dienes
•
List substituents in alphabetical order
CH3
CH3C CHCH
CH2CH3
CCH2CH3
5-ethyl-2-methyl-2,4-heptadiene
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IUPAC Nomenclature of
Dienes
•
Place numbers indicating the double
bond positions either in front of the parent
compound or in the middle of the name
immediately before the diene suffix
CH3
CH3C CHCH
CH2CH3
CCH2CH3
5-ethyl-2-methyl-2,4-heptadiene
or 5-ethyl-2-methyl-hepta-2,4-diene
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The E, Z System of
Nomenclature
Br
Cl
C
H
Br
CH3
C
C
CH3
H
C
Cl
Which isomer is cis and which is trans?
A more definitive nomenclature is needed!
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The E, Z System of
Nomenclature
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First prioritize the groups bonded to the two sp2
carbons
If the higher priority group for each carbon is on
the same side of the double bond, it is the Z
isomer (for Zusammen, German for “together”)
If the higher priority group for each carbon is on
the opposite side of the double bond, it is the E
isomer (for Entgegen, German for “opposite”)
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The E, Z
Prioritization Rules
•
•
Relative priorities depend first on the
atomic number of the atom (not the
formula weight of the group) bonded to
the sp2 carbon
In the case of a tie, the atomic numbers
of the atoms bonded to the tied atoms
are considered next (e.g. C, C, & H
beats C, H, & H)
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The E, Z
Prioritization Rules
•
•
If an atom is doubly bonded to another
atom, the system treats it as if it were
bonded to two such atoms
In the case of isotopes, the isotope
with the greater mass number has the
higher priority
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Relative Stabilities of Alkenes
Chapter 4
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Relative Stabilities of Alkenes
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The more alkyl substituents attached to
a double bond the more stable the
double bond.
Trans alkenes more stable than cis
alkenes
Not difficult concepts but should be
learned now in order to understand
Chapter 9 later.
Chapter 4
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Reactivity Considerations
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Electrophiles react with nucleophiles
An alkene has electron density above
and below the  bond making it
electron-rich and therefore a
nucleophile
Therefore alkenes react with
electrophiles
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Reaction Mechanisms

We use curved arrows to indicate the
movement of pairs of electrons as two
molecules, ions or atoms interact
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Reaction Mechanisms

Curved arrows are drawn only from
the electron-rich site to the electron
deficient site
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Thermodynamics

When G° is negative the reaction is
exergonic
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Thermodynamics

When G° is positive the reaction is
endergonic
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Kinetics
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Knowing the G° of a reaction will not
tell us how fast it will occur or if it will
occur at all
We need to know the rate of reaction
The rate of a reaction is related to the
height of the energy barrier for the
reaction, G‡, the free energy of
activation
Chapter 4
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Free Energy of Activation
Chapter 4
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Rate-Determining Step
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Formation of the carbocation intermediate
is the slower of the two steps
It is the rate-determining step
Chapter 4
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Rate-Determining Step
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Carbocation intermediates are consumed
by bromide ions as fast as they are
formed
The rate of the overall reaction is
determined by the slow first step
Chapter 4
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Transition States and
Intermediates
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It is important to distinguish between a transition
state and a reaction intermediate
A transition state
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is a local maximum in the reaction coordinate
diagram
has partially formed and partially broken bonds
has only fleeting existence
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Transition States and
Intermediates
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An intermediate
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is at a local minimum energy in the reaction
coordinate diagram
may be isolated in some cases
Chapter 4
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