1. No category

Chapter 10
Conjugation in Alkadienes and
Allylic Systems
conjugare is a Latin verb meaning "to
link or yoke together"
Dr. Wolf's CHM 201 & 202
10-1
The Double Bond as a Substituent
C
C
C+
allylic carbocation
Dr. Wolf's CHM 201 & 202
10-2
The Double Bond as a Substituent
C
C
C+
allylic carbocation
Dr. Wolf's CHM 201 & 202
C
C
C•
allylic radical
10-3
The Double Bond as a Substituent
C
C
C
C+
C
allylic carbocation
allylic radical
C
C
C
C•
C
conjugated diene
Dr. Wolf's CHM 201 & 202
10-4
The Allyl Group
H
H
H
C
C
C
H
H
Dr. Wolf's CHM 201 & 202
10-5
Vinylic versus Allylic
C
C
C
vinylic carbons
Dr. Wolf's CHM 201 & 202
allylic
carbon
10-6
Vinylic versus Allylic
H
C
C
H
C
H
vinylic hydrogens are attached to vinylic carbons
Dr. Wolf's CHM 201 & 202
10-7
Vinylic versus Allylic
H
H
C
C
C
H
allylic hydrogens are attached to allylic carbons
Dr. Wolf's CHM 201 & 202
10-8
Vinylic versus Allylic
X
C
C
X
C
X
vinylic substituents are attached to vinylic carbons
Dr. Wolf's CHM 201 & 202
10-9
Vinylic versus Allylic
X
X
C
C
C
X
allylic substituents are attached to allylic carbons
Dr. Wolf's CHM 201 & 202
10-10
Allylic Carbocations
+
C
C
C
Dr. Wolf's CHM 201 & 202
10-11
Allylic Carbocations
the fact that a tertiary allylic halide undergoes
solvolysis (SN1) faster than a simple tertiary
alkyl halide
CH3
CH3
H2C
CH
C
CH3
123
Cl
CH3
C
Cl
CH3
1
relative rates: (ethanolysis, 45°C)
Dr. Wolf's CHM 201 & 202
10-12
Allylic Carbocations
provides good evidence for the conclusion that
allylic carbocations are more stable than
other carbocations
CH3
CH3
H2C
CH
C+
CH3
CH3
C+
CH3
formed faster
Dr. Wolf's CHM 201 & 202
10-13
Allylic Carbocations
provides good evidence for the conclusion that
allylic carbocations are more stable than
other carbocations
CH3
CH3
H2C
CH
C+
CH3
CH3
C+
CH3
H2C=CH— stabilizes C+ better than CH3—
Dr. Wolf's CHM 201 & 202
10-14
Stabilization of Allylic Carbocations
Delocalization of electrons in the double
bond stabilizes the carbocation
resonance model
orbital overlap model
Dr. Wolf's CHM 201 & 202
10-15
Resonance Model
CH3
H2C
CH
C+
CH3
Dr. Wolf's CHM 201 & 202
CH3
+
H2C
CH
C
CH3
10-16
Resonance Model
CH3
H2C
CH
CH3
+
H2C
C+
CH3
d+
H2C
CH
C
CH3
CH3
CH
C d+
CH3
Dr. Wolf's CHM 201 & 202
10-17
Orbital Overlap Model
d+
Dr. Wolf's CHM 201 & 202
d+
10-18
Orbital Overlap Model
Dr. Wolf's CHM 201 & 202
10-19
Orbital Overlap Model
Dr. Wolf's CHM 201 & 202
10-20
Orbital Overlap Model
Dr. Wolf's CHM 201 & 202
10-21
SN1 Reactions of
Allylic Halides
Dr. Wolf's CHM 201 & 202
10-22
Hydrolysis of an Allylic Halide
CH3
H2C
CH
C
Cl
CH3
H2O
Na2CO3
CH3
CH3
H2C
CH
(85%)
Dr. Wolf's CHM 201 & 202
C
CH3
OH
+
HOCH2
CH
(15%)
C
CH3
10-23
Corollary Experiment
CH3
ClCH2
CH
C
CH3
H2O
Na2CO3
CH3
CH3
H2C
CH
(85%)
Dr. Wolf's CHM 201 & 202
C
CH3
OH
+
HOCH2
CH
(15%)
C
CH3
10-24
CH3
CH3
H2C
CH
C
Cl
and ClCH2
CH
CH3
C
CH3
give the same products because they
form the same carbocation
Dr. Wolf's CHM 201 & 202
10-25
CH3
CH3
H2C
CH
Cl
C
and ClCH2
CH
C
CH3
CH3
give the same products because they
form the same carbocation
CH3
H2C
CH
C+
CH3
Dr. Wolf's CHM 201 & 202
CH3
+
H2C
CH
C
CH3
10-26
more positive charge on tertiary carbon;
therefore more tertiary alcohol in product
CH3
H2C
CH
C+
CH3
Dr. Wolf's CHM 201 & 202
CH3
+
H2C
CH
C
CH3
10-27
(85%)
H2C
(15%)
CH3
CH
OH +
C
HOCH2
CH
CH3
C
CH3
CH3
more positive charge on tertiary carbon;
therefore more tertiary alcohol in product
CH3
H2C
CH
C+
CH3
Dr. Wolf's CHM 201 & 202
CH3
+
H2C
CH
C
CH3
10-28
SN2 Reactions
of Allylic Halides
Dr. Wolf's CHM 201 & 202
10-29
Allylic SN2 Reactions
•Allylic halides also undergo SN2 reactions
•faster than simple primary alkyl halides.
H2C
CH2 Cl
CH
H3C
CH2 CH2 Cl
80
1
relative rates: (I-, acetone)
Dr. Wolf's CHM 201 & 202
10-30
Allylic SN2 Reactions
•Two factors:
•Steric
• Trigonal carbon smaller than tetrahedral carbon.
H2C
CH2 Cl
CH
H3C
CH2 CH2 Cl
80
1
relative rates: (I-, acetone)
Dr. Wolf's CHM 201 & 202
10-31
Allylic SN2 reactions
•Two factors:
•Electronic
• Electron delocalization lowers LUMO energy
• which means lower activation energy.
H2C
CH2 Cl
CH
H3C
CH2 CH2 Cl
80
1
relative rates: (I-, acetone)
Dr. Wolf's CHM 201 & 202
10-32
Allylic Free Radicals
C•
C
C
Dr. Wolf's CHM 201 & 202
10-33
Allylic free radicals are stabilized by
electron delocalization
C•
C
C
Dr. Wolf's CHM 201 & 202
•C
C
C
10-34
Free-radical stabilities are related to
bond-dissociation energies
CH3CH2CH2—H
H2C
CHCH2—H
410 kJ/mol
•
CH3CH2CH2 + H•
368 kJ/mol
•
CHCH2 + H•
H2C
C—H bond is weaker in propene because
resulting radical (allyl) is more stable than
radical (propyl) from propane
Dr. Wolf's CHM 201 & 202
10-35
Allylic Halogenation
Dr. Wolf's CHM 201 & 202
10-36
Chlorination of Propene
addition
ClCH2CHCH3
H2C
CHCH3
Cl
+ Cl2
H2C
500 °C
CHCH2Cl
+ HCl
substitution
Dr. Wolf's CHM 201 & 202
10-37
Allylic Halogenation
selective for replacement of allylic hydrogen
free radical mechanism
allylic radical is intermediate
Dr. Wolf's CHM 201 & 202
10-38
Hydrogen-atom abstraction step
H
H
H
C
C
H
C
410 kJ/mol
H
..
. Cl:
..
368 kJ/mol
H
allylic C—H bond weaker than vinylic
chlorine atom abstracts allylic H in
propagation step
Dr. Wolf's CHM 201 & 202
10-39
Hydrogen-atom abstraction step
H
H
C•
C
H
C
410 kJ/mol
H
..
:
H : Cl
..
368 kJ/mol
H
Dr. Wolf's CHM 201 & 202
10-40
N-Bromosuccinimide
reagent used (instead of Br2) for allylic bromination
Br
O
NBr +
heat
O
+
NH
CCl4
O
Dr. Wolf's CHM 201 & 202
(82-87%)
O
10-41
Limited Scope
Allylic halogenation is only used when:
all of the allylic hydrogens are equivalent
and
the resonance forms of allylic radical
are equivalent
Dr. Wolf's CHM 201 & 202
10-42
Example
H H
Cyclohexene
satisfies both
requirements
All allylic
hydrogens are
equivalent
H
Dr. Wolf's CHM 201 & 202
H
10-43
Example
H H
Cyclohexene
satisfies both
requirements
All allylic
hydrogens are
equivalent
H
H
•
H
H
H
H
H
•
H
Both resonance forms are equivalent
Dr. Wolf's CHM 201 & 202
10-44
Example
2-Butene
CH3CH
CHCH3
All allylic
hydrogens are
equivalent
But
CH3CH
CH
•
CH2
•
CH3CH
CH
CH2
Two resonance forms are not equivalent;
gives mixture of isomeric allylic bromides.
Dr. Wolf's CHM 201 & 202
10-45
Allylic Anions
Dr. Wolf's CHM 201 & 202
10-46
Allylic anions are stabilized by
electron delocalization
CH3
H2C
CH
C-
CH3
Dr. Wolf's CHM 201 & 202
CH3
-
H2C
CH
C
CH3
10-47
Acidity of Propene
H3C
CH
CH2
H3C
pKa ~ 62
pKa ~ 43
H2C
CH
CH2
CH2 CH3
H2C
CH2 CH3
Propene is significantly more acidic than propane.
Dr. Wolf's CHM 201 & 202
10-48
Resonance Model
H2C
CH
H2C
CH2
dH2C
CH
CH
CH2
dCH2
Charge is delocalized to both terminal carbons,
stabilizing the conjugate base.
Dr. Wolf's CHM 201 & 202
10-49
Classes of Dienes
Dr. Wolf's CHM 201 & 202
10-50
Classification of Dienes
isolated diene
conjugated diene
C
Dr. Wolf's CHM 201 & 202
cumulated diene
10-51
Nomenclature
(2E,5E)-2,5-heptadiene
(2E,4E)-2,4-heptadiene
C
Dr. Wolf's CHM 201 & 202
3,4-heptadiene
10-52
Relative Stabilities
of Dienes
Dr. Wolf's CHM 201 & 202
10-53
Heats of Hydrogenation
252 kJ/mol
Dr. Wolf's CHM 201 & 202
1,3-pentadiene is
26 kJ/mol more
stable than
1,4-pentadiene,
but some of this
stabilization is
because it also
contains a more
highly substituted
double bond
226 kJ/mol
10-54
Heats of Hydrogenation
126 kJ/mol
252 kJ/mol
Dr. Wolf's CHM 201 & 202
115 kJ/mol
226 kJ/mol
10-55
Heats of Hydrogenation
126 kJ/mol
126 kJ/mol
252 kJ/mol
Dr. Wolf's CHM 201 & 202
111 kJ/mol
115 kJ/mol
226 kJ/mol
10-56
Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
when terminal double bond is conjugated with
other double bond, its heat of hydrogenation
is 15 kJ/mol less than when isolated
Dr. Wolf's CHM 201 & 202
10-57
Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
this extra 15 kJ/mol is known by several terms
stabilization energy
delocalization energy
resonance energy
Dr. Wolf's CHM 201 & 202
10-58
Heats of Hydrogenation
Cumulated double bonds have relatively
high heats of hydrogenation
H2C
C
CH2 + 2H2
CH3CH2CH3
DH° = -295 kJ
H2C
CH2CH3 + H2
CH3CH2CH3
DH° = -125 kJ
Dr. Wolf's CHM 201 & 202
10-59
Bonding
in Conjugated Dienes
Dr. Wolf's CHM 201 & 202
10-60
Isolated diene
1,4-pentadiene
1,3-pentadiene
Conjugated diene
Dr. Wolf's CHM 201 & 202
10-61
Isolated diene
p bonds are
independent of
each other
1,3-pentadiene
Conjugated diene
Dr. Wolf's CHM 201 & 202
10-62
Isolated diene
p bonds are
independent of
each other
p orbitals overlap
to give extended p
bond
encompassing
four carbons
Conjugated diene
Dr. Wolf's CHM 201 & 202
10-63
Isolated diene
less electron
delocalization;
less stable
more electron
delocalization;
more stable
Conjugated diene
Dr. Wolf's CHM 201 & 202
10-64
Conformations of Dienes
H
H
H
H
H
H
H
s-trans
H
H
H
HH
s-cis
s prefix designates conformation around single bond
s prefix is lower case (different from Cahn-IngoldPrelog S which designates configuration and is upper
case)
Dr. Wolf's CHM 201 & 202
10-65
Conformations of Dienes
H
H
H
H
H
H
H
s-trans
H
H
H
HH
s-cis
s prefix designates conformation around single bond
s prefix is lower case (different from Cahn-IngoldPrelog S which designates configuration and is upper
case)
Dr. Wolf's CHM 201 & 202
10-66
Conformations of Dienes
s-trans
s-cis
Both conformations allow electron delocalization via
overlap of p orbitals to give extended p system
Dr. Wolf's CHM 201 & 202
10-67
s-trans is more stable than s-cis
Interconversion of conformations requires two
p bonds to be at right angles to each other
and prevents conjugation
12 kJ/mol
Dr. Wolf's CHM 201 & 202
10-68
Dr. Wolf's CHM 201 & 202
10-69
16 kJ/mol
12 kJ/mol
Dr. Wolf's CHM 201 & 202
10-70
Bonding in Allenes
Dr. Wolf's CHM 201 & 202
10-71
Cumulated Dienes
C
C
C
cumulated dienes are less stable than
isolated and conjugated dienes
(see Problem 10.7 on p 375)
Dr. Wolf's CHM 201 & 202
10-72
Structure of Allene
118.4°
131 pm
linear arrangement of carbons
nonplanar geometry
Dr. Wolf's CHM 201 & 202
10-73
Structure of Allene
118.4°
131 pm
linear arrangement of carbons
nonplanar geometry
Dr. Wolf's CHM 201 & 202
10-74
Bonding in Allene
sp 2
Dr. Wolf's CHM 201 & 202
sp
sp 2
10-75
Bonding in Allene
Dr. Wolf's CHM 201 & 202
10-76
Bonding in Allene
Dr. Wolf's CHM 201 & 202
10-77
Bonding in Allene
Dr. Wolf's CHM 201 & 202
10-78
Chiral Allenes
Allenes of the type shown are chiral
X
A
C
C
C
Y
B
A  B; X  Y
Have a stereogenic axis
Dr. Wolf's CHM 201 & 202
10-79
Stereogenic Axis
analogous to difference between:
a screw with a right-hand thread and one
with a left-hand thread
a right-handed helix and a left-handed helix
Dr. Wolf's CHM 201 & 202
10-80
Preparation of Dienes
Dr. Wolf's CHM 201 & 202
10-81
1,3-Butadiene
590-675°C
CH3CH2CH2CH3
H2C
chromiaalumina
CHCH
CH2
+ 2H2
More than 4 billion pounds of 1,3-butadiene
prepared by this method in U.S. each year
used to prepare synthetic rubber (See "Diene
Polymers" box)
Dr. Wolf's CHM 201 & 202
10-82
Dehydration of Alcohols
KHSO4
OH
Dr. Wolf's CHM 201 & 202
heat
10-83
Dehydration of Alcohols
KHSO4
OH
heat
major product;
88% yield
Dr. Wolf's CHM 201 & 202
10-84
Dehydrohalogenation of Alkyl Halides
KOH
Br
Dr. Wolf's CHM 201 & 202
heat
10-85
Dehydrohalogenation of Alkyl Halides
KOH
Br
heat
major product;
78% yield
Dr. Wolf's CHM 201 & 202
10-86
Reactions of Dienes
isolated dienes: double bonds react independently
of one another
cumulated dienes: specialized topic
conjugated dienes: reactivity pattern requires
us to think of conjugated diene system as a
functional group of its own
Dr. Wolf's CHM 201 & 202
10-87
Addition of Hydrogen Halides
to
Conjugated Dienes
Dr. Wolf's CHM 201 & 202
10-88
Electrophilic Addition to Conjugated Dienes
H
X
+
H
Proton adds to end of diene system
Carbocation formed is allylic
Dr. Wolf's CHM 201 & 202
10-89
H
Example:
H
H
H
H
H
HCl
H
H
Cl
H
H
H
H
H
Dr. Wolf's CHM 201 & 202
?
H
?
H
H
Cl
H
H
H
H
10-90
H
Example:
H
H
H
H
H
HCl
H
H
Cl
H
H
H
H
H
Dr. Wolf's CHM 201 & 202
10-91
via:
H
H
H
H
+
H
H
H
H
X
H
H
H
H
H
H
H
H
+
H
H
H
Dr. Wolf's CHM 201 & 202
H
H
10-92
and:
H
H
H
Cl
+
H
H
H
H
H
H
H
H
H
H
Cl–
H
3-Chlorocyclopentene
H
H
H
+
H
H
H
H
H
Dr. Wolf's CHM 201 & 202
H
Cl
H
H
H
H
H
10-93
1,2-Addition versus 1,4-Addition
1,2-addition of XY
Y
X
Dr. Wolf's CHM 201 & 202
10-94
1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
Y
Y
X
Dr. Wolf's CHM 201 & 202
X
10-95
1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
Y
Y
X
via
X
+
X
Dr. Wolf's CHM 201 & 202
10-96
HBr Addition to 1,3-Butadiene
H2C
CH2
CHCH
HBr
CH3CHCH
CH2
+
CH3CH
CHCH2Br
Br
electrophilic addition
1,2 and 1,4-addition both observed
product ratio depends on temperature
Dr. Wolf's CHM 201 & 202
10-97
Rationale
3-Bromo-1-butene is formed faster than
1-bromo-2-butene because allylic carbocations
react with nucleophiles preferentially at the carbon
that bears the greater share of positive charge.
CH3CHCH
CH2
+
CH3CH
CHCH2Br
CH3CH
+
CHCH2
Br
via:
+
CH3CHCH
Dr. Wolf's CHM 201 & 202
CH2
10-98
Rationale
3-Bromo-1-butene is formed faster than
1-bromo-2-butene because allylic carbocations
react with nucleophiles preferentially at the carbon
that bears the greater share of positive charge.
CH3CHCH
CH2
+
CH3CH
CHCH2Br
Br
formed faster
Dr. Wolf's CHM 201 & 202
10-99
Rationale
1-Bromo-2-butene is more stable than
3-bromo-1-butene because it has a
more highly substituted double bond.
CH3CHCH
CH2
+
CH3CH
CHCH2Br
Br
more stable
Dr. Wolf's CHM 201 & 202
10-100
Rationale
The two products equilibrate at 25°C.
Once equilibrium is established, the more
stable isomer predominates.
CH3CHCH
CH2
Br
major product at -80°C
(formed faster)
Dr. Wolf's CHM 201 & 202
CH3CH
CHCH2Br
major product at 25°C
(more stable)
10-101
Kinetic Control
versus
Thermodynamic Control
• Kinetic control: major product is the one
formed at the fastest rate
• Thermodynamic control: major product is the
one that is the most stable
Dr. Wolf's CHM 201 & 202
10-102
+
CH3CHCH
CH3CH
CH2
+
CHCH2
HBr
H2C
Dr. Wolf's CHM 201 & 202
CHCH
CH2
10-103
+
CH3CHCH
CH3CH
higher
activation
energy
CH2
+
CHCH2
CH3CHCH
formed
more
slowly
CH2
Br
CH3CH
Dr. Wolf's CHM 201 & 202
CHCH2Br
10-104
Addition of hydrogen chloride to
2-methyl-1,3-butadiene is a kinetically controlled
reaction and gives one product in much greater
amounts than any isomers. What is this product?
+
Dr. Wolf's CHM 201 & 202
HCl
?
10-105
Think mechanistically.
+
Protonation occurs:
at end of diene system
in direction that gives most stable carbocation
HCl
Kinetically controlled product corresponds to attack by
chloride ion at carbon that has the greatest share of
positive charge in the carbocation
Dr. Wolf's CHM 201 & 202
10-106
Think mechanistically
H
+
Cl
+
one resonance form is
tertiary carbocation;
other is primary
Dr. Wolf's CHM 201 & 202
10-107
Think mechanistically
H
+
Cl
Cl
H
+
+
+
one resonance form is
tertiary carbocation;
one resonance form is
secondary carbocation;
other is primary
other is primary
Dr. Wolf's CHM 201 & 202
10-108
Think mechanistically
H
Cl
More stable carbocation
+
+
one resonance form is
tertiary carbocation;
other is primary
Dr. Wolf's CHM 201 & 202
Is attacked by chloride ion
at carbon that bears
greater share of positive
charge
10-109
Think mechanistically
H
+
Cl
+
one resonance form is
tertiary carbocation;
Cl–
Cl
major
product
other is primary
Dr. Wolf's CHM 201 & 202
10-110
Halogen Addition to Dienes
gives mixtures of 1,2 and
1,4-addition products
Dr. Wolf's CHM 201 & 202
10-111
Example
H2C
CH2
CHCH
Br2
BrCH2CHCH
CH2
+
BrCH2CH
CHCH2Br
Br
(37%)
Dr. Wolf's CHM 201 & 202
(63%)
10-112
The Diels-Alder Reaction
Synthetic method for preparing
compounds containing a cyclohexene ring
Dr. Wolf's CHM 201 & 202
10-113
In general...
+
conjugated
alkene
diene
(dienophile)
Dr. Wolf's CHM 201 & 202
cyclohexene
10-114
via
transition state
Dr. Wolf's CHM 201 & 202
10-115
Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202
10-116
Mechanistic features
concerted mechanism
cycloaddition
pericyclic reaction
a concerted reaction that proceeds
through a cyclic transition state
Dr. Wolf's CHM 201 & 202
10-117
Recall the general reaction...
+
alkene
conjugated
(dienophile)
diene
cyclohexene
The equation as written is somewhat
misleading because ethylene is a relatively
unreactive dienophile.
Dr. Wolf's CHM 201 & 202
10-118
What makes a reactive dienophile?
The most reactive dienophiles have an
electron-withdrawing group (EWG) directly
attached to the double bond.
EWG
C
Dr. Wolf's CHM 201 & 202
C
Typical EWGs
C
O
C
N
10-119
Example
O
H2C
CHCH
CH2 + H2C
benzene
CH
CH
100°C
O
CH
(100%)
Dr. Wolf's CHM 201 & 202
10-120
Example
O
H2C
CHCH
CH2 + H2C
benzene
via:
CH
CH
100°C
O
O
CH
CH
(100%)
Dr. Wolf's CHM 201 & 202
10-121
Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202
10-122
Example
H2C
O
CH2
CHC
+
CH3
benzene
O
100°C O
O
H3C
O
(100%)
Dr. Wolf's CHM 201 & 202
O
10-123
Example
H2C
O
CH2
CHC
+
CH3
benzene
via:
O
H3C
100°C O
O
H3C
O
O
O
Dr. Wolf's CHM 201 & 202
O
(100%)
O
10-124
Acetylenic Dienophile
O
O
H2C
CHCH
CH2 + CH3CH2OCC
benzene
CCOCH2CH3
100°C
O
COCH2CH3
(98%)
COCH2CH3
Dr. Wolf's CHM 201 & 202
O
10-125
Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202
10-126
Diels-Alder Reaction is Stereospecific*
syn addition to alkene
cis-trans relationship of substituents on alkene
retained in cyclohexene product
*A stereospecific reaction is one in which
stereoisomeric starting materials give
stereoisomeric products; characterized by
terms like syn addition, anti elimination,
inversion of configuration, etc.
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Example
O
C6H5
H2C
CHCH
CH2 +
COH
C
H
C
H
H
C6H5
COH
only product
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H
O
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Example
O
COH
H
H2C
CHCH
CH2 +
C
C6H5
C
H
C6H5
H
COH
only product
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H
O
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Cyclic dienes yield bridged bicyclic
Diels-Alder adducts.
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Diels-Alder Reaction
Dr. Wolf's CHM 201 & 202
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Diels-Alder Reaction
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O
COCH3
H
+
C
CH3OC
O
C
H
H
O
COCH3
H
COCH3
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O
10-133
H
O
COCH3
H
O
COCH3
H
is the
same as
COCH3
H
COCH3
O
O
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The p Molecular Orbitals
of
Ethylene and 1,3-Butadiene
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Orbitals and Chemical Reactions
• A deeper understanding of chemical reactivity
can be gained by focusing on the frontier
orbitals of the reactants.
• Electrons flow from the highest occupied
molecular orbital (HOMO) of one reactant to
the lowest unoccupied molecular orbital
(LUMO) of the other.
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Orbitals and Chemical Reactions
• We can illustrate HOMO-LUMO interactions
by way of the Diels-Alder reaction between
ethylene and 1,3-butadiene.
• We need only consider only the p electrons of
ethylene and 1,3-butadiene. We can ignore
the framework of s bonds in each molecule.
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The p MOs of Ethylene
• red and blue colors
distinguish sign of
wave function
• bonding p MO is
antisymmetric with
respect to plane of
molecule
Bonding p orbital of ethylene;
two electrons in this orbital
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The p MOs of Ethylene
Antibonding p orbital of ethylene;
no electrons in this orbital
LUMO
HOMO
Bonding p orbital of ethylene;
two electrons in this orbital
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The p MOs of 1,3-Butadiene
• Four p orbitals contribute to the p system of
1,3-butadiene; therefore, there are four p
molecular orbitals.
• Two of these orbitals are bonding; two are
antibonding.
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The Two Bonding p MOs of 1,3-Butadiene
HOMO
4 p electrons; 2 in
each orbital
Lowest energy orbital
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The Two Antibonding p MOs of 1,3-Butadiene
Highest energy orbital
LUMO
Both antibonding
orbitals are vacant
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A p Molecular Orbital Analysis
of the
Diels-Alder Reaction
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MO Analysis of Diels-Alder Reaction
• Inasmuch as electron-withdrawing groups
increase the reactivity of a dienophile, we
assume electrons flow from the HOMO of the
diene to the LUMO of the dienophile.
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MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
HOMO of 1,3-butadiene
and LUMO of ethylene
are in phase with one
another
allows s bond formation
between the alkene and
the diene
LUMO of ethylene (dienophile)
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MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
LUMO of ethylene (dienophile)
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A "forbidden" reaction
H2C
H2C
+
CH2
CH2
• The dimerization of ethylene to give
cyclobutane does not occur under conditions
of typical Diels-Alder reactions. Why not?
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A "forbidden" reaction
H2C
H2C
+
CH2
CH2
HOMO-LUMO
mismatch of two
ethylene molecules
precludes single-step
formation of two new
s bonds
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HOMO of
one ethylene
molecule
LUMO of
other ethylene
molecule
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End of Chapter 10
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