Chapter 10 Conjugation in Alkadienes and Allylic Systems

1
Chapter 10Conjugation in Alkadienes andAllylic
Systems

• conjugare is a Latin verb meaning "to link or
  yoke together"

2
The Double Bond as a Substituent
allylic carbocation
3
The Double Bond as a Substituent
allylic carbocation
allylic radical
4
The Double Bond as a Substituent
allylic carbocation
allylic radical
conjugated diene
5
The Allyl Group
6
Vinylic versus Allylic
C
C
C
allylic carbon
vinylic carbons
7
Vinylic versus Allylic
H
C
C
H
C
H
vinylic hydrogens are attached to vinylic carbons
8
Vinylic versus Allylic
allylic hydrogens are attached to allylic carbons
9
Vinylic versus Allylic
X
C
C
X
C
X
vinylic substituents are attached to vinylic
carbons
10
Vinylic versus Allylic
X
X
C
C
X
C
allylic substituents are attached to allylic
carbons
11
Allylic Carbocations
12
Allylic Carbocations

• the fact that a tertiary allylic halide
  undergoessolvolysis (SN1) faster than a simple
  tertiaryalkyl halide

CH3
Cl
CH3
CH3
123
1
relative rates (ethanolysis, 45C)
13
Allylic Carbocations

• provides good evidence for the conclusion
  thatallylic carbocations are more stable
  thanother carbocations

CH3
CH3


C
C
H2C
CH
CH3
CH3
CH3
formed faster
14
Allylic Carbocations

• provides good evidence for the conclusion
  thatallylic carbocations are more stable
  thanother carbocations

CH3

C
C
CH3
CH3
H2CCH stabilizes C better than CH3
15
Stabilization of Allylic Carbocations

• Delocalization of electrons in the doublebond
  stabilizes the carbocation
• resonance model orbital overlap model

16
Resonance Model
CH3
17
Resonance Model
CH3
CH3
d
d
C
H2C
CH
CH3
18
Orbital Overlap Model
?
?
19
Orbital Overlap Model
20
Orbital Overlap Model
21
Orbital Overlap Model
22
SN1 Reactions of Allylic Halides
23
Hydrolysis of an Allylic Halide
H2O
Na2CO3
CH3
C
CH

HOCH2
CH3
(15)
(85)
24
Corollary Experiment
H2O
Na2CO3

(15)
(85)
25
and
give the same products because they form the
same carbocation
26
and
give the same products because they form the
same carbocation
CH3
CH3


C
C
H2C
CH
H2C
CH
CH3
CH3
27
more positive charge on tertiary
carbontherefore more tertiary alcohol in product
CH3
CH3


C
C
H2C
CH
H2C
CH
CH3
CH3
28
(85)
(15)
CH3
CH3

C
CH
OH
HOCH2
CH3
CH3
more positive charge on tertiary
carbontherefore more tertiary alcohol in product
CH3
CH3


C
C
H2C
CH
H2C
CH
CH3
CH3
29
SN2 Reactions of Allylic Halides
30
Allylic SN2 Reactions

• Allylic halides also undergo SN2 reactions
• faster than simple primary alkyl halides.

Cl
H3C
CH2
CH2
1
80
relative rates (I-, acetone)
31
Allylic SN2 Reactions

• Two factors
• Steric
• Trigonal carbon smaller than tetrahedral
  carbon.

Cl
CH2
H3C
CH2
1
80
relative rates (I-, acetone)
32
Allylic SN2 reactions

• Two factors
• Electronic
• Electron delocalization lowers LUMO energy
• which means lower activation energy.

Cl
CH2
H3C
CH2
1
80
relative rates (I-, acetone)
33
Allylic Free Radicals
34
Allylic free radicals are stabilized byelectron
delocalization
35
Free-radical stabilities are related
tobond-dissociation energies
410 kJ/mol


CH3CH2CH2H
CH3CH2CH2
H

368 kJ/mol

H

• CH bond is weaker in propene because resulting
  radical (allyl) is more stable than radical
  (propyl) from propane

36
Allylic Halogenation
37
Chlorination of Propene
addition

Cl2
500 C
HCl
substitution
38
Allylic Halogenation

• selective for replacement of allylic hydrogen
• free radical mechanism
• allylic radical is intermediate

39
Hydrogen-atom abstraction step
H
H
410 kJ/mol
368 kJ/mol
H

• allylic CH bond weaker than vinylic
• chlorine atom abstracts allylic H in propagation
  step

40
Hydrogen-atom abstraction step
H
H

C
C
H
H
C
410 kJ/mol
368 kJ/mol
H
41
N-Bromosuccinimide

• reagent used (instead of Br2) for allylic
  bromination

Br
heat


CCl4
(82-87)
42
Limited Scope
Allylic halogenation is only used when

• all of the allylic hydrogens are equivalent
• andthe resonance forms of allylic radicalare
  equivalent

43
Example
Cyclohexene satisfies both requirements
All allylichydrogens areequivalent
44
Example
Cyclohexene satisfies both requirements
All allylichydrogens areequivalent
Both resonance forms are equivalent
45
Example
All allylichydrogens areequivalent
2-Butene
But
Two resonance forms are not equivalentgives
mixture of isomeric allylic bromides.
46
Allylic Anions
47
Allylic anions are stabilized byelectron
delocalization
CH3
48
Acidity of Propene
CH3
H3C
CH2
pKa 43
pKa 62
-
CH3
H2C
CH2
Propene is significantly more acidic than propane.
49
Resonance Model
-
CH2
H2C
CH
Charge is delocalized to both terminal
carbons, stabilizing the conjugate base.
50
Classes of Dienes
51
Classification of Dienes

• isolated diene
• conjugated diene
• cumulated diene

52
Nomenclature

• (2E,5E)-2,5-heptadiene
• (2E,4E)-2,4-heptadiene
• 3,4-heptadiene

53
Relative Stabilitiesof Dienes
54
Heats of Hydrogenation

• 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

252 kJ/mol
226 kJ/mol
55
Heats of Hydrogenation
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
56
Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
57
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

58
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

59
Heats of Hydrogenation
Cumulated double bonds have relatively high
heats of hydrogenation

C
2H2
CH3CH2CH3
H2C
CH2
DH -295 kJ

CH3CH2CH3
H2
DH -125 kJ
60
Bondingin Conjugated Dienes
61
Isolated diene
1,4-pentadiene
1,3-pentadiene
Conjugated diene
62
Isolated diene
p bonds are independent of each other
1,3-pentadiene
Conjugated diene
63
Isolated diene
p bonds are independent of each other
p orbitals overlap to give extended p bond
encompassing four carbons
Conjugated diene
64
Isolated diene
less electron delocalization less stable
more electron delocalization more stable
Conjugated diene
65
Conformations of Dienes
s-trans
s-cis

• s prefix designates conformation around single
  bond
• s prefix is lower case (different from
  Cahn-Ingold-Prelog S which designates
  configuration and is upper case)

66
Conformations of Dienes
s-trans
s-cis

• s prefix designates conformation around single
  bond
• s prefix is lower case (different from
  Cahn-Ingold-Prelog S which designates
  configuration and is upper case)

67
Conformations of Dienes
s-trans
s-cis

• Both conformations allow electron delocalization
  via overlap of p orbitals to give extended p
  system

68
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
69
(No Transcript)
70
16 kJ/mol
12 kJ/mol
71
Bonding in Allenes
72
Cumulated Dienes

• cumulated dienes are less stable thanisolated
  and conjugated dienes
• (see Problem 10.7 on p 375)

73
Structure of Allene
118.4
131 pm

• linear arrangement of carbons
• nonplanar geometry

74
Structure of Allene
118.4
131 pm

• linear arrangement of carbons
• nonplanar geometry

75
Bonding in Allene
sp
sp 2
sp 2
76
Bonding in Allene
77
Bonding in Allene
78
Bonding in Allene
79
Chiral Allenes

• Allenes of the type shown are chiral

A
X
Y
B
A ¹ B X ¹ Y
Have a stereogenic axis
80
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

81
Preparation of Dienes
82
1,3-Butadiene
590-675C
CH3CH2CH2CH3
chromia- alumina

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)

83
Dehydration of Alcohols
KHSO4
heat
84
Dehydration of Alcohols
KHSO4
heat

• major product 88 yield

85
Dehydrohalogenation of Alkyl Halides
KOH
heat
86
Dehydrohalogenation of Alkyl Halides
KOH
heat

• major product 78 yield

87
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

88
Addition of Hydrogen HalidestoConjugated Dienes
89
Electrophilic Addition to Conjugated Dienes

H
X
H

• Proton adds to end of diene system
• Carbocation formed is allylic

90
Example
HCl
?
?
91
Example
HCl
92
via
H
X
93
and
Cl
3-Chlorocyclopentene
H
H
Cl
H
H
H
H
H
94
1,2-Addition versus 1,4-Addition
1,2-addition of XY
95
1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
96
1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
via
97
HBr Addition to 1,3-Butadiene
HBr

• electrophilic addition
• 1,2 and 1,4-addition both observed
• product ratio depends on temperature

98
Rationale

• 3-Bromo-1-butene is formed faster
  than1-bromo-2-butene because allylic
  carbocations react with nucleophiles
  preferentially at the carbon that bears the
  greater share of positive charge.

via


99
Rationale

• 3-Bromo-1-butene is formed faster
  than1-bromo-2-butene because allylic
  carbocations react with nucleophiles
  preferentially at the carbon that bears the
  greater share of positive charge.

formed faster
100
Rationale
1-Bromo-2-butene is more stable
than3-bromo-1-butene because it has amore
highly substituted double bond.

• more stable

101
Rationale
The two products equilibrate at 25C.Once
equilibrium is established, the morestable
isomer predominates.
major product at -80C
major product at 25C
(formed faster)
(more stable)
102
Kinetic ControlversusThermodynamic 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

103
HBr
104

higher activation energy
CH3CHCH
CH2

CH3CH
CHCH2
formed more slowly
105

• 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?

HCl
?
106

• Think mechanistically.
• Protonation occurs at end of diene system in
  direction that gives most stable carbocation
• Kinetically controlled product corresponds to
  attack by chloride ion at carbon that has the
  greatest share of positive charge in the
  carbocation

HCl
107
Think mechanistically

• one resonance form is tertiary carbocation
  other is primary

108
Think mechanistically
Cl
H

• one resonance form is secondary carbocation
  other is primary

one resonance form is tertiary carbocation
other is primary
109
Think mechanistically

• More stable carbocation
• Is attacked by chloride ion at carbon that bears
  greater share of positive charge

one resonance form is tertiary carbocation
other is primary
110
Think mechanistically
Cl
Cl


one resonance form is tertiary carbocation
other is primary
majorproduct
111
Halogen Addition to Dienes

• gives mixtures of 1,2 and 1,4-addition products

112
Example
Br2

(37)
(63)
113
The Diels-Alder Reaction

• Synthetic method for preparing compounds
  containing a cyclohexene ring

114
In general...

conjugated diene
alkene (dienophile)
cyclohexene
115
via
transition state
116
Diels-Alder Reaction
117
Mechanistic features

• concerted mechanism
• cycloaddition
• pericyclic reaction
• a concerted reaction that proceeds through a
  cyclic transition state

118
Recall the general reaction...

alkene (dienophile)
conjugated diene
cyclohexene

• The equation as written is somewhat misleading
  because ethylene is a relatively unreactive
  dienophile.

119
What makes a reactive dienophile?

• The most reactive dienophiles have an
  electron-withdrawing group (EWG) directly
  attached to the double bond.

Typical EWGs
120
Example

H2C
CH
(100)
121
Example

H2C
CH
via
(100)
122
Diels-Alder Reaction
123
Example

benzene
100C
(100)
124
Example

benzene
100C
(100)
125
Acetylenic Dienophile

(98)
126
Diels-Alder Reaction
127
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.
128
Example
O
C6H5
COH
H
H
only product
129
Example
only product
130
Cyclic dienes yield bridged bicyclicDiels-Alder
adducts.
131
Diels-Alder Reaction

• Dr. Wolf's CHM 201 202

132
Diels-Alder Reaction
133


134

• is thesame as

135
The p Molecular OrbitalsofEthylene and
1,3-Butadiene
136
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.

137
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.

138
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 ethylenetwo electrons in
this orbital
139
The p MOs of Ethylene
Antibonding p orbital of ethyleneno electrons
in this orbital

• LUMOHOMO

Bonding p orbital of ethylenetwo electrons in
this orbital
140
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.

141
The Two Bonding p MOs of 1,3-Butadiene
HOMO
4 p electrons 2 ineach orbital
Lowest energy orbital
142
The Two Antibonding p MOs of 1,3-Butadiene
Highest energy orbital
LUMO
Both antibondingorbitals are vacant
143
A p Molecular Orbital Analysisof
theDiels-Alder Reaction
144
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.

145
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)
146
MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
LUMO of ethylene (dienophile)
147
A "forbidden" reaction

• The dimerization of ethylene to give cyclobutane
  does not occur under conditions of typical
  Diels-Alder reactions. Why not?

148
A "forbidden" reaction
HOMO of one ethylenemolecule
HOMO-LUMOmismatch of twoethylene
moleculesprecludes single-stepformation of two
news bonds
LUMO of other ethylenemolecule
149
End of Chapter 10