Chapter 10 Conjugation in Alkadienes and Allylic Systems

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Chapter 10Conjugation in Alkadienes andAllylic
Systems

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

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The Double Bond as a Substituent
allylic carbocation
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The Double Bond as a Substituent
allylic carbocation
allylic radical
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The Double Bond as a Substituent
allylic carbocation
allylic radical
conjugated diene
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10.1The Allyl Group
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Vinylic versus Allylic
C
C
C
allylic carbon
vinylic carbons
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Vinylic versus Allylic
H
C
C
H
C
H
vinylic hydrogens are attached to vinylic carbons
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Vinylic versus Allylic
allylic hydrogens are attached to allylic carbons
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Vinylic versus Allylic
X
C
C
X
C
X
vinylic substituents are attached to vinylic
carbons
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Vinylic versus Allylic
X
X
C
C
X
C
allylic substituents are attached to allylic
carbons
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10.2Allylic Carbocations
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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)
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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
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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
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Stabilization of Allylic Carbocations

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

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Resonance Model
CH3
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Resonance Model
CH3
CH3
d
d
C
H2C
CH
CH3
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Orbital Overlap Model
d
d
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Orbital Overlap Model
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Orbital Overlap Model
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Orbital Overlap Model
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Hydrolysis of an Allylic Halide
H2O
Na2CO3
CH3
C
CH

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

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


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


C
C
H2C
CH
H2C
CH
CH3
CH3
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(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
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10.3Allylic Free Radicals
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Allylic free radicals are stabilized byelectron
delocalization
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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

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10.4Allylic Halogenation
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Chlorination of Propene
addition

Cl2
500 C
HCl
substitution
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Allylic Halogenation

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

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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

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Hydrogen-atom abstraction step
H
H

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

• reagent used (instead of Br2) for allylic
  bromination

Br
heat


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

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

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Example
Cyclohexene satisfies both requirements
All allylichydrogens areequivalent
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Example
Cyclohexene satisfies both requirements
All allylichydrogens areequivalent
Both resonance forms are equivalent
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Example
All allylichydrogens areequivalent
2-Butene
But
Two resonance forms are not equivalentgives
mixture of isomeric allylic bromides.