Chapter 10 Conjugation in Alkadienes and Allylic Systems - PowerPoint PPT Presentation

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Chapter 10 Conjugation in Alkadienes and Allylic Systems

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Title: 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
10.1The 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
10.2Allylic 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
d
d
19
Orbital Overlap Model
20
Orbital Overlap Model
21
Orbital Overlap Model
22
Hydrolysis of an Allylic Halide
H2O
Na2CO3
CH3
C
CH

HOCH2
CH3
(15)
(85)
23
Corollary Experiment
H2O
Na2CO3

(15)
(85)
24
and
give the same products because they form the
same carbocation
25
and
give the same products because they form the
same carbocation
CH3
CH3


C
C
H2C
CH
H2C
CH
CH3
CH3
26
more positive charge on tertiary
carbontherefore more tertiary alcohol in product
CH3
CH3


C
C
H2C
CH
H2C
CH
CH3
CH3
27
(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
28
10.3Allylic Free Radicals
29
Allylic free radicals are stabilized byelectron
delocalization
30
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

31
10.4Allylic Halogenation
32
Chlorination of Propene
addition

Cl2
500 C
HCl
substitution
33
Allylic Halogenation
  • selective for replacement of allylic hydrogen
  • free radical mechanism
  • allylic radical is intermediate

34
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

35
Hydrogen-atom abstraction step
H
H

C
C
H
H
C
410 kJ/mol
368 kJ/mol
H
36
N-Bromosuccinimide
  • reagent used (instead of Br2) for allylic
    bromination

Br
heat


CCl4
(82-87)
37
Limited Scope
Allylic halogenation is only used when
  • all of the allylic hydrogens are equivalent
  • andthe resonance forms of allylic radicalare
    equivalent

38
Example
Cyclohexene satisfies both requirements
All allylichydrogens areequivalent
39
Example
Cyclohexene satisfies both requirements
All allylichydrogens areequivalent
Both resonance forms are equivalent
40
Example
All allylichydrogens areequivalent
2-Butene
But
Two resonance forms are not equivalentgives
mixture of isomeric allylic bromides.
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