The Double Bond as a Substituent

1
The Double Bond as a Substituent
2
The Allyl Group
3
Vinylic versus Allylic
C
C
C
4
Vinylic versus Allylic
H
C
C
H
C
H
vinylic hydrogens are attached to vinylic carbons
5
Vinylic versus Allylic
allylic hydrogens are attached to allylic carbons
6
Vinylic versus Allylic
X
C
C
X
C
X
vinylic substituents are attached to vinylic
carbons
7
Vinylic versus Allylic
X
X
C
C
X
C
allylic substituents are attached to allylic
carbons
8
Allylic Carbocations
9
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)
10
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
11
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 does CH3
12
Stabilization of Allylic Carbocations

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

13
Resonance Model
14
Orbital Overlap Model
?
?
15
Orbital Overlap Model
16
Orbital Overlap Model
17
Orbital Overlap Model
18
Hydrolysis of an Allylic Halide
H2O
Na2CO3
19
Corollary Experiment
H2O
Na2CO3
20
and
give the same products because they form the
same carbocation
21
more positive charge on tertiary
carbontherefore more tertiary alcohol in product
CH3
CH3


C
C
H2C
CH
H2C
CH
CH3
CH3
22
Allylic Free Radicals
23
Allylic free radicals are stabilized byelectron
delocalization
24
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

25
Allylic Halogenation
26
Chlorination of Propene
addition

Cl2
500 C or hv
HCl
substitution
27
Allylic Halogenation

• selective for substitution of allylic hydrogen
• free radical mechanism
• allylic radical is intermediate
• The first step in the reaction is initiation
  which forms chlorine radical, Cl

28
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

29
Hydrogen-atom abstraction step
H
H

C
C
H
H
C
H
30
Propagation step
H
H

C
C
H
H
C
H
31
Propagation step
H
H
Cl
C
C
H
H
C
H
32
N-Bromosuccinimide

• reagent used (instead of Br2) for allylic
  bromination

33
Limited Scope
Allylic halogenation is only used when

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

34
Example
Cyclohexene satisfies both requirements
All allylichydrogens areequivalent
35
Example
All allylichydrogens areequivalent
2-Butene
36
Classes of Dienes
37
Classification of Dienes

• isolated diene
• conjugated diene
• cumulated diene

38
Nomenclature

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

39
Relative Stabilities of Dienes
40
Heats of Hydrogenation

• 1,3-pentadiene is 26 kJ/mol more stable than
  1,4-pentadiene

252 kJ/mol
226 kJ/mol
41
Bondingin Conjugated Dienes
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Isolated diene
1,4-pentadiene
1,3-pentadiene
Conjugated diene
43
Isolated diene
? bonds are independent of each other
1,3-pentadiene
Conjugated diene
44
Isolated diene
? bonds are independent of each other
p orbitals overlap to give extended ? bond
encompassing four carbons
Conjugated diene
45
Isolated diene
less electron delocalization less stable
more electron delocalization more stable
Conjugated diene
46
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)

47
Conformations of Dienes
s-trans
s-cis

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

48
s-trans is more stable than s-cis

• Interconversion of conformations requires two ?
  bonds to be at right angles to each other and
  prevents conjugation

12 kJ/mol
49
(No Transcript)
50
16 kJ/mol
12 kJ/mol
51
Bonding in Allenes
52
Cumulated Dienes

• cumulated dienes are less stable thanisolated
  and conjugated dienes

53
Structure of Allene
118.4
131 pm

• linear arrangement of carbons
• nonplanar geometry

54
Bonding in Allene
sp
sp 2
sp 2
55
Bonding in Allene
56
Chiral Allenes

• Allenes of the type shown are chiral

A
X
Y
B
A ? B X ? Y
Have a chirality axis
57
Chirality 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