Chapter
Chapter 13
13 Conjugated
Conjugated
Unsaturated
Unsaturated Systems
Systems
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Chapter 13
13.1 Introduction
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Chapter 13
Allyl radical
CH2 CH CH2
C
C
C
Allyl cation
CH2 CH CH2
C
C
C
1,3-Butadiene CH2 CH CH CH2
C
C
C
C
Molecules with delocalized π bonds are called
conjugated unsaturated systems.
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Chapter 13
Conjugated unsaturated systems have a p orbital
on a carbon adjacent to a double bond
The p orbital can come from another double or
triple bond
The p orbital may be the empty p orbital of a
carbocation or a p orbital with a single electron
in it (a radical)
Conjugation affords special stability to the
molecule
Conjugated molecules can be detected using
UV spectroscopy
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Chapter 13
13.2 Allylic Substitution and the Allyl
Radical
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Chapter 13
Reaction of propene with bromine varies
depending on reaction conditions
At low temperature the halogen adds across the
double bond
At high temperature or at very low concentration of
halogen an allylic substitution occurs
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Chapter 13
CH2 CH CH3 + X2
hv
or
CH2 CH CH2X + HX
Allylic substitution
Allylic hydrogen atoms
H
H
CH2 CH CH2
C
C
H
H
C
H
Allylic radical
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Chapter 13
13.2A Allylic Chlorination (High
Temperature)
¾
Allylic chlorination can be performed at high
temperature in the gas phase
CH2 CH CH3 + Cl2
400°C
CH2 CH CH2Cl + HCl
Relative stability of free radicals:
Allylic or allyl > 3° > 2° >1°> vinylic or vinyl
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Chapter 13
13.2B Allylic Bromination with N-Bromosuccinimide (Low Concentration of Br2)
O
light or ROOR
CH2 CH CH3 +
N Br
O
N-Bromosuccinimide
(NBS)
CCl4
CH2 CH CH2Br
O
+
N H
O
¾
¾
NBS provides a continuous low concentration of
bromine for the radical reaction
A low bromine concentration favors allylic substitution
over alkene addition
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¾
Chapter 13
The radical reaction is initiated by a small amount of
bromine radical formed by exposure of NBS to light
or peroxides
O
O
N Br + HBr
N H + Br2
O
O
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Chapter 13
13.3 The Stability of the Allyl Radical
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Chapter 13
13.3A Molecular Orbital Description of
the Allyl Radical
H C
H
C
H
sp2 hybridized
H
C
H
Conjugated unsaturated system
antibonding
orbital
nonbonding
orbital
Bonding
orbital
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¾
Chapter 13
The three p orbitals of the allylic system combine to
form three molecular orbitals
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Chapter 13
13.3B Resonance Description of the
Allyl Radical
CH2 CH CH2
A
CH2 CH CH2
B
Resonance structures
1
2
1
2
CH2 CH CH2
C
A and B are equivalent resonance structures.
The allyl radical is even more stable than a tertiary
radical.
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Chapter 13
13.4 The Allyl Cation
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The allyl cation is intermediate in stability between a
tertiary and secondary carbocation
CH2 CH CH2
sp2 hybridized
H C
H
C
H
H
C
antibonding
orbital
nonbonding
orbital
H
Conjugated unsaturated system
Bonding
orbital
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Chapter 13
Stability arises from the delocalization of the positive
charge over C1 and C3
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Chapter 13
Resonance theory predicts that the allyl cation is a
hybrid of equivalent structures D and E
CH2 CH CH2
CH2 CH CH2
D
E
Resonance structures
1
2
1
2
CH2 CH CH2
F
Both molecular orbital theory and resonance theory
suggest that structure F is the best representation for the
allyl cation
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13.5 Summary of Rules for Resonance
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13.5A Rules for Writing Resonance
Structures
1. Resonance structures exist only on paper.
2. In writing resonance structures we are only
allowed to move electrons.
3. All of the structures must be proper Lewis
structures.
4. All resonance structures must have the same
number of unpaired electrons.
5. All atoms that are a part of the delocalized
system must lie in a plane or be nearly planar
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Chapter 13
6. The energy of the actual molecule is lower
than the energy that might be estimated for
any contributing structure.
7. Equivalent resonance structures make equal
contributions to the hybrid, and a system
described by them has a large resonance
stabilization.
8. The more stable a structure is (when taken by
itself), the greater is its contribution to the
hybrid.
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Chapter 13
13.5B Estimating the Relative Stability
of Resonance Structures
a) The more covalent bonds a structure has, the
more stable it is.
b) Structures in which all of the atoms have a
complete valence shell of electrons(i.e., the
noble gas structure) are especially stable and
make large contributions to the hybrid.
c) Charge separation decreases stability.
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Chapter 13
CH2 CH CH CH2
1
CH2 CH CH CH2
2
CH2 CH CH CH2
4
CH2 CH CH CH2
3
CH2 CH CH CH2
5
CH2 CH CH CH2
6
CH2
CH CH CH2
7
Relative stability: 1 > 4, 5, 6, 7 > 2, 3
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