Transcript Chapter 10 Conjugation in Alkadienes and Allylic Systems

Chapter 10
Conjugation in Alkadienes and
Allylic Systems
conjugare is a Latin verb meaning "to
link or yoke together"
The Double Bond as a Substituent
C
C
C+
allylic carbocation
The Double Bond as a Substituent
C
C
C+
allylic carbocation
C
C
C•
allylic radical
The Double Bond as a Substituent
C
C
C
C+
C
allylic carbocation
allylic radical
C
C
C
C•
C
conjugated diene
10.1
The Allyl Group
H
H
H
C
C
C
H
H
Vinylic versus Allylic
C
C
C
vinylic carbons
allylic
carbon
Vinylic versus Allylic
H
C
C
H
C
H
vinylic hydrogens are attached to vinylic carbons
Vinylic versus Allylic
H
H
C
C
C
H
allylic hydrogens are attached to allylic carbons
Vinylic versus Allylic
X
C
C
X
C
X
vinylic substituents are attached to vinylic carbons
Vinylic versus Allylic
X
X
C
C
C
X
allylic substituents are attached to allylic carbons
10.2
Allylic Carbocations
C
C
C
+
Allylic Carbocations
the fact that a tertiary allylic halide undergoes
solvolysis (SN1) faster than a simple tertiary
alkyl halide
CH3
CH3
H2C
CH
C
CH3
123
Cl
CH3
C
CH3
1
relative rates: (ethanolysis, 45°C)
Cl
Allylic Carbocations
provides good evidence for the conclusion that
allylic carbocations are more stable than
other carbocations
CH3
CH3
H2C
CH
C+
CH3
formed faster
CH3
C+
CH3
Allylic Carbocations
provides good evidence for the conclusion that
allylic carbocations are more stable than
other carbocations
CH3
CH3
H2C
CH
C+
CH3
CH3
C+
CH3
H2C=CH— stabilizes C+ better than CH3—
Stabilization of Allylic Carbocations
Delocalization of electrons in the double
bond stabilizes the carbocation
resonance model
orbital overlap model
Resonance Model
CH3
H2C
CH
C+
CH3
CH3
+
H2C
CH
C
CH3
Resonance Model
CH3
H2C
CH
CH3
+
H2C
C+
CH3
d+
H2C
CH
C
CH3
CH3
CH
C d+
CH3
Orbital Overlap Model
d+
d+
Orbital Overlap Model
Orbital Overlap Model
Orbital Overlap Model
Hydrolysis of an Allylic Halide
CH3
H2C
CH
C
Cl
CH3
H2O
Na2CO3
CH3
CH3
H2C
CH
(85%)
C
CH3
OH
+
HOCH2
CH
(15%)
C
CH3
Corollary Experiment
CH3
ClCH2
CH
C
CH3
H2O
Na2CO3
CH3
CH3
H2C
CH
(85%)
C
CH3
OH
+
HOCH2
CH
(15%)
C
CH3
CH3
CH3
H2C
CH
C
Cl
and ClCH2
CH
CH3
give the same products because they
form the same carbocation
C
CH3
CH3
CH3
H2C
CH
Cl
C
and ClCH2
CH
C
CH3
CH3
give the same products because they
form the same carbocation
CH3
H2C
CH
C+
CH3
CH3
+
H2C
CH
C
CH3
more positive charge on tertiary carbon;
therefore more tertiary alcohol in product
CH3
H2C
CH
C+
CH3
CH3
+
H2C
CH
C
CH3
(85%)
H2C
(15%)
CH3
CH
OH +
C
HOCH2
CH
CH3
C
CH3
CH3
more positive charge on tertiary carbon;
therefore more tertiary alcohol in product
CH3
H2C
CH
C+
CH3
CH3
+
H2C
CH
C
CH3
10.3
Allylic Free Radicals
C•
C
C
Allylic free radicals are stabilized by
electron delocalization
C•
C
C
•C
C
C
Free-radical stabilities are related to
bond-dissociation energies
CH3CH2CH2—H
H2C
CHCH2—H
410 kJ/mol
•
CH3CH2CH2 + H•
368 kJ/mol
•
CHCH2 + H•
H2C
C—H bond is weaker in propene because
resulting radical (allyl) is more stable than
radical (propyl) from propane
10.4
Allylic Halogenation
Chlorination of Propene
addition
ClCH2CHCH3
H2C
CHCH3
Cl
+ Cl2
H2C
500 °C
CHCH2Cl
+ HCl
substitution
Allylic Halogenation
selective for replacement of allylic hydrogen
free radical mechanism
allylic radical is intermediate
Hydrogen-atom abstraction step
H
H
H
C
C
H
C
410 kJ/mol
H
..
. Cl:
..
368 kJ/mol
H
allylic C—H bond weaker than vinylic
chlorine atom abstracts allylic H in
propagation step
Hydrogen-atom abstraction step
H
H
C•
C
H
C
410 kJ/mol
H
..
:
H : Cl
..
368 kJ/mol
H
N-Bromosuccinimide
reagent used (instead of Br2) for allylic bromination
Br
O
NBr +
heat
O
+
NH
CCl4
O
O
(82-87%)
Limited Scope
Allylic halogenation is only used when:
all of the allylic hydrogens are equivalent
and
the resonance forms of allylic radical
are equivalent
Example
H
H
Cyclohexene
satisfies both
requirements
All allylic
hydrogens are
equivalent
H
H
Example
H
H
Cyclohexene
satisfies both
requirements
All allylic
hydrogens are
equivalent
H
H
•
H
H
H
H
•
H
Both resonance forms are equivalent
H
Example
2-Butene
CH3CH
CHCH3
All allylic
hydrogens are
equivalent
But
CH3CH
CH
•
CH2
•
CH3CH
CH
Two resonance forms are not equivalent;
gives mixture of isomeric allylic bromides.
CH2