Transcript conjugated dienes

10.8
Classes of Dienes
Dr. Wolf's CHM 201 & 202
10-1
Classification of Dienes
isolated diene
conjugated diene
C
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cumulated diene
10-2
Nomenclature
(2E,5E)-2,5-heptadiene
(2E,4E)-2,4-heptadiene
C
Dr. Wolf's CHM 201 & 202
3,4-heptadiene
10-3
10.9
Relative Stabilities
of Dienes
Dr. Wolf's CHM 201 & 202
10-4
Heats of Hydrogenation
252 kJ/mol
Dr. Wolf's CHM 201 & 202
1,3-pentadiene is
26 kJ/mol more
stable than
1,4-pentadiene,
but some of this
stabilization is
because it also
contains a more
highly substituted
double bond
226 kJ/mol
10-5
Heats of Hydrogenation
126 kJ/mol
252 kJ/mol
Dr. Wolf's CHM 201 & 202
115 kJ/mol
226 kJ/mol
10-6
Heats of Hydrogenation
126 kJ/mol
126 kJ/mol
252 kJ/mol
Dr. Wolf's CHM 201 & 202
111 kJ/mol
115 kJ/mol
226 kJ/mol
10-7
Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
when terminal double bond is conjugated with
other double bond, its heat of hydrogenation
is 15 kJ/mol less than when isolated
Dr. Wolf's CHM 201 & 202
10-8
Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
this extra 15 kJ/mol is known by several terms
stabilization energy
delocalization energy
resonance energy
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10-9
Heats of Hydrogenation
Cumulated double bonds have relatively
high heats of hydrogenation
H2C
C
CH2 + 2H2
CH3CH2CH3
DH° = -295 kJ
H2C
CH2CH3 + H2
CH3CH2CH3
DH° = -125 kJ
Dr. Wolf's CHM 201 & 202
10-10
10.10
Bonding
in Conjugated Dienes
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10-11
Isolated diene
1,4-pentadiene
1,3-pentadiene
Conjugated diene
Dr. Wolf's CHM 201 & 202
10-12
Isolated diene
p bonds are
independent of
each other
1,3-pentadiene
Conjugated diene
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10-13
Isolated diene
p bonds are
independent of
each other
p orbitals overlap
to give extended p
bond
encompassing
four carbons
Conjugated diene
Dr. Wolf's CHM 201 & 202
10-14
Isolated diene
less electron
delocalization;
less stable
more electron
delocalization;
more stable
Conjugated diene
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10-15
Conformations of Dienes
H
H
H
H
H
H
H
s-trans
H
H
H
HH
s-cis
s prefix designates conformation around single bond
s prefix is lower case (different from Cahn-IngoldPrelog S which designates configuration and is upper
case)
Dr. Wolf's CHM 201 & 202
10-16
Conformations of Dienes
H
H
H
H
H
H
H
s-trans
H
H
H
HH
s-cis
s prefix designates conformation around single bond
s prefix is lower case (different from Cahn-IngoldPrelog S which designates configuration and is upper
case)
Dr. Wolf's CHM 201 & 202
10-17
Conformations of Dienes
s-trans
s-cis
Both conformations allow electron delocalization via
overlap of p orbitals to give extended p system
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10-18
s-trans is more stable than s-cis
Interconversion of conformations requires two
p bonds to be at right angles to each other
and prevents conjugation
12 kJ/mol
Dr. Wolf's CHM 201 & 202
10-19
Dr. Wolf's CHM 201 & 202
10-20
16 kJ/mol
12 kJ/mol
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10-21
10.11
Bonding in Allenes
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10-22
Cumulated Dienes
C
C
C
cumulated dienes are less stable than
isolated and conjugated dienes
(see Problem 10.7 on p 375)
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10-23
Structure of Allene
118.4°
131 pm
linear arrangement of carbons
nonplanar geometry
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10-24
Structure of Allene
118.4°
131 pm
linear arrangement of carbons
nonplanar geometry
Dr. Wolf's CHM 201 & 202
10-25
Bonding in Allene
sp 2
Dr. Wolf's CHM 201 & 202
sp
sp 2
10-26
Bonding in Allene
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10-27
Bonding in Allene
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10-28
Bonding in Allene
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10-29
Chiral Allenes
Allenes of the type shown are chiral
X
A
C
C
C
Y
B
A  B; X  Y
Have a stereogenic axis
Dr. Wolf's CHM 201 & 202
10-30
Stereogenic 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
Dr. Wolf's CHM 201 & 202
10-31
10.12
Preparation of Dienes
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10-32
1,3-Butadiene
590-675°C
CH3CH2CH2CH3
H2C
chromiaalumina
CHCH
CH2
+ 2H2
More than 4 billion pounds of 1,3-butadiene
prepared by this method in U.S. each year
used to prepare synthetic rubber (See "Diene
Polymers" box)
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10-33
Dehydration of Alcohols
KHSO4
OH
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heat
10-34
Dehydration of Alcohols
KHSO4
OH
heat
major product;
88% yield
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10-35
Dehydrohalogenation of Alkyl Halides
KOH
Br
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heat
10-36
Dehydrohalogenation of Alkyl Halides
KOH
Br
heat
major product;
78% yield
Dr. Wolf's CHM 201 & 202
10-37
Reactions of Dienes
isolated dienes: double bonds react independently
of one another
cumulated dienes: specialized topic
conjugated dienes: reactivity pattern requires
us to think of conjugated diene system as a
functional group of its own
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10-38
10.13
Addition of Hydrogen Halides
to
Conjugated Dienes
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10-39
Electrophilic Addition to Conjugated Dienes
H
X
+
H
Proton adds to end of diene system
Carbocation formed is allylic
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10-40
H
Example:
H
H
H
H
H
HCl
H
H
Cl
H
H
H
H
H
Dr. Wolf's CHM 201 & 202
?
H
?
H
H
Cl
H
H
H
H
10-41
H
Example:
H
H
H
H
H
HCl
H
H
Cl
H
H
H
H
H
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10-42
via:
H
H
H
H
+
H
H
H
H
X
H
H
H
H
H
H
H
H
+
H
H
H
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H
H
10-43
and:
H
H
H
Cl
+
H
H
H
H
H
H
H
H
H
H
Cl–
H
3-Chlorocyclopentene
H
H
H
+
H
H
H
H
H
Dr. Wolf's CHM 201 & 202
H
Cl
H
H
H
H
H
10-44
1,2-Addition versus 1,4-Addition
1,2-addition of XY
Y
X
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10-45
1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
Y
Y
X
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X
10-46
1,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
Y
Y
X
via
X
+
X
Dr. Wolf's CHM 201 & 202
10-47
HBr Addition to 1,3-Butadiene
H2C
CH2
CHCH
HBr
CH3CHCH
CH2
+
CH3CH
CHCH2Br
Br
electrophilic addition
1,2 and 1,4-addition both observed
product ratio depends on temperature
Dr. Wolf's CHM 201 & 202
10-48
Rationale
3-Bromo-1-butene is formed faster than
1-bromo-2-butene because allylic carbocations
react with nucleophiles preferentially at the carbon
that bears the greater share of positive charge.
CH3CHCH
CH2
+
CH3CH
CHCH2Br
CH3CH
+
CHCH2
Br
via:
+
CH3CHCH
Dr. Wolf's CHM 201 & 202
CH2
10-49
Rationale
3-Bromo-1-butene is formed faster than
1-bromo-2-butene because allylic carbocations
react with nucleophiles preferentially at the carbon
that bears the greater share of positive charge.
CH3CHCH
CH2
+
CH3CH
CHCH2Br
Br
formed faster
Dr. Wolf's CHM 201 & 202
10-50
Rationale
1-Bromo-2-butene is more stable than
3-bromo-1-butene because it has a
more highly substituted double bond.
CH3CHCH
CH2
+
CH3CH
CHCH2Br
Br
more stable
Dr. Wolf's CHM 201 & 202
10-51
Rationale
The two products equilibrate at 25°C.
Once equilibrium is established, the more
stable isomer predominates.
CH3CHCH
CH2
Br
major product at -80°C
(formed faster)
Dr. Wolf's CHM 201 & 202
CH3CH
CHCH2Br
major product at 25°C
(more stable)
10-52
Kinetic Control
versus
Thermodynamic Control
• Kinetic control: major product is the one
formed at the fastest rate
• Thermodynamic control: major product is the
one that is the most stable
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10-53
+
CH3CHCH
CH3CH
CH2
+
CHCH2
HBr
H2C
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CHCH
CH2
10-54
+
CH3CHCH
CH3CH
higher
activation
energy
CH2
+
CHCH2
CH3CHCH
formed
more
slowly
CH2
Br
CH3CH
Dr. Wolf's CHM 201 & 202
CHCH2Br
10-55
Addition of hydrogen chloride to
2-methyl-1,3-butadiene is a kinetically controlled
reaction and gives one product in much greater
amounts than any isomers. What is this product?
+
Dr. Wolf's CHM 201 & 202
HCl
?
10-56
Think mechanistically.
+
Protonation occurs:
at end of diene system
in direction that gives most stable carbocation
HCl
Kinetically controlled product corresponds to attack by
chloride ion at carbon that has the greatest share of
positive charge in the carbocation
Dr. Wolf's CHM 201 & 202
10-57
Think mechanistically
H
+
Cl
+
one resonance form is
tertiary carbocation;
other is primary
Dr. Wolf's CHM 201 & 202
10-58
Think mechanistically
H
+
Cl
Cl
H
+
+
+
one resonance form is
tertiary carbocation;
one resonance form is
secondary carbocation;
other is primary
other is primary
Dr. Wolf's CHM 201 & 202
10-59
Think mechanistically
H
Cl
More stable carbocation
+
+
one resonance form is
tertiary carbocation;
other is primary
Dr. Wolf's CHM 201 & 202
Is attacked by chloride ion
at carbon that bears
greater share of positive
charge
10-60
Think mechanistically
H
+
Cl
+
one resonance form is
tertiary carbocation;
Cl–
Cl
major
product
other is primary
Dr. Wolf's CHM 201 & 202
10-61