Title: Chapter 10 Conjugation in Alkadienes and Allylic Systems
1Chapter 10Conjugation in Alkadienes andAllylic
Systems
- conjugare is a Latin verb meaning "to link or
yoke together"
2The Double Bond as a Substituent
allylic carbocation
3The Double Bond as a Substituent
allylic carbocation
allylic radical
4The Double Bond as a Substituent
allylic carbocation
allylic radical
conjugated diene
5The Allyl Group
6Vinylic versus Allylic
C
C
C
allylic carbon
vinylic carbons
7Vinylic versus Allylic
H
C
C
H
C
H
vinylic hydrogens are attached to vinylic carbons
8Vinylic versus Allylic
allylic hydrogens are attached to allylic carbons
9Vinylic versus Allylic
X
C
C
X
C
X
vinylic substituents are attached to vinylic
carbons
10Vinylic versus Allylic
X
X
C
C
X
C
allylic substituents are attached to allylic
carbons
11Allylic Carbocations
12Allylic 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)
13Allylic 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
14Allylic 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
15Stabilization of Allylic Carbocations
- Delocalization of electrons in the doublebond
stabilizes the carbocation - resonance model orbital overlap model
16Resonance Model
CH3
17Resonance Model
CH3
CH3
d
d
C
H2C
CH
CH3
18Orbital Overlap Model
?
?
19Orbital Overlap Model
20Orbital Overlap Model
21Orbital Overlap Model
22SN1 Reactions of Allylic Halides
23Hydrolysis of an Allylic Halide
H2O
Na2CO3
CH3
C
CH
HOCH2
CH3
(15)
(85)
24Corollary Experiment
H2O
Na2CO3
(15)
(85)
25and
give the same products because they form the
same carbocation
26and
give the same products because they form the
same carbocation
CH3
CH3
C
C
H2C
CH
H2C
CH
CH3
CH3
27more positive charge on tertiary
carbontherefore more tertiary alcohol in product
CH3
CH3
C
C
H2C
CH
H2C
CH
CH3
CH3
28(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
29SN2 Reactions of Allylic Halides
30Allylic SN2 Reactions
- Allylic halides also undergo SN2 reactions
- faster than simple primary alkyl halides.
Cl
H3C
CH2
CH2
1
80
relative rates (I-, acetone)
31Allylic SN2 Reactions
- Two factors
- Steric
- Trigonal carbon smaller than tetrahedral
carbon.
Cl
CH2
H3C
CH2
1
80
relative rates (I-, acetone)
32Allylic SN2 reactions
- Two factors
- Electronic
- Electron delocalization lowers LUMO energy
- which means lower activation energy.
Cl
CH2
H3C
CH2
1
80
relative rates (I-, acetone)
33Allylic Free Radicals
34Allylic free radicals are stabilized byelectron
delocalization
35Free-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
36Allylic Halogenation
37Chlorination of Propene
addition
Cl2
500 C
HCl
substitution
38Allylic Halogenation
- selective for replacement of allylic hydrogen
- free radical mechanism
- allylic radical is intermediate
39Hydrogen-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
40Hydrogen-atom abstraction step
H
H
C
C
H
H
C
410 kJ/mol
368 kJ/mol
H
41N-Bromosuccinimide
- reagent used (instead of Br2) for allylic
bromination
Br
heat
CCl4
(82-87)
42Limited Scope
Allylic halogenation is only used when
- all of the allylic hydrogens are equivalent
- andthe resonance forms of allylic radicalare
equivalent
43Example
Cyclohexene satisfies both requirements
All allylichydrogens areequivalent
44Example
Cyclohexene satisfies both requirements
All allylichydrogens areequivalent
Both resonance forms are equivalent
45Example
All allylichydrogens areequivalent
2-Butene
But
Two resonance forms are not equivalentgives
mixture of isomeric allylic bromides.
46Allylic Anions
47Allylic anions are stabilized byelectron
delocalization
CH3
48Acidity of Propene
CH3
H3C
CH2
pKa 43
pKa 62
-
CH3
H2C
CH2
Propene is significantly more acidic than propane.
49Resonance Model
-
CH2
H2C
CH
Charge is delocalized to both terminal
carbons, stabilizing the conjugate base.
50Classes of Dienes
51Classification of Dienes
- isolated diene
- conjugated diene
- cumulated diene
52Nomenclature
- (2E,5E)-2,5-heptadiene
- (2E,4E)-2,4-heptadiene
- 3,4-heptadiene
53Relative Stabilitiesof Dienes
54Heats of Hydrogenation
- 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
252 kJ/mol
226 kJ/mol
55Heats of Hydrogenation
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
56Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
57Heats 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
58Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
- this extra 15 kJ/mol is known by several
terms stabilization energy delocalization
energy resonance energy
59Heats of Hydrogenation
Cumulated double bonds have relatively high
heats of hydrogenation
C
2H2
CH3CH2CH3
H2C
CH2
DH -295 kJ
CH3CH2CH3
H2
DH -125 kJ
60Bondingin Conjugated Dienes
61Isolated diene
1,4-pentadiene
1,3-pentadiene
Conjugated diene
62Isolated diene
p bonds are independent of each other
1,3-pentadiene
Conjugated diene
63Isolated diene
p bonds are independent of each other
p orbitals overlap to give extended p bond
encompassing four carbons
Conjugated diene
64Isolated diene
less electron delocalization less stable
more electron delocalization more stable
Conjugated diene
65Conformations 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)
66Conformations 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)
67Conformations of Dienes
s-trans
s-cis
- Both conformations allow electron delocalization
via overlap of p orbitals to give extended p
system
68s-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
69(No Transcript)
7016 kJ/mol
12 kJ/mol
71Bonding in Allenes
72Cumulated Dienes
- cumulated dienes are less stable thanisolated
and conjugated dienes - (see Problem 10.7 on p 375)
73Structure of Allene
118.4
131 pm
- linear arrangement of carbons
- nonplanar geometry
74Structure of Allene
118.4
131 pm
- linear arrangement of carbons
- nonplanar geometry
75Bonding in Allene
sp
sp 2
sp 2
76Bonding in Allene
77Bonding in Allene
78Bonding in Allene
79Chiral Allenes
- Allenes of the type shown are chiral
A
X
Y
B
A ¹ B X ¹ Y
Have a stereogenic axis
80Stereogenic 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
81Preparation of Dienes
821,3-Butadiene
590-675C
CH3CH2CH2CH3
chromia- alumina
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)
83Dehydration of Alcohols
KHSO4
heat
84Dehydration of Alcohols
KHSO4
heat
85Dehydrohalogenation of Alkyl Halides
KOH
heat
86Dehydrohalogenation of Alkyl Halides
KOH
heat
87Reactions 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
88Addition of Hydrogen HalidestoConjugated Dienes
89Electrophilic Addition to Conjugated Dienes
H
X
H
- Proton adds to end of diene system
- Carbocation formed is allylic
90Example
HCl
?
?
91Example
HCl
92via
H
X
93and
Cl
3-Chlorocyclopentene
H
H
Cl
H
H
H
H
H
941,2-Addition versus 1,4-Addition
1,2-addition of XY
951,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
961,2-Addition versus 1,4-Addition
1,2-addition of XY
1,4-addition of XY
via
97HBr Addition to 1,3-Butadiene
HBr
- electrophilic addition
- 1,2 and 1,4-addition both observed
- product ratio depends on temperature
98Rationale
- 3-Bromo-1-butene is formed faster
than1-bromo-2-butene because allylic
carbocations react with nucleophiles
preferentially at the carbon that bears the
greater share of positive charge.
via
99Rationale
- 3-Bromo-1-butene is formed faster
than1-bromo-2-butene because allylic
carbocations react with nucleophiles
preferentially at the carbon that bears the
greater share of positive charge.
formed faster
100Rationale
1-Bromo-2-butene is more stable
than3-bromo-1-butene because it has amore
highly substituted double bond.
101Rationale
The two products equilibrate at 25C.Once
equilibrium is established, the morestable
isomer predominates.
major product at -80C
major product at 25C
(formed faster)
(more stable)
102Kinetic ControlversusThermodynamic 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
103HBr
104higher activation energy
CH3CHCH
CH2
CH3CH
CHCH2
formed more slowly
105- 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?
HCl
?
106- Think mechanistically.
- Protonation occurs at end of diene system in
direction that gives most stable carbocation - Kinetically controlled product corresponds to
attack by chloride ion at carbon that has the
greatest share of positive charge in the
carbocation
HCl
107Think mechanistically
- one resonance form is tertiary carbocation
other is primary
108Think mechanistically
Cl
H
- one resonance form is secondary carbocation
other is primary
one resonance form is tertiary carbocation
other is primary
109Think mechanistically
- More stable carbocation
- Is attacked by chloride ion at carbon that bears
greater share of positive charge
one resonance form is tertiary carbocation
other is primary
110Think mechanistically
Cl
Cl
one resonance form is tertiary carbocation
other is primary
majorproduct
111Halogen Addition to Dienes
- gives mixtures of 1,2 and 1,4-addition products
112Example
Br2
(37)
(63)
113The Diels-Alder Reaction
- Synthetic method for preparing compounds
containing a cyclohexene ring
114In general...
conjugated diene
alkene (dienophile)
cyclohexene
115via
transition state
116Diels-Alder Reaction
117Mechanistic features
- concerted mechanism
- cycloaddition
- pericyclic reaction
- a concerted reaction that proceeds through a
cyclic transition state
118Recall the general reaction...
alkene (dienophile)
conjugated diene
cyclohexene
- The equation as written is somewhat misleading
because ethylene is a relatively unreactive
dienophile.
119What makes a reactive dienophile?
- The most reactive dienophiles have an
electron-withdrawing group (EWG) directly
attached to the double bond.
Typical EWGs
120Example
H2C
CH
(100)
121Example
H2C
CH
via
(100)
122Diels-Alder Reaction
123Example
benzene
100C
(100)
124Example
benzene
100C
(100)
125Acetylenic Dienophile
(98)
126Diels-Alder Reaction
127Diels-Alder Reaction is Stereospecific
- syn addition to alkene
- cis-trans relationship of substituents on alkene
retained in cyclohexene product
A stereospecific reaction is one in which
stereoisomeric starting materials give
stereoisomeric products characterized by
terms like syn addition, anti elimination,
inversion of configuration, etc.
128Example
O
C6H5
COH
H
H
only product
129Example
only product
130Cyclic dienes yield bridged bicyclicDiels-Alder
adducts.
131Diels-Alder Reaction
132Diels-Alder Reaction
133 134 135The p Molecular OrbitalsofEthylene and
1,3-Butadiene
136Orbitals and Chemical Reactions
- A deeper understanding of chemical reactivity can
be gained by focusing on the frontier orbitals of
the reactants. - Electrons flow from the highest occupied
molecular orbital (HOMO) of one reactant to the
lowest unoccupied molecular orbital (LUMO) of the
other.
137Orbitals and Chemical Reactions
- We can illustrate HOMO-LUMO interactions by way
of the Diels-Alder reaction between ethylene and
1,3-butadiene. - We need only consider only the p electrons of
ethylene and 1,3-butadiene. We can ignore the
framework of s bonds in each molecule.
138The p MOs of Ethylene
- red and blue colors distinguish sign of wave
function - bonding p MO is antisymmetric with respect to
plane of molecule
Bonding p orbital of ethylenetwo electrons in
this orbital
139The p MOs of Ethylene
Antibonding p orbital of ethyleneno electrons
in this orbital
Bonding p orbital of ethylenetwo electrons in
this orbital
140The p MOs of 1,3-Butadiene
- Four p orbitals contribute to the p system of
1,3-butadiene therefore, there are four p
molecular orbitals. - Two of these orbitals are bonding two are
antibonding.
141The Two Bonding p MOs of 1,3-Butadiene
HOMO
4 p electrons 2 ineach orbital
Lowest energy orbital
142The Two Antibonding p MOs of 1,3-Butadiene
Highest energy orbital
LUMO
Both antibondingorbitals are vacant
143A p Molecular Orbital Analysisof
theDiels-Alder Reaction
144MO Analysis of Diels-Alder Reaction
- Inasmuch as electron-withdrawing groups increase
the reactivity of a dienophile, we assume
electrons flow from the HOMO of the diene to the
LUMO of the dienophile.
145MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
- HOMO of 1,3-butadiene and LUMO of ethylene are
in phase with one another - allows s bond formation between the alkene and
the diene
LUMO of ethylene (dienophile)
146MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
LUMO of ethylene (dienophile)
147A "forbidden" reaction
- The dimerization of ethylene to give cyclobutane
does not occur under conditions of typical
Diels-Alder reactions. Why not?
148A "forbidden" reaction
HOMO of one ethylenemolecule
HOMO-LUMOmismatch of twoethylene
moleculesprecludes single-stepformation of two
news bonds
LUMO of other ethylenemolecule
149End of Chapter 10