chapter 10 conjugation in alkadienes and allylic systems
DESCRIPTION
Chapter 10 Conjugation in Alkadienes and Allylic Systems. conjugare is a Latin verb meaning "to link or yoke together". C. C. C. +. The Double Bond as a Substituent. allylic carbocation. C. C. C. C. C. C. +. •. The Double Bond as a Substituent. allylic carbocation. - PowerPoint PPT PresentationTRANSCRIPT
Chapter 10Chapter 10Conjugation in Alkadienes andConjugation in Alkadienes and
Allylic SystemsAllylic Systems
conjugare is a Latin verb meaning "to conjugare is a Latin verb meaning "to
link or yoke together"link or yoke together"
allylic carbocationallylic carbocation
The Double Bond as a SubstituentThe Double Bond as a Substituent
CC++CC
CC
allylic carbocationallylic carbocation allylic radicalallylic radical
The Double Bond as a SubstituentThe Double Bond as a Substituent
CC++CC
CC ••CCCC
CC
allylic carbocationallylic carbocation allylic radicalallylic radical
conjugated dieneconjugated diene
The Double Bond as a SubstituentThe Double Bond as a Substituent
CC++CC
CC ••CCCC
CC
CCCCCC
CC
CC
CCCC
HH
HH
HH
HH
HH
10.110.1
The Allyl GroupThe Allyl Group
vinylic carbonsvinylic carbonsallylicallylic
carboncarbon
Vinylic versus AllylicVinylic versus Allylic
CC
CCCC
vinylic hydrogens are attached to vinylic carbonsvinylic hydrogens are attached to vinylic carbons
Vinylic versus AllylicVinylic versus Allylic
CC
CCCC
HH
HH
HH
allylic hydrogens are attached to allylic carbonsallylic hydrogens are attached to allylic carbons
Vinylic versus AllylicVinylic versus Allylic
CC
CCCC
HH
HH
HH
Vinylic versus AllylicVinylic versus Allylic
CC
CCCC
XX
XX
XX
vinylic substituents are attached to vinylic carbonsvinylic substituents are attached to vinylic carbons
Vinylic versus AllylicVinylic versus Allylic
CC
CCCC
XX
XX
XX
allylic substituents are attached to allylic carbonsallylic substituents are attached to allylic carbons
10.210.2
Allylic CarbocationsAllylic Carbocations
CC
CCCC
++
the fact that a tertiary allylic halide undergoesthe fact that a tertiary allylic halide undergoessolvolysis (Ssolvolysis (SNN1) faster than a simple tertiary1) faster than a simple tertiary
alkyl halide alkyl halide
ClCl
CHCH33
CHCH33
CHCH33
relative rates: (ethanolysis, 45°C)relative rates: (ethanolysis, 45°C)
123123 11
Allylic CarbocationsAllylic Carbocations
CCClCl
CHCH33
CHCH33
CCHH22CC CHCH
provides good evidence for the conclusion thatprovides good evidence for the conclusion thatallylic carbocations are more stable thanallylic carbocations are more stable thanother carbocations other carbocations
CHCH33
CHCH33
CHCH33
formed fasterformed faster
Allylic CarbocationsAllylic Carbocations
CC CCHH22CC CHCH ++ ++
CHCH33
CHCH33
provides good evidence for the conclusion thatprovides good evidence for the conclusion thatallylic carbocations are more stable thanallylic carbocations are more stable thanother carbocations other carbocations
CHCH33
HH22C=CH— stabilizes C+ better than CHC=CH— stabilizes C+ better than CH33——
Allylic CarbocationsAllylic Carbocations
CC CC
CHCH33
CHCH33
HH22CC CHCH ++ ++
CHCH33
CHCH33
Delocalization of electrons in the doubleDelocalization of electrons in the doublebond stabilizes the carbocationbond stabilizes the carbocation
resonance modelresonance modelorbital overlap modelorbital overlap model
Stabilization of Allylic CarbocationsStabilization of Allylic Carbocations
Resonance ModelResonance Model
CHCH33
CHCH33
HH22CC CHCH ++CC
CHCH33
CHCH33
HH22CC CHCH++
CC
Resonance ModelResonance Model
CHCH33
CHCH33
HH22CC CHCH ++CC
CHCH33
CHCH33
HH22CC CHCH++
CC
CHCH33
CHCH33
HH22CC CHCH ++CC++
Orbital Overlap ModelOrbital Overlap Model
++++
Orbital Overlap ModelOrbital Overlap Model
Orbital Overlap ModelOrbital Overlap Model
Orbital Overlap ModelOrbital Overlap Model
HH22OO NaNa22COCO33
(85%)(85%)(15%)(15%)
Hydrolysis of an Allylic HalideHydrolysis of an Allylic Halide
ClCl
CHCH33
CHCH33
CCHH22CC CHCH
OHOH
CHCH33
CHCH33
CCHH22CC CHCH ++
CHCH33
HOCHHOCH22 CHCH CC
CHCH33
HH22OO NaNa22COCO33
(85%)(85%)(15%)(15%)
Corollary ExperimentCorollary Experiment
OHOH
CHCH33
CHCH33
CCHH22CC CHCH ++
CHCH33
HOCHHOCH22 CHCH CC
CHCH33
CHCH33
ClCHClCH22 CHCH CC
CHCH33
CHCH33
ClClCHCH22 CHCH CC
CHCH33
ClCl
CHCH33
CHCH33
CCHH22CC CHCH andand
give the same products because they give the same products because they form the same carbocationform the same carbocation
CHCH33
ClClCHCH22 CHCH CC
CHCH33
ClCl
CHCH33
CHCH33
CCHH22CC CHCH andand
give the same products because they give the same products because they form the same carbocationform the same carbocation
CHCH33
CHCH33
HH22CC CHCH ++CC
CHCH33
CHCH33
HH22CC CHCH++
CC
more positive charge on tertiary carbon;more positive charge on tertiary carbon;therefore more tertiary alcohol in producttherefore more tertiary alcohol in product
CHCH33
CHCH33
HH22CC CHCH ++CC
CHCH33
CHCH33
HH22CC CHCH++
CC
CHCH33
HOHOCHCH22 CHCH CC
CHCH33
OHOH
CHCH33
CHCH33
CCHH22CC CHCH
more positive charge on tertiary carbon;more positive charge on tertiary carbon;therefore more tertiary alcohol in producttherefore more tertiary alcohol in product
CHCH33
CHCH33
HH22CC CHCH ++CC
CHCH33
CHCH33
HH22CC CHCH++
CC
++
(85%)(85%) (15%)(15%)
10.310.3
Allylic Free RadicalsAllylic Free Radicals
CC
CCCC ••
Allylic free radicals are stabilized byAllylic free radicals are stabilized byelectron delocalizationelectron delocalization
CC
CCCC •• CC
CCCC••
Free-radical stabilities are related toFree-radical stabilities are related tobond-dissociation energiesbond-dissociation energies
CHCH33CHCH22CHCH22—H—H410 kJ/mol410 kJ/mol
CHCH33CHCH22CHCH22 ++ H•H•••
368 kJ/mol368 kJ/mol++ H•H•
••CHCHCHCH22—H—HHH22CC CHCHCHCH22HH22CC
C—H bond is weaker in propene because C—H bond is weaker in propene because resulting radical (allyl) is more stable than resulting radical (allyl) is more stable than radical (propyl) from propaneradical (propyl) from propane
10.410.4
Allylic HalogenationAllylic Halogenation
ClCHClCH22CHCHCHCH33
ClCl
additionaddition
500 °C500 °C
substitutionsubstitution
CHCHCHCH33HH22CC ++ ClCl22
CHCHCHCH22ClClHH22CC
+ HCl+ HCl
Chlorination of PropeneChlorination of Propene
selective for replacement of allylic hydrogenselective for replacement of allylic hydrogen
free radical mechanismfree radical mechanism
allylic radical is intermediateallylic radical is intermediate
Allylic HalogenationAllylic Halogenation
410 kJ/mol410 kJ/mol
Hydrogen-atom abstraction stepHydrogen-atom abstraction step
CC
CCCC
HH
HH
HH
HH
HH
HH368 kJ/mol368 kJ/mol
ClCl::..........
allylic C—H bond weaker than vinylicallylic C—H bond weaker than vinylic
chlorine atom abstracts allylic H in chlorine atom abstracts allylic H in propagation steppropagation step
410 kJ/mol410 kJ/mol
Hydrogen-atom abstraction stepHydrogen-atom abstraction step
CC
CCCC
HH
HH
HH
HH
HH368 kJ/mol368 kJ/mol
HH ClCl:::: ........
••
BrBr
++heatheat
CClCCl44
(82-87%)(82-87%)
++
N-BromosuccinimideN-Bromosuccinimide
OO
OO
NNBrBr
OO
OO
NNHH
all of the allylic hydrogens are equivalent all of the allylic hydrogens are equivalent
andandthe resonance forms of allylic radicalthe resonance forms of allylic radical
are equivalent are equivalent
Limited ScopeLimited Scope
Allylic halogenation is only used when:Allylic halogenation is only used when:
ExampleExample
Cyclohexene Cyclohexene satisfies both satisfies both requirementsrequirements
All allylicAll allylichydrogens arehydrogens areequivalentequivalent
HH HH
HHHH
ExampleExample
Cyclohexene Cyclohexene satisfies both satisfies both requirementsrequirements
All allylicAll allylichydrogens arehydrogens areequivalentequivalent
Both resonance forms are equivalentBoth resonance forms are equivalent
HH
HH
HH
••
HH
HH
HH••
HH HH
HHHH
ExampleExample
2-Butene2-ButeneAll allylicAll allylichydrogens arehydrogens areequivalentequivalent
Two resonance forms are not equivalent;Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides.gives mixture of isomeric allylic bromides.
CHCH33CHCH CHCHCHCH33
ButBut
••CHCH33CHCH CHCH CHCH22
••CHCH33CHCH CHCH CHCH22