chapter 10 conjugation in alkadienes and allylic systems

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