10-1 Dr. Wolf's CHM 201 & 202 Chapter 10 Conjugation in Alkadienes and Allylic Systems conjugare is a Latin verb meaning "to link or yoke together"

10-1Dr. Wolf's CHM 201 & 202

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"

10-2Dr. Wolf's CHM 201 & 202

allylic carbocationallylic carbocation

The Double Bond as a SubstituentThe Double Bond as a SubstituentThe Double Bond as a SubstituentThe Double Bond as a Substituent

CC++CC

CC

10-3Dr. Wolf's CHM 201 & 202

allylic carbocationallylic carbocation allylic radicalallylic radical


CC++CC

CC ••CCCC

CC

10-4Dr. Wolf's CHM 201 & 202

allylic carbocationallylic carbocation allylic radicalallylic radical

conjugated dieneconjugated diene


CC++CC

CC ••CCCC

CC

CCCCCC

CC

10-5Dr. Wolf's CHM 201 & 202

CC

CCCC

HH

HH

HH

HH

HH

The Allyl GroupThe Allyl Group

10-6Dr. Wolf's CHM 201 & 202

vinylic carbonsvinylic carbonsallylicallylic

carboncarbon

Vinylic versus AllylicVinylic versus AllylicVinylic versus AllylicVinylic versus Allylic

CC

CCCC

10-7Dr. Wolf's CHM 201 & 202

vinylic hydrogens are attached to vinylic carbonsvinylic hydrogens are attached to vinylic carbons


CC

CCCC

HH

HH

HH

10-8Dr. Wolf's CHM 201 & 202

allylic hydrogens are attached to allylic carbonsallylic hydrogens are attached to allylic carbons


CC

CCCC

HH

HH

HH

10-9Dr. Wolf's CHM 201 & 202


CC

CCCC

XX

XX

XX

vinylic substituents are attached to vinylic carbonsvinylic substituents are attached to vinylic carbons

10-10Dr. Wolf's CHM 201 & 202


CC

CCCC

XX

XX

XX

allylic substituents are attached to allylic carbonsallylic substituents are attached to allylic carbons

10-11Dr. Wolf's CHM 201 & 202

Allylic CarbocationsAllylic Carbocations

CC

CCCC ++

10-12Dr. Wolf's CHM 201 & 202

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 CarbocationsAllylic CarbocationsAllylic Carbocations

CCClCl

CHCH33

CHCH33

CCHH22CC CHCH

10-13Dr. Wolf's CHM 201 & 202

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


CC CCHH22CC CHCH ++ ++

CHCH33

CHCH33

10-14Dr. Wolf's CHM 201 & 202

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——


CC CC

CHCH33

CHCH33

HH22CC CHCH ++ ++

CHCH33

CHCH33

10-15Dr. Wolf's CHM 201 & 202

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 CarbocationsStabilization of Allylic CarbocationsStabilization of Allylic Carbocations

10-16Dr. Wolf's CHM 201 & 202

Resonance ModelResonance ModelResonance ModelResonance Model

CHCH33

CHCH33

HH22CC CHCH ++CC

CHCH33

CHCH33

HH22CC CHCH++

CC

10-17Dr. Wolf's CHM 201 & 202

Resonance ModelResonance ModelResonance ModelResonance Model

CHCH33

CHCH33

HH22CC CHCH ++CC

CHCH33

CHCH33

HH22CC CHCH++

CC

CHCH33

CHCH33

HH22CC CHCH ++CC++

10-18Dr. Wolf's CHM 201 & 202

Orbital Overlap Model

++++

10-19Dr. Wolf's CHM 201 & 202


10-20Dr. Wolf's CHM 201 & 202


10-21Dr. Wolf's CHM 201 & 202


10-22Dr. Wolf's CHM 201 & 202

SN1 Reactions of Allylic Halides

10-23Dr. Wolf's CHM 201 & 202

HH22OO NaNa22COCO33

(85%)(85%)(15%)(15%)

Hydrolysis of an Allylic HalideHydrolysis of an Allylic HalideHydrolysis of an Allylic HalideHydrolysis of an Allylic Halide

ClCl

CHCH33

CHCH33

CCHH22CC CHCH

OHOH

CHCH33

CHCH33

CCHH22CC CHCH ++

CHCH33

HOCHHOCH22 CHCH CC

CHCH33

10-24Dr. Wolf's CHM 201 & 202

HH22OO NaNa22COCO33

(85%)(85%)(15%)(15%)

Corollary ExperimentCorollary ExperimentCorollary ExperimentCorollary Experiment

OHOH

CHCH33

CHCH33

CCHH22CC CHCH ++

CHCH33

HOCHHOCH22 CHCH CC

CHCH33

CHCH33

ClCHClCH22 CHCH CC

CHCH33

10-25Dr. Wolf's CHM 201 & 202

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


10-26Dr. Wolf's CHM 201 & 202

CHCH33

ClClCHCH22 CHCH CC

CHCH33

ClCl

CHCH33

CHCH33

CCHH22CC CHCH andand



CHCH33

CHCH33

HH22CC CHCH ++CC

CHCH33

CHCH33

HH22CC CHCH++

CC

10-27Dr. Wolf's CHM 201 & 202

more positive charge on tertiary carbon;more positive charge on tertiary carbon;therefore more tertiary alcohol in producttherefore more tertiary alcohol in productmore 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

10-28Dr. Wolf's CHM 201 & 202

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 productmore 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%)

10-29Dr. Wolf's CHM 201 & 202

SN2 Reactions of Allylic Halides

10-30Dr. Wolf's CHM 201 & 202

•Allylic halides also undergo SN2 reactions

•faster than simple primary alkyl halides.

relative rates: (Irelative rates: (I--, acetone), acetone)

8080 11

Allylic SN2 Reactions

ClClCHCH22HH22CC CHCH ClClCHCH22

HH33CC CHCH22

10-31Dr. Wolf's CHM 201 & 202

•Two factors:•Steric• Trigonal carbon smaller than tetrahedral carbon.


8080 11

Allylic SN2 Reactions

ClClCHCH22HH22CC CHCH ClClCHCH22

HH33CC CHCH22

10-32Dr. Wolf's CHM 201 & 202

•Two factors:•Electronic• Electron delocalization lowers LUMO energy

• which means lower activation energy.


8080 11

Allylic SN2 reactions

ClClCHCH22HH33CC CHCH22ClClCHCH22

HH22CC CHCH

10-33Dr. Wolf's CHM 201 & 202

Allylic Free RadicalsAllylic Free Radicals

CC

CCCC ••

10-34Dr. Wolf's CHM 201 & 202

Allylic free radicals are stabilized byAllylic free radicals are stabilized byelectron delocalizationelectron delocalization

Allylic free radicals are stabilized byAllylic free radicals are stabilized byelectron delocalizationelectron delocalization

CC

CCCC •• CC

CCCC••

10-35Dr. Wolf's CHM 201 & 202

Free-radical stabilities are related toFree-radical stabilities are related tobond-dissociation energiesbond-dissociation energies

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

10-36Dr. Wolf's CHM 201 & 202

Allylic HalogenationAllylic Halogenation

10-37Dr. Wolf's CHM 201 & 202

ClCHClCH22CHCHCHCH33

ClCl

additionaddition

500 °C500 °C

substitutionsubstitution

CHCHCHCH33HH22CC ++ ClCl22

CHCHCHCH22ClClHH22CC

+ HCl+ HCl

Chlorination of PropeneChlorination of PropeneChlorination of PropeneChlorination of Propene

10-38Dr. Wolf's CHM 201 & 202

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 HalogenationAllylic HalogenationAllylic Halogenation

10-39Dr. Wolf's CHM 201 & 202

410 kJ/mol410 kJ/mol

Hydrogen-atom abstraction stepHydrogen-atom abstraction stepHydrogen-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

10-40Dr. Wolf's CHM 201 & 202

410 kJ/mol410 kJ/mol

Hydrogen-atom abstraction stepHydrogen-atom abstraction stepHydrogen-atom abstraction stepHydrogen-atom abstraction step

CC

CCCC

HH

HH

HH

HH

HH368 kJ/mol368 kJ/mol

HH ClCl:::: ........

••

10-41Dr. Wolf's CHM 201 & 202

BrBr

++heatheat

CClCCl44

(82-87%)(82-87%)

++

OO

OO

NNBrBr

OO

OO

NHNH

N-BromosuccinimideN-BromosuccinimideN-BromosuccinimideN-Bromosuccinimide

10-42Dr. Wolf's CHM 201 & 202

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 ScopeLimited ScopeLimited Scope

Allylic halogenation is only used when:Allylic halogenation is only used when:

10-43Dr. Wolf's CHM 201 & 202

ExampleExampleExampleExample

Cyclohexene Cyclohexene satisfies both satisfies both requirementsrequirements

HH HH

HHHH

All allylicAll allylichydrogens arehydrogens areequivalentequivalent

10-44Dr. Wolf's CHM 201 & 202


Cyclohexene Cyclohexene satisfies both satisfies both requirementsrequirements

HH HH

HHHH

All allylicAll allylichydrogens arehydrogens areequivalentequivalent

HH

HH

HH

••

HH

HH

HH••

Both resonance forms are equivalentBoth resonance forms are equivalent

10-45Dr. Wolf's CHM 201 & 202


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

10-46Dr. Wolf's CHM 201 & 202

Allylic AnionsAllylic Anions

10-47Dr. Wolf's CHM 201 & 202

CHCH33

CHCH33

HH22CC CHCH --CC

CHCH33

CHCH33

HH22CC CHCH--

CC Allylic anions are stabilized byelectron delocalization

10-48Dr. Wolf's CHM 201 & 202

Acidity of Propene

HH22CC CHCH--

CHCH22

Propene is significantly more acidic than propane.Propene is significantly more acidic than propane.

HH33CC CHCH CHCH22HH33CC CHCH22 CHCH33

HH22CC CHCH22

--CHCH33

pKa ~ 43pKa ~ 43 pKa ~ 62pKa ~ 62

10-49Dr. Wolf's CHM 201 & 202

Resonance Model

HH22CC CHCH--CHCH22HH22CC CHCH

--CHCH22

CHCH22HH22CC CHCH----

Charge is delocalized to both terminal carbons,Charge is delocalized to both terminal carbons,

stabilizing the conjugate base.stabilizing the conjugate base.

10-50Dr. Wolf's CHM 201 & 202

Classes of DienesClasses of Dienes

10-51Dr. Wolf's CHM 201 & 202

isolated dieneisolated diene

conjugated dieneconjugated diene

cumulated dienecumulated diene

CC Classification of DienesClassification of DienesClassification of DienesClassification of Dienes

10-52Dr. Wolf's CHM 201 & 202

(2(2EE,5,5EE)-2,5-heptadiene)-2,5-heptadiene

(2(2EE,4,4EE)-2,4-heptadiene)-2,4-heptadiene

3,4-heptadiene3,4-heptadiene

CC NomenclatureNomenclatureNomenclatureNomenclature

10-53Dr. Wolf's CHM 201 & 202

Relative StabilitiesRelative Stabilities

of Dienesof Dienes

10-54Dr. Wolf's CHM 201 & 202

252 kJ/mol252 kJ/mol 226 kJ/mol226 kJ/mol

1,3-pentadiene is 1,3-pentadiene is 26 kJ/mol more 26 kJ/mol more stable than stable than 1,4-pentadiene, 1,4-pentadiene, but some of this but some of this stabilization is stabilization is because it also because it also contains a more contains a more highly substituted highly substituted double bonddouble bond

Heats of HydrogenationHeats of HydrogenationHeats of HydrogenationHeats of Hydrogenation

10-55Dr. Wolf's CHM 201 & 202




10-56Dr. Wolf's CHM 201 & 202



126 kJ/mol126 kJ/mol111 kJ/mol111 kJ/mol


10-57Dr. Wolf's CHM 201 & 202


Heats of HydrogenationHeats of HydrogenationHeats of HydrogenationHeats of Hydrogenation when terminal double bond is conjugated with when terminal double bond is conjugated with other double bond, its heat of hydrogenation other double bond, its heat of hydrogenation is 15 kJ/mol less than when isolatedis 15 kJ/mol less than when isolated

10-58Dr. Wolf's CHM 201 & 202


Heats of HydrogenationHeats of HydrogenationHeats of HydrogenationHeats of Hydrogenation this extra 15 kJ/mol is known by several termsthis extra 15 kJ/mol is known by several terms

stabilization energystabilization energydelocalization energydelocalization energyresonance energyresonance energy

10-59Dr. Wolf's CHM 201 & 202

Cumulated double bonds have relatively Cumulated double bonds have relatively high heats of hydrogenationhigh heats of hydrogenation

HH° = -295 kJ° = -295 kJ


HH22CC CHCH22CC ++ 2H2H22 CHCH33CHCH22CHCH33

HH° = -125 kJ° = -125 kJ

HH22CC CHCH22CHCH33 ++ HH22 CHCH33CHCH22CHCH33

10-60Dr. Wolf's CHM 201 & 202

Bondingin Conjugated Dienes

10-61Dr. Wolf's CHM 201 & 202

Isolated dieneIsolated diene

Conjugated dieneConjugated diene

1,4-pentadiene1,4-pentadiene


10-62Dr. Wolf's CHM 201 & 202



bonds are bonds are independent of independent of

each othereach other


10-63Dr. Wolf's CHM 201 & 202



bonds are bonds are independent of independent of

each othereach other

pp orbitals overlap orbitals overlap to give extended to give extended

bond bond encompassing encompassing four carbonsfour carbons

10-64Dr. Wolf's CHM 201 & 202



less electron less electron delocalization; delocalization;

less stableless stable

more electron more electron delocalization; delocalization;

more stablemore stable

10-65Dr. Wolf's CHM 201 & 202

s-s-transtrans s-s-ciscis

Conformations of DienesConformations of DienesConformations of DienesConformations of Dienes

ss prefix designates prefix designates conformationconformation around single bond around single bond

ss prefix is lower case (different from Cahn-Ingold- prefix is lower case (different from Cahn-Ingold-Prelog Prelog SS which designates which designates configurationconfiguration and is upper and is upper case)case)

HHHH HH

HH HH

HH HHHH

HH

HH HH

HH

10-66Dr. Wolf's CHM 201 & 202



ss prefix designates prefix designates conformationconformation around single bond around single bond

ss prefix is lower case (different from Cahn-Ingold- prefix is lower case (different from Cahn-Ingold-Prelog Prelog SS which designates which designates configurationconfiguration and is upper and is upper case)case)

HHHH HH

HH HH

HH HHHH

HH

HH HH

HH

10-67Dr. Wolf's CHM 201 & 202



Both conformations allow electron delocalization via Both conformations allow electron delocalization via overlap of overlap of pp orbitals to give extended orbitals to give extended system system

10-68Dr. Wolf's CHM 201 & 202

ss-trans is more stable than -trans is more stable than ss-cis-cisss-trans is more stable than -trans is more stable than ss-cis-cis

12 kJ/mol12 kJ/mol

Interconversion of conformations requires two Interconversion of conformations requires two bonds to be at right angles to each other bonds to be at right angles to each other and prevents conjugationand prevents conjugation

10-69Dr. Wolf's CHM 201 & 202

10-70Dr. Wolf's CHM 201 & 202

16 kJ/mol16 kJ/mol

12 kJ/mol12 kJ/mol

10-71Dr. Wolf's CHM 201 & 202

Bonding in Allenes

10-72Dr. Wolf's CHM 201 & 202

cumulated dienes are less stable thancumulated dienes are less stable thanisolated and conjugated dienesisolated and conjugated dienes

(see Problem 10.7 on p 375)(see Problem 10.7 on p 375)

Cumulated DienesCumulated DienesCumulated DienesCumulated Dienes

CCCC CC

10-73Dr. Wolf's CHM 201 & 202

131 pm131 pm

Structure of AlleneStructure of AlleneStructure of AlleneStructure of Allene

118.4°118.4°

linear arrangement of carbonslinear arrangement of carbons

nonplanar geometrynonplanar geometry

10-74Dr. Wolf's CHM 201 & 202

Structure of AlleneStructure of AlleneStructure of AlleneStructure of Allene

131 pm131 pm

118.4°118.4°

linear arrangement of carbonslinear arrangement of carbons

nonplanar geometrynonplanar geometry

10-75Dr. Wolf's CHM 201 & 202

spsp 2 2spsp

Bonding in AlleneBonding in AlleneBonding in AlleneBonding in Allene

spsp 2 2

10-76Dr. Wolf's CHM 201 & 202


10-77Dr. Wolf's CHM 201 & 202


10-78Dr. Wolf's CHM 201 & 202


10-79Dr. Wolf's CHM 201 & 202

Allenes of the type shown are chiralAllenes of the type shown are chiral

AA

BB

XX

YY

A A BB;; X X YY

Have a stereogenic axisHave a stereogenic axis

Chiral AllenesChiral AllenesChiral AllenesChiral Allenes

CCCC CC

10-80Dr. Wolf's CHM 201 & 202

analogous to difference between: analogous to difference between:

a screw with a right-hand thread and one a screw with a right-hand thread and one with a left-hand threadwith a left-hand thread

a right-handed helix and a left-handed helix a right-handed helix and a left-handed helix

Stereogenic AxisStereogenic AxisStereogenic AxisStereogenic Axis

10-81Dr. Wolf's CHM 201 & 202

Preparation of DienesPreparation of Dienes

10-82Dr. Wolf's CHM 201 & 202

CHCH33CHCH22CHCH22CHCH33

590-675°C590-675°C

chromia-chromia-aluminaalumina

More than 4 billion pounds of 1,3-butadiene More than 4 billion pounds of 1,3-butadiene prepared by this method in U.S. each yearprepared by this method in U.S. each year

used to prepare synthetic rubber (See "Diene used to prepare synthetic rubber (See "Diene Polymers" box)Polymers" box)

1,3-Butadiene1,3-Butadiene1,3-Butadiene1,3-Butadiene

HH22CC CHCHCHCH CHCH22

++ 2H2H22

10-83Dr. Wolf's CHM 201 & 202

KHSOKHSO44

heatheat

Dehydration of AlcoholsDehydration of AlcoholsDehydration of AlcoholsDehydration of Alcohols

OHOH

10-84Dr. Wolf's CHM 201 & 202

KHSOKHSO44

heatheat

Dehydration of AlcoholsDehydration of AlcoholsDehydration of AlcoholsDehydration of Alcohols

OHOH

major product; major product; 88% yield 88% yield

10-85Dr. Wolf's CHM 201 & 202

KOHKOH

heatheat

Dehydrohalogenation of Alkyl HalidesDehydrohalogenation of Alkyl HalidesDehydrohalogenation of Alkyl HalidesDehydrohalogenation of Alkyl Halides

BrBr

10-86Dr. Wolf's CHM 201 & 202

KOHKOH

heatheat BrBr

major product; major product; 78% yield 78% yield

Dehydrohalogenation of Alkyl HalidesDehydrohalogenation of Alkyl HalidesDehydrohalogenation of Alkyl HalidesDehydrohalogenation of Alkyl Halides

10-87Dr. Wolf's CHM 201 & 202

isolated dienesisolated dienes: double bonds react independently : double bonds react independently of one anotherof one another

cumulated dienes: cumulated dienes: specialized topic specialized topic

conjugated dienes:conjugated dienes: reactivity pattern requires reactivity pattern requires us to think of conjugated diene system as a us to think of conjugated diene system as a functional group of its ownfunctional group of its own

Reactions of DienesReactions of DienesReactions of DienesReactions of Dienes

10-88Dr. Wolf's CHM 201 & 202

Addition of Hydrogen HalidesAddition of Hydrogen Halides

toto

Conjugated DienesConjugated Dienes

10-89Dr. Wolf's CHM 201 & 202

ProtonProton adds to end of diene system adds to end of diene system

Carbocation formed is allylicCarbocation formed is allylic

Electrophilic Addition to Conjugated DienesElectrophilic Addition to Conjugated DienesElectrophilic Addition to Conjugated DienesElectrophilic Addition to Conjugated Dienes HH XX

HH

++

10-90Dr. Wolf's CHM 201 & 202

HHClCl

Example:Example:Example:Example: HH

HH

HH

HH

HH

HH

ClCl

HHHH

HH

HH

HH

HHHH

HH

HH

HHHH

HH

ClCl

HHHH

?? ??

10-91Dr. Wolf's CHM 201 & 202

HHClCl

Example:Example:Example:Example: HH

HH

HH

HH

HH

HH

ClCl

HHHH

HH

HH

HH

HHHH

10-92Dr. Wolf's CHM 201 & 202

via:via: HHHH

HH

HH

HH

HHHH

++

HH

HH

HH

HH

HH

HH

HH XX HH

HH

HH

HH

HH

HHHH

++

10-93Dr. Wolf's CHM 201 & 202

and:and: HHHH

HH

HH

HH

HHHH

++ HHHH

HH

HH

HH

HHHH

++

ClCl––

ClCl

HHHH

HH

HH

HH

HHHH

HH

HHHH

HH

HH

HHHH

ClCl

3-Chlorocyclopentene3-Chlorocyclopentene

10-94Dr. Wolf's CHM 201 & 202

1,2-Addition versus 1,4-1,2-Addition versus 1,4-AdditionAddition


1,2-addition of XY1,2-addition of XY

XX

YY

10-95Dr. Wolf's CHM 201 & 202



1,2-addition of XY1,2-addition of XY 1,4-addition of XY1,4-addition of XY

XX

YY XX

YY

10-96Dr. Wolf's CHM 201 & 202

viavia



1,2-addition of XY1,2-addition of XY 1,4-addition of XY1,4-addition of XY

XX

YY XX

YY XX

++

10-97Dr. Wolf's CHM 201 & 202

electrophilic additionelectrophilic addition1,2 and 1,4-addition both observed1,2 and 1,4-addition both observedproduct ratio depends on temperatureproduct ratio depends on temperature

HBr Addition to 1,3-ButadieneHBr Addition to 1,3-ButadieneHBr Addition to 1,3-ButadieneHBr Addition to 1,3-Butadiene


HBrHBr

BrBr

CHCH22CHCH33CHCHCHCH CHCHCHCH22BrBrCHCH33CHCH++

10-98Dr. Wolf's CHM 201 & 202

3-Bromo-1-butene is formed faster than3-Bromo-1-butene is formed faster than1-bromo-2-butene because allylic carbocations 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. that bears the greater share of positive charge.

RationaleRationaleRationaleRationale

BrBr


CHCH22CHCH33CHCHCHCH CHCHCHCH22CHCH33CHCH

via:via:++ ++

10-99Dr. Wolf's CHM 201 & 202

3-Bromo-1-butene is formed faster than3-Bromo-1-butene is formed faster than1-bromo-2-butene because allylic carbocations 1-bromo-2-butene because allylic carbocations react with nucleophiles preferentially at the carbon react with nucleophiles preferentially at the carbon that bears the greater share of positive charge. that bears the greater share of positive charge.


BrBr


formed fasterformed faster

10-100Dr. Wolf's CHM 201 & 202

more stablemore stable


BrBr


1-Bromo-2-butene is more stable than1-Bromo-2-butene is more stable than3-bromo-1-butene because it has a3-bromo-1-butene because it has amore highly substituted double bond.more highly substituted double bond.

10-101Dr. Wolf's CHM 201 & 202

major product at -80°Cmajor product at -80°C


major product at 25°Cmajor product at 25°C

The two products equilibrate at 25°C.The two products equilibrate at 25°C.Once equilibrium is established, the moreOnce equilibrium is established, the morestable isomer predominates.stable isomer predominates.

BrBr

CHCH22CHCH33CHCHCHCH CHCHCHCH22BrBrCHCH33CHCH

(formed faster)(formed faster) (more stable)(more stable)

10-102Dr. Wolf's CHM 201 & 202

Kinetic ControlKinetic Controlversusversus

Thermodynamic ControlThermodynamic Control

Kinetic ControlKinetic Controlversusversus

Thermodynamic ControlThermodynamic Control

• Kinetic control: major product is the one Kinetic control: major product is the one formed at the fastest rateformed at the fastest rate

• Thermodynamic control: major product is the Thermodynamic control: major product is the one that is the most stableone that is the most stable

10-103Dr. Wolf's CHM 201 & 202

H2C CHCH CH2

HBr

CH2CH3CHCH CHCH2CH3CH+ +

10-104Dr. Wolf's CHM 201 & 202

CH2CH3CHCH+

CHCH2CH3CH+

Br

CH2CH3CHCH

CHCH2BrCH3CH

higher activation energy

formed more slowly

10-105Dr. Wolf's CHM 201 & 202

Addition of hydrogen chloride to Addition of hydrogen chloride to 2-methyl-1,3-butadiene is a kinetically controlled 2-methyl-1,3-butadiene is a kinetically controlled reaction and gives one product in much greater reaction and gives one product in much greater amounts than any isomers. What is this product?amounts than any isomers. What is this product?

++ HClHCl ??

10-106Dr. Wolf's CHM 201 & 202

Think mechanistically.Think mechanistically.

Protonation occurs:Protonation occurs:

at end of diene systemat end of diene system

in direction that gives most stable carbocationin direction that gives most stable carbocation

Kinetically controlled product corresponds to attack by Kinetically controlled product corresponds to attack by

chloride ion at carbon that has the greatest share of chloride ion at carbon that has the greatest share of

positive charge in the carbocationpositive charge in the carbocation

++ HClHCl

10-107Dr. Wolf's CHM 201 & 202

Think mechanistically Think mechanistically HH ClCl

++ ++

one resonance form is one resonance form is

tertiary carbocation; tertiary carbocation;

other is primaryother is primary

10-108Dr. Wolf's CHM 201 & 202


++ ++


secondary carbocation; secondary carbocation;





ClCl HH

++

++

10-109Dr. Wolf's CHM 201 & 202


++ ++




More stable carbocationMore stable carbocation

Is attacked by chloride ion Is attacked by chloride ion

at carbon that bears at carbon that bears

greater share of positive greater share of positive

chargecharge

10-110Dr. Wolf's CHM 201 & 202


++ ++




ClClClCl––

majormajorproductproduct

10-111Dr. Wolf's CHM 201 & 202

gives mixtures of 1,2 and gives mixtures of 1,2 and 1,4-addition products1,4-addition products

Halogen Addition to DienesHalogen Addition to Dienes

10-112Dr. Wolf's CHM 201 & 202



BrBr22

BrBr

CHCH22BrBrCHCH22CHCHCHCH CHCHCHCH22BrBrBrBrCHCH22CHCH++

(37%)(37%) (63%)(63%)

10-113Dr. Wolf's CHM 201 & 202

The Diels-Alder ReactionThe Diels-Alder Reaction

Synthetic method for preparing Synthetic method for preparing compounds containing a cyclohexene ringcompounds containing a cyclohexene ring

10-114Dr. Wolf's CHM 201 & 202

conjugated conjugated dienediene

alkene alkene (dienophile)(dienophile)

cyclohexenecyclohexene

++

In general...In general...In general...In general...

10-115Dr. Wolf's CHM 201 & 202

transition statetransition state

viaviaviavia

10-116Dr. Wolf's CHM 201 & 202

Diels-Alder Reaction

10-117Dr. Wolf's CHM 201 & 202

concerted mechanismconcerted mechanism

cycloadditioncycloaddition

pericyclic reactionpericyclic reaction

a concerted reaction that proceeds a concerted reaction that proceeds through a cyclic transition statethrough a cyclic transition state

Mechanistic featuresMechanistic featuresMechanistic featuresMechanistic features

10-118Dr. Wolf's CHM 201 & 202

conjugated conjugated dienediene

alkene alkene (dienophile)(dienophile)

cyclohexenecyclohexene

++

Recall the general reaction...Recall the general reaction...Recall the general reaction...Recall the general reaction...

The equation as written is somewhat The equation as written is somewhat misleading because ethylene is a relatively misleading because ethylene is a relatively unreactive dienophile.unreactive dienophile.

10-119Dr. Wolf's CHM 201 & 202

What makes a reactive dienophile?What makes a reactive dienophile?What makes a reactive dienophile?What makes a reactive dienophile?

The most reactive dienophiles have an The most reactive dienophiles have an electron-withdrawing group (electron-withdrawing group (EWGEWG) directly ) directly attached to the double bond.attached to the double bond.

Typical EWGs Typical EWGs

CC OO

CC NN

CC CC

EWGEWG

10-120Dr. Wolf's CHM 201 & 202

++

benzenebenzene 100°C100°C

(100%)(100%)

HH22CC CHCHCHCH CHCH22 HH22CC CHCH CHCH

OO CHCH

OO


10-121Dr. Wolf's CHM 201 & 202

++


(100%)(100%)

HH22CC CHCHCHCH CHCH22 HH22CC CHCH CHCH

OO CHCH

OO


CHCH

OOvia:via:

10-122Dr. Wolf's CHM 201 & 202


10-123Dr. Wolf's CHM 201 & 202

++


(100%)(100%)

OO

OO

OO


HH22CC CHCCHC CHCH22

CHCH33 HH33CC

OO

OO

OO

10-124Dr. Wolf's CHM 201 & 202

++


(100%)(100%)

OO

OO

OO


HH22CC CHCCHC CHCH22

CHCH33 HH33CC

OO

OO

OO

via:via:

HH33CCOO

OO

OO

10-125Dr. Wolf's CHM 201 & 202

++


(98%)(98%)


Acetylenic DienophileAcetylenic DienophileAcetylenic DienophileAcetylenic Dienophile

OO

CCOCHCCOCH22CHCH33CHCH33CHCH22OCCOCC

OO COCHCOCH22CHCH33

COCHCOCH22CHCH33

OO

OO

10-126Dr. Wolf's CHM 201 & 202


10-127Dr. Wolf's CHM 201 & 202

syn addition to alkenesyn addition to alkene

cis-trans relationship of substituents on alkene cis-trans relationship of substituents on alkene retained in cyclohexene productretained in cyclohexene product

Diels-Alder Reaction is Stereospecific*Diels-Alder Reaction is Stereospecific*Diels-Alder Reaction is Stereospecific*Diels-Alder Reaction is Stereospecific*

*A stereospecific reaction is one in which *A stereospecific reaction is one in which stereoisomeric starting materials give stereoisomeric starting materials give stereoisomeric products; characterized by stereoisomeric products; characterized by terms like syn addition, anti elimination, terms like syn addition, anti elimination, inversion of configuration, etc.inversion of configuration, etc.

10-128Dr. Wolf's CHM 201 & 202

only productonly product

++HH22CC CHCHCHCH CHCH22


CC CC

CC66HH55 COHCOH

HH HH

OO

HH

CC66HH55

HH

COHCOH

OO

10-129Dr. Wolf's CHM 201 & 202

only productonly product

++HH22CC CHCHCHCH CHCH22


CC CC

CC66HH55

COHCOHHH

HH

OO

HH

CC66HH55

HH

COHCOH

OO

10-130Dr. Wolf's CHM 201 & 202

Cyclic dienes yield bridged bicyclic

Diels-Alder adducts.

Cyclic dienes yield bridged bicyclic

Diels-Alder adducts.

10-131Dr. Wolf's CHM 201 & 202


Dr. Wolf's CHM 201 & 202

10-132Dr. Wolf's CHM 201 & 202


10-133Dr. Wolf's CHM 201 & 202

++

CC CC

COCHCOCH33HH

HH

OO

CHCH33OCOC

OO HH

HH

COCHCOCH33

OO

COCHCOCH33

OO

10-134Dr. Wolf's CHM 201 & 202

is theis thesame assame as

HH

HH

COCHCOCH33

OO

COCHCOCH33

OO

HH

HH

COCHCOCH33

OO

COCHCOCH33

OO

10-135Dr. Wolf's CHM 201 & 202

The The Molecular Orbitals Molecular Orbitals

ofof

Ethylene and 1,3-ButadieneEthylene and 1,3-Butadiene

10-136Dr. Wolf's CHM 201 & 202

Orbitals and Chemical ReactionsOrbitals and Chemical ReactionsOrbitals and Chemical ReactionsOrbitals and Chemical Reactions

• A deeper understanding of chemical reactivity A deeper understanding of chemical reactivity can be gained by focusing on the can be gained by focusing on the frontier frontier orbitals orbitals of the reactants.of the reactants.

• Electrons flow from the highest occupied Electrons flow from the highest occupied molecular orbital (HOMO) of one reactant to molecular orbital (HOMO) of one reactant to the lowest unoccupied molecular orbital the lowest unoccupied molecular orbital (LUMO) of the other.(LUMO) of the other.

10-137Dr. Wolf's CHM 201 & 202

Orbitals and Chemical ReactionsOrbitals and Chemical ReactionsOrbitals and Chemical ReactionsOrbitals and Chemical Reactions

• We can illustrate HOMO-LUMO interactions We can illustrate HOMO-LUMO interactions by way of the Diels-Alder reaction between by way of the Diels-Alder reaction between ethylene and 1,3-butadiene.ethylene and 1,3-butadiene.

• We need only consider only the We need only consider only the electrons of electrons of ethylene and 1,3-butadiene. We can ignore ethylene and 1,3-butadiene. We can ignore the framework of the framework of bonds in each molecule. bonds in each molecule.

10-138Dr. Wolf's CHM 201 & 202

The The MOs of Ethylene MOs of EthyleneThe The MOs of Ethylene MOs of Ethylene

• red and blue colors red and blue colors distinguish sign of distinguish sign of wave functionwave function

• bonding bonding MO is MO is antisymmetric with antisymmetric with respect to plane of respect to plane of moleculemolecule

Bonding Bonding orbital of ethylene; orbital of ethylene;two electrons in this orbitaltwo electrons in this orbital

10-139Dr. Wolf's CHM 201 & 202

The The MOs of Ethylene MOs of EthyleneThe The MOs of Ethylene MOs of Ethylene

Bonding Bonding orbital of ethylene; orbital of ethylene;two electrons in this orbitaltwo electrons in this orbital

Antibonding Antibonding orbital of ethylene; orbital of ethylene;no electrons in this orbitalno electrons in this orbital

LUMOLUMO

HOMOHOMO

10-140Dr. Wolf's CHM 201 & 202

• FourFour pp orbitals contribute to the orbitals contribute to the system of system of 1,3-butadiene; therefore, there are 1,3-butadiene; therefore, there are fourfour molecular orbitals.molecular orbitals.

• Two of these orbitals are bonding; two are Two of these orbitals are bonding; two are antibonding.antibonding.

The The MOs of 1,3-Butadiene MOs of 1,3-ButadieneThe The MOs of 1,3-Butadiene MOs of 1,3-Butadiene

10-141Dr. Wolf's CHM 201 & 202

The Two Bonding The Two Bonding MOs of 1,3-Butadiene MOs of 1,3-ButadieneThe Two Bonding The Two Bonding MOs of 1,3-Butadiene MOs of 1,3-Butadiene

Lowest energy orbitalLowest energy orbital

4 4 electrons; 2 in electrons; 2 ineach orbitaleach orbital

HOMOHOMO

10-142Dr. Wolf's CHM 201 & 202

The Two Antibonding The Two Antibonding MOs of 1,3-Butadiene MOs of 1,3-ButadieneThe Two Antibonding The Two Antibonding MOs of 1,3-Butadiene MOs of 1,3-Butadiene

Highest energy orbitalHighest energy orbital

Both antibondingBoth antibondingorbitals are vacantorbitals are vacant

LUMOLUMO

10-143Dr. Wolf's CHM 201 & 202

A A Molecular Orbital Analysis Molecular Orbital Analysis

of theof the

Diels-Alder ReactionDiels-Alder Reaction

10-144Dr. Wolf's CHM 201 & 202

MO Analysis of Diels-Alder ReactionMO Analysis of Diels-Alder ReactionMO Analysis of Diels-Alder ReactionMO Analysis of Diels-Alder Reaction

• Inasmuch as electron-withdrawing groups Inasmuch as electron-withdrawing groups increase the reactivity of a dienophile, we increase the reactivity of a dienophile, we assume electrons flow from the HOMO of the assume electrons flow from the HOMO of the diene to the LUMO of the dienophile.diene to the LUMO of the dienophile.

10-145Dr. Wolf's CHM 201 & 202

LUMO of ethylene (dienophile)LUMO of ethylene (dienophile)

HOMO of 1,3-butadieneHOMO of 1,3-butadiene


HOMO of 1,3-butadiene HOMO of 1,3-butadiene and LUMO of ethylene and LUMO of ethylene are in phase with one are in phase with one anotheranother

allows allows bond formation bond formation between the alkene and between the alkene and the dienethe diene

10-146Dr. Wolf's CHM 201 & 202

LUMO of ethylene (dienophile)LUMO of ethylene (dienophile)

HOMO of 1,3-butadieneHOMO of 1,3-butadiene


10-147Dr. Wolf's CHM 201 & 202

A "forbidden" reactionA "forbidden" reactionA "forbidden" reactionA "forbidden" reaction

• The dimerization of ethylene to give The dimerization of ethylene to give cyclobutane does not occur under conditions cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not?of typical Diels-Alder reactions. Why not?

HH22CC CHCH22

HH22CC CHCH22

++

10-148Dr. Wolf's CHM 201 & 202

A "forbidden" reactionA "forbidden" reactionA "forbidden" reactionA "forbidden" reaction HH22CC CHCH22

HH22CC CHCH22

++

HOMO of HOMO of one ethyleneone ethylenemoleculemolecule

LUMO of LUMO of other ethyleneother ethylenemoleculemolecule

HOMO-LUMOHOMO-LUMOmismatch of twomismatch of twoethylene moleculesethylene moleculesprecludes single-stepprecludes single-stepformation of two newformation of two newbondsbonds

10-149Dr. Wolf's CHM 201 & 202

End of Chapter 10

Documents

10-1 Dr. Wolf's CHM 201 & 202 Chapter 10 Conjugation in Alkadienes and Allylic Systems conjugare is a Latin verb meaning "to link or yoke together"