10-1 dr. wolf's chm 201 & 202 10.8 classes of dienes

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

10.8 10.8

Classes of DienesClasses of Dienes

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

isolated dieneisolated diene

conjugated dieneconjugated diene

cumulated dienecumulated diene

CC Classification of DienesClassification of DienesClassification of DienesClassification of Dienes

10-3Dr. 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

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

10.910.9

Relative StabilitiesRelative Stabilities

of Dienesof Dienes

10-5Dr. 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

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




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



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


10-8Dr. 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

10-9Dr. 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

10-10Dr. 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

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

10.10Bonding

in Conjugated Dienes

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

Isolated dieneIsolated diene

Conjugated dieneConjugated diene

1,4-pentadiene1,4-pentadiene


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



bonds are bonds are independent of independent of

each othereach other


10-14Dr. 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

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



less electron less electron delocalization; delocalization;

less stableless stable

more electron more electron delocalization; delocalization;

more stablemore stable

10-16Dr. 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

10-17Dr. 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

10-18Dr. 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

10-19Dr. 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

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

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

16 kJ/mol16 kJ/mol

12 kJ/mol12 kJ/mol

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

10.11Bonding in Allenes

10-23Dr. 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

10-24Dr. 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

10-25Dr. 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

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

spsp 2 2spsp

Bonding in AlleneBonding in AlleneBonding in AlleneBonding in Allene

spsp 2 2

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


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


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


10-30Dr. 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

10-31Dr. 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

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

10.1210.12

Preparation of DienesPreparation of Dienes

10-33Dr. 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

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

KHSOKHSO44

heatheat

Dehydration of AlcoholsDehydration of AlcoholsDehydration of AlcoholsDehydration of Alcohols

OHOH

10-35Dr. 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

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

KOHKOH

heatheat

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

BrBr

10-37Dr. 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

10-38Dr. 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

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

10.1310.13

Addition of Hydrogen HalidesAddition of Hydrogen Halides

toto

Conjugated DienesConjugated Dienes

10-40Dr. 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

++

10-41Dr. 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

?? ??

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

HHClCl

Example:Example:Example:Example: HH

HH

HH

HH

HH

HH

ClCl

HHHH

HH

HH

HH

HHHH

10-43Dr. 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

++

10-44Dr. 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

10-45Dr. 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

10-46Dr. 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

10-47Dr. 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

++

10-48Dr. 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

HH22CC CHCHCHCH CHCH22

HBrHBr

BrBr

CHCH22CHCH33CHCHCHCH CHCHCHCH22BrBrCHCH33CHCH++

10-49Dr. 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:++ ++

10-50Dr. 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

10-51Dr. 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.

10-52Dr. 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)

10-53Dr. 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

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

H2C CHCH CH2

HBr

CH2CH3CHCH CHCH2CH3CH+ +

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

CH2CH3CHCH+

CHCH2CH3CH+

Br

CH2CH3CHCH

CHCH2BrCH3CH

higher activation energy

formed more slowly

10-56Dr. 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 ??

10-57Dr. 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

10-58Dr. 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

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


++ ++


secondary carbocation; secondary carbocation;





ClCl HH

++

++

10-60Dr. 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

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


++ ++




ClClClCl––

majormajorproductproduct

10-1 dr. wolf's chm 201 & 202 10.8 classes of dienes

Documents

kjmol slide

isolated slide

stable slide

pentadiene slide

conjugation slide

allenes slide

conjugated dienes

extended system slide