Chapter 10 conjugation in alkadienes and allylic systems
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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.

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Chapter 10 Conjugation in Alkadienes and Allylic Systems

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Chapter 10 conjugation in alkadienes and allylic systems

Chapter 10Conjugation in Alkadienes andAllylic Systems

conjugare is a Latin verb meaning "to link or yoke together"

Dr. Wolf's CHM 201 & 202


The double bond as a substituent

C

C

C

+

The Double Bond as a Substituent

allylic carbocation

Dr. Wolf's CHM 201 & 202


The double bond as a substituent1

C

C

C

C

C

C

+

The Double Bond as a Substituent

allylic carbocation

allylic radical

Dr. Wolf's CHM 201 & 202


The double bond as a substituent2

C

C

C

C

C

C

+

C

C

C

C

The Double Bond as a Substituent

allylic carbocation

allylic radical

conjugated diene

Dr. Wolf's CHM 201 & 202


10 1 the allyl group

H

H

C

C

H

H

C

H

10.1The Allyl Group

Dr. Wolf's CHM 201 & 202


Vinylic versus allylic

Vinylic versus Allylic

C

C

C

allylic

carbon

vinylic carbons

Dr. Wolf's CHM 201 & 202


Vinylic versus allylic1

Vinylic versus Allylic

H

C

C

H

C

H

vinylic hydrogens are attached to vinylic carbons

Dr. Wolf's CHM 201 & 202


Vinylic versus allylic2

H

H

C

C

H

C

Vinylic versus Allylic

allylic hydrogens are attached to allylic carbons

Dr. Wolf's CHM 201 & 202


Vinylic versus allylic3

Vinylic versus Allylic

X

C

C

X

C

X

vinylic substituents are attached to vinylic carbons

Dr. Wolf's CHM 201 & 202


Vinylic versus allylic4

Vinylic versus Allylic

X

X

C

C

X

C

allylic substituents are attached to allylic carbons

Dr. Wolf's CHM 201 & 202


10 2 allylic carbocations

+

C

C

C

10.2Allylic Carbocations

Dr. Wolf's CHM 201 & 202


Allylic carbocations

CH3

Cl

H2C

C

C

CH

CH3

Allylic 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, 45°C)

Dr. Wolf's CHM 201 & 202


Allylic carbocations1

Allylic Carbocations

provides good evidence for the conclusion thatallylic carbocations are more stable thanother carbocations

CH3

CH3

+

+

H2C

C

C

CH

CH3

CH3

CH3

formed faster

Dr. Wolf's CHM 201 & 202


Allylic carbocations2

CH3

+

H2C

CH

CH3

Allylic Carbocations

provides good evidence for the conclusion thatallylic carbocations are more stable thanother carbocations

CH3

+

C

C

CH3

CH3

H2C=CH— stabilizes C+ better than CH3—

Dr. Wolf's CHM 201 & 202


Stabilization of allylic carbocations

Stabilization of Allylic Carbocations

Delocalization of electrons in the doublebond stabilizes the carbocation

resonance modelorbital overlap model

Dr. Wolf's CHM 201 & 202


Resonance model

CH3

+

+

C

C

H2C

H2C

CH

CH

CH3

CH3

Resonance Model

CH3

Dr. Wolf's CHM 201 & 202


Resonance model1

CH3

+

+

C

C

H2C

H2C

CH

CH

CH3

CH3

Resonance Model

CH3

CH3

d+

d+

C

H2C

CH

CH3

Dr. Wolf's CHM 201 & 202


Orbital overlap model

Orbital Overlap Model

+

+

Dr. Wolf's CHM 201 & 202


Orbital overlap model1

Orbital Overlap Model

Dr. Wolf's CHM 201 & 202


Orbital overlap model2

Orbital Overlap Model

Dr. Wolf's CHM 201 & 202


Orbital overlap model3

Orbital Overlap Model

Dr. Wolf's CHM 201 & 202


10 3 s n 1 reactions of allylic halides

10.3SN1 Reactions of Allylic Halides

Dr. Wolf's CHM 201 & 202


Hydrolysis of an allylic halide

CH3

Cl

H2C

C

CH

CH3

CH3

OH

H2C

C

CH

CH3

Hydrolysis of an Allylic Halide

H2O

Na2CO3

CH3

C

CH

+

HOCH2

CH3

(15%)

(85%)

Dr. Wolf's CHM 201 & 202


Corollary experiment

CH3

C

CH

ClCH2

CH3

CH3

CH3

C

CH

HOCH2

OH

H2C

C

CH

CH3

CH3

Corollary Experiment

H2O

Na2CO3

+

(15%)

(85%)

Dr. Wolf's CHM 201 & 202


Chapter 10 conjugation in alkadienes and allylic systems

CH3

CH3

C

CH

Cl

ClCH2

H2C

C

CH

CH3

CH3

and

give the same products because they form the same carbocation

Dr. Wolf's CHM 201 & 202


Chapter 10 conjugation in alkadienes and allylic systems

CH3

CH3

C

CH

Cl

ClCH2

H2C

C

CH

CH3

CH3

and

give the same products because they form the same carbocation

CH3

CH3

+

+

C

C

H2C

H2C

CH

CH

CH3

CH3

Dr. Wolf's CHM 201 & 202


Chapter 10 conjugation in alkadienes and allylic systems

more positive charge on tertiary carbon;therefore more tertiary alcohol in product

CH3

CH3

+

+

C

C

H2C

H2C

CH

CH

CH3

CH3

Dr. Wolf's CHM 201 & 202


Chapter 10 conjugation in alkadienes and allylic systems

H2C

C

CH

(85%)

(15%)

CH3

CH3

+

C

CH

OH

HOCH2

CH3

CH3

more positive charge on tertiary carbon;therefore more tertiary alcohol in product

CH3

CH3

+

+

C

C

H2C

H2C

CH

CH

CH3

CH3

Dr. Wolf's CHM 201 & 202


Allylic halides are more reactive in s n 2 reactions

CHCH2I

CHCH2—Cl

H2C

H2C

Allylic Halides are more reactive in SN2 reactions

Allyl chloride is 80x more reactive than

1-Cloropropane

KI

CH3CH2CH2—Cl

CH3CH2CH2I

KI

Dr. Wolf's CHM 201 & 202


10 4 s n 2 reactions of allylic halides

10.4SN2 Reactions of Allylic Halides

Dr. Wolf's CHM 201 & 202


Allylic s n 2 reactions

Cl

H2C

CH2

CH

Allylic SN2 Reactions

  • Allylic halides also undergo SN2 reactions

  • faster than simple primary alkyl halides.

Cl

H3C

CH2

CH2

1

80

relative rates: (I-, acetone)

Dr. Wolf's CHM 201 & 202


Allylic s n 2 reactions1

Cl

H2C

CH2

CH

Allylic SN2 Reactions

  • Two factors:

  • Steric

  • Trigonal carbon smaller than tetrahedral carbon.

Cl

H3C

CH2

CH2

1

80

relative rates: (I-, acetone)

Dr. Wolf's CHM 201 & 202


Allylic s n 2 reactions2

Cl

H2C

CH2

CH

Allylic SN2 reactions

  • Two factors:

  • Electronic

  • Electron delocalization lowers LUMO energy

  • which means lower activation energy.

Cl

H3C

CH2

CH2

1

80

relative rates: (I-, acetone)

Dr. Wolf's CHM 201 & 202


10 5 allylic free radicals

C

C

C

10.5Allylic Free Radicals

Dr. Wolf's CHM 201 & 202


Allylic free radicals are stabilized by electron delocalization

C

C

C

C

C

C

Allylic free radicals are stabilized byelectron delocalization

Dr. Wolf's CHM 201 & 202


Free radical stabilities are related to bond dissociation energies

CHCH2

CHCH2—H

H2C

H2C

Free-radical stabilities are related tobond-dissociation energies

C—H bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane

410 kJ/mol

+

CH3CH2CH2—H

CH3CH2CH2

H•

368 kJ/mol

+

H•

Dr. Wolf's CHM 201 & 202


10 6 allylic halogenation

10.6Allylic Halogenation

Dr. Wolf's CHM 201 & 202


Chlorination of propene

ClCH2CHCH3

Cl

CHCH3

H2C

CHCH2Cl

H2C

Chlorination of Propene

addition

+

Cl2

500 °C

+ HCl

substitution

Dr. Wolf's CHM 201 & 202


Allylic halogenation

Allylic Halogenation

selective for replacement of allylic hydrogen

free radical mechanism

allylic radical is intermediate

Dr. Wolf's CHM 201 & 202


Hydrogen atom abstraction step

H

H

..

.

Cl

:

C

C

..

H

C

Hydrogen-atom abstraction step

H

allylic C—H bond weaker than vinylic

chlorine atom abstracts allylic H in propagation step

H

410 kJ/mol

368 kJ/mol

H

Dr. Wolf's CHM 201 & 202


Hydrogen atom abstraction step1

..

Cl

:

:

H

..

Hydrogen-atom abstraction step

H

H

C

C

H

H

C

410 kJ/mol

368 kJ/mol

H

Dr. Wolf's CHM 201 & 202


N bromosuccinimide

O

O

NBr

NH

O

O

N-Bromosuccinimide

reagent used (instead of Br2) for allylic bromination

Br

heat

+

+

CCl4

(82-87%)

Dr. Wolf's CHM 201 & 202


Limited scope

Limited Scope

Allylic halogenation is only used when:

all of the allylic hydrogens are equivalent

andthe resonance forms of allylic radicalare equivalent

Dr. Wolf's CHM 201 & 202


Example

H

H

H

H

Example

Cyclohexene satisfies both requirements

All allylichydrogens areequivalent

Dr. Wolf's CHM 201 & 202


Example1

H

H

H

H

H

H

H

H

H

H

Example

Cyclohexene satisfies both requirements

All allylichydrogens areequivalent

Both resonance forms are equivalent

Dr. Wolf's CHM 201 & 202


Example2

CH3CH

CHCH3

CH3CH

CH

CH2

CH3CH

CH

CH2

Example

All allylichydrogens areequivalent

2-Butene

But

Two resonance forms are not equivalent;gives mixture of isomeric allylic bromides.

Dr. Wolf's CHM 201 & 202


10 7 allylic anions

10.7 Allylic Anions

Dr. Wolf's CHM 201 & 202


Chapter 10 conjugation in alkadienes and allylic systems

CH3

-

-

C

C

H2C

H2C

CH

CH

CH3

CH3

Allylic anions are stabilized byelectron delocalization

CH3

Dr. Wolf's CHM 201 & 202


Acidity of propene

-

CH2

CH2

H2C

H3C

CH

CH

Acidity of Propene

CH3

H3C

CH2

pKa ~ 62

pKa ~ 43

-

CH3

H2C

CH2

Propene is significantly more acidic than propane.

Dr. Wolf's CHM 201 & 202


Resonance model2

-

CH2

H2C

CH

-

-

CH2

H2C

CH

Resonance Model

-

CH2

H2C

CH

Charge is delocalized to both terminal carbons,

stabilizing the conjugate base.

Dr. Wolf's CHM 201 & 202


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