chapter 7. electron delocalization and resonance

7/29/2019 Chapter 7. Electron Delocalization and Resonance

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Organic Chemistry

4th Edition

Paula Yurkanis Bruice

Chapter 7

ElectronDelocalization

and Resonance

More about Molecular

Orbital Theory

Irene Lee

Case Western Reserve UniversityCleveland, OH

2004, Prentice Hall


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Localized Versus Delocalized Electrons

CH3 NH2 CH3 CH CH2

localized

electrons

localized

electrons

delocalized

electronsCH3C

O

O

-

-


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Benzene

A planar molecule

Has six identical carboncarbon bonds

Each p electron is shared by all six carbons

The p electrons are delocalized


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Resonance Contributors and the

Resonance Hybrid

Resonance contributors are imaginary, but the

resonance hybrid is real


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p electrons cannot delocalize in

nonplanar molecules


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Drawing Resonance Contributors


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Rules for Drawing Resonance

Contributors

1. Only electrons move

2. Only p electrons and lone-pair electrons move

3. The total number of electrons in the molecule does

not change

4. The numbers of paired and unpaired electrons do

not change


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The electrons can be moved in one of the following ways:

1. Move p electrons toward a positive charge ortoward a p bond

2. Move lone-pair electrons toward a p bond

3. Move a single nonbonding electron toward a p bond


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Resonance contributors are obtained by moving p

electrons toward a positive charge:

CH3

CH CH CHCH3

CH3

CH CH CHCH3

CH3CHCH CHCH3

++

CH3CH CHCH CH CH2 CH3CH

CH CH CH CH2 CH3CH CH CH CH CH2

CH3CHCH CH CH CH2

+ + +

CH2 CH2 CH2 CH2 CH2

CH2

+

+

+

+

resonance hybrid

resonance hybrid

resonance hybrid


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Moving p electrons toward a p bond


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Moving a nonbonding pair of electrons toward a p bond


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Resonance Structures for the Allylic

Radical and for the Benzyl Radical


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Note

Electrons move toward an sp2 carbon but never towardan sp3 carbon

Electrons are neither added to nor removed from themolecule when resonance contributors are drawn

Radicals can also have delocalized electrons if the

unpaired electron is on a carbon adjacent to an sp2atom


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The Difference Between Delocalized

and Localized Electrons


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CH2 CH CHCH3 CH2 CH CHCH3

CH2 CH CH2CHCH3

X

an sp3 hybridized carbon

cannot accept electrons

delocalized electrons

localized electrons


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Resonance contributors with separated charges are

less stable

R C

O

OH R C

O-

OH+

R C

O

O- R C

O-

O

more

stable

equally stable


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Electrons always move toward the more electronegative

atom


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When there is only one way to move the electrons,

because electron delocalization makes a molecule morestable

CH2 CH OCH3 CH2 CH OCH3

movement of the electrons away from the more

electronegative atom is better than no movement at all


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Features that decrease the predicted stability of a

contributing resonance structure

1. An atom with an incomplete octet

2. A negative charge that is not on the most

electronegative atom

3. A positive charge that is not on the most

electropositive atom

4. Charge separation


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Resonance Energy

A measure of the extra stability a compound gains from

having delocalized electrons


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Benzene is stabilized by electron delocalization


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Summary

The greater the predicted stability of a resonance

contributor, the more it contributes to the resonancehybrid

The greater the number of relatively stable resonancecontributors, the greater is the resonance energy

The more nearly equivalent the resonance contributors,

the greater is the resonance energy


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Resonance-Stabilized Cations

R l i S bili i f All li d


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Relative Stabilities of Allylic and

Benzylic Cations

R l ti St biliti f C b ti


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Relative Stabilities of Carbocations

R l ti St biliti f R di l


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Relative Stabilities of Radicals

S Ch i l C f


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Some Chemical Consequences of

Electron Delocalization


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Relative reactivities toward HBr

CH2 C

OCH3

CH3

CH2 C

CH3

CH3

CH2 C

CH2OCH3

CH3

> >

A B C

Delocalized electrons can affect the reactivity of a

compound

Compound A is the most reactive


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Compound A is the most reactive

CH2 C

OCH3

CH3

CH2 C

OCH3

CH3

HBr CH3 C

OCH3

CH3

CH3 C

OCH3

CH3

+ Br-

A.

CH2 C

CH3

CH3

HBr CH3 C

CH3

CH3

+ Br-

CH2 C

CH2OCH3

CH3 HBr

CH3 C

CH2OCH3

CH3

+ Br-

B.

C.


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Why is RCO2H more acidic than ROH?

Electron withdrawal by the double-bonded oxygen

decreases the electron density of the negatively

charged oxygen, thereby stabilizing the conjugated base

(the carboxylate)


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Increased resonance stabilization of the conjugated base

A t f th A idit f Ph l b


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Account for the Acidity of Phenol by

Resonance Stabilization

A t f th A idit f P t t d


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Account for the Acidity of Protonated

Aniline by Resonance Stabilization

A Molec lar Orbital Description of


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A Molecular Orbital Description of

Stability

Bonding MO: constructive (in-phase) overlap

Antibonding MO: destructive (out-of-phase) overlap

The Molecular Orbitals of


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1,3-Butadiene

Symmetry in Molecular Orbitals


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Symmetry in Molecular Orbitals

y1 and y3 in 1,3-butadiene are symmetrical molecular

orbitals

y2 and y4 in 1,3-butadiene are fully asymmetrical orbitals


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HOMO = the highest occupied molecular orbital

LUMO = the lowest unoccupied orbital

The highest-energy molecular orbital of 1,3-butadiene

that contains electrons is y2 (HOMO)

The lowest-energy molecular orbital of 1,3-butadiene

that does not contain electrons is y3 (LUMO)


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Consider the p molecular orbitals of 1,4-pentadiene:

This compound has fourp electrons that are completely

separated from one another

The Molecular Orbitals of the Allyl


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The Molecular Orbitals of the Allyl

System

No overlap between the p orbitals: the nonbonding MO

y2 is the nonbonding MO

Resonance structures of the allyl cation the allyl radical


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Resonance structures of the allyl cation, the allyl radical,

and the allyl anion

CH2 CH CH2 CH2 CH CH2 CH2CH CH2

+ +

CH2CH CH2

.

.

CH2 CH CH2 CH2 CH CH2

CH2 CH CH2 CH2 CH CH2 CH2 CH CH2

- -



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1,3,5-Hexatriene


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Benzene has six p molecular orbitals


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Benzene is unusually stable because of large


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Benzene is unusually stable because of large

delocalization energies

chapter 7. electron delocalization and resonance

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