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DelocalizedElectrons and Their

Effect on Stability,pKa, and theProducts of a

Reaction

Paula Yurkanis Bruice

University of California,

Santa Barbara

Chapter 8

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Localized Electrons

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Delocalized Electrons

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What is the Structure of Benzene?

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Degree of Unsaturation = 4

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Two Disubstituted Products

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Four Disubstituted Products

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Two are in Rapid Equilibrium

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It was Confirmed that Benzene

is Cyclic in 1901

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The Structure of Benzene

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Showing Delocalized Electrons

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Resonance Contributors

Resonance Hybrid

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Resonance Contributors

Resonance Hybrid

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Cyclooctatriene Does Not Have

Delocalized Electrons

Cyclooctatetraene is not planar.

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Delocalized Electrons

Delocalized electrons result from theporbital of one atom

overlapping theporbitals of two adjacent atoms.

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Rules for Drawing Resonance

Contributors

1. Only electrons move.

2. Only electrons and lone-pair electrons move.

3. The total number of electrons in the molecule

does not change.

4. An sp3carbon cannot accept electrons;it already has an octet.

5. Do not break sigma bonds.

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Move Electrons to an

sp2Carbon

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Cannot Move Electrons to an

sp3Carbon

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Move Electrons to an sp2Carbon

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Move Lone-Pair Electrons to

an sp2Carbon

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Move Electrons to an spCarbon

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How Delocalized Electrons Affect

Protein Structure

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A is More Stable Than B

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A and B are Equally Stable

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F is More Stable Than G;

G is More Stable Than E

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H is More Stable Than I

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K is More Stable Than J

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Delocalization Energy

The delocalization energy is the extra stability a compound has

as a result of having delocalized electrons.

Electron delocalization is also called resonance.

Delocalization energy is also called resonance energy.

The resonance hybrid is more stable than any of its

resonance contributors is predicted to be.

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The Greater the Number of Relatively Stable Resonance

Contributors,The Greater the Delocalization Energy

The delocalization energy is greater for the carboxylate ion

than for the carboxylic acid.

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The Number of Relatively Stable Resonance

Contributors is What is Important

Little delocalization energy

Significant delocalization energy

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The greater the predicted stability of a resonance

contributor, the more it contributes to the resonance hybrid.

The greater the number of relatively stable resonance

contributors, the greater the delocalization energy.

The more nearly equivalent the resonance contributors,

the greater the delocalization energy.

Summary

D l li d El t M k

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Delocalized Electrons Make a

Compound More Stable

The delocalization energy of benzene is 36 kcal/mol.

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Criteria for a Compound to Be Aromatic

It must have an uninterrupted cloud of

electrons.(cyclic, planar, every ring atom must have aporbital).

The cloud must have an odd number of pairs of electrons.

E amples of Compo nds

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Examples of Compounds

That are Not Aromatic

Cyclobutadiene has an even number of pairs of electrons.

Cyclooctatetraene has an even number of pairs of electrons and

it is not planar.

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Nonaromatic and Aromatic Compounds

Resonance Contributors and the

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Resonance Contributors and the

Resonance Hybrid

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Aromatic Compounds

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Heterocyclic Aromatic Compounds

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Orbital Structure of Pyridine

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Orbital Structure of Pyrrole and Furan

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Resonance Contributors of Pyrrole

The aromatic ring of pyridine is less electron dense than benzenebecause the nitrogen withdraws electrons from the ring.

The aromatic ring of pyrrole is more electron dense than benzene

because the nitrogen donates electrons into the ring.

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Resonance Contributors of Furan

More Aromatic

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More Aromatic

Heterocyclic Compounds

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Relative Stabilities

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Antiaromatic Compounds

An MO Description of

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An MO Description of

Aromaticity and Antiaromaticity

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Conjugated and Isolated Dienes

The Smaller the Heat of Hydrogenation

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The Smaller the Heat of Hydrogenation,

the More Stable the Compound

Conjugated Dienes Have

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Conjugated Dienes Have

Delocalized Electrons

An sp2 sp2 Bond Is Stronger

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Ansp2sp2Bond Is Stronger

Than an sp2sp3Bond

Carbon-Carbon Bond Length

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Carbon-Carbon Bond Length

Depends on Hybridization

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Cumulated Double Bonds

Organic Compounds That

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Organic Compounds That

Conduct Electricity

Allyl and Benzyl

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Allyl and Benzyl

Allylic and Benzylic

Resonance Contributors for

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Resonance Contributors for

an Allylic Cation

Resonance Contributors for

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Resonance Contributors for

a Benzylic Cation

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Relative Stabilities of Carbocations

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Relative Stabilities of Carbocations

A MO Di f E h

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An MO Diagram for Ethene

Resonance Contributors of

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Resonance Contributors of

1,3-Butadiene

A MO Di f 1 3 B t di

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An MO Diagram for 1,3-Butadiene

Symmetric and Antisymmetric

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Symmetric and Antisymmetric

Molecular Orbitals

MO Diagrams for 1,4-Pentadiene and

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MO Diagrams for 1,4 Pentadiene and

1,3-Butadiene

Delocalized Electrons Affect

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Delocalized Electrons Affect

pKaValues

Proton Loss is Accompanied by

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Proton Loss is Accompanied by

Greater Delocalization Energy

The negative charge is on one oxygen.

The negative charge is shared by two oxygens.

Ph l Al h l

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Phenols versusAlcohols

N El t D l li ti

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No Electron Delocalization

Wh Ph l M A idi

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Why Phenols are More Acidic

The base has greater delocalization energy than the conjugate acid.

Protonated Anilines versus

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o o a ed es e sus

Protonated Amines

N D l li d El t

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No Delocalized Electrons

Why Protonated Anilines

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y

are More Acidic

The base has greater delocalization energy than the conjugate acid.

Important!!!

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Important!!!

Withdrawing Electrons by Resonance

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Withdrawing Electrons by Resonance

Donating Electrons by Resonance

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Donating Electrons by Resonance

Only One Product is Formed Because

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y

of Electron Delocalization

Isolated and Conjugated Dienes

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Isolated and Conjugated Dienes

Reactions of Isolated Dienes

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Reactions of Isolated Dienes

Mechanism

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Mechanism

Double Bonds Can Have

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Different Reactivities

Reactions of Conjugated Dienes

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Reactions of Conjugated Dienes

1 2 Addition and 1 4 Addition

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1,2-Addition and 1,4-Addition

Mechanism for the Reaction of

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a Conjugated Diene

Which Carbon Gets the Proton?

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Which Carbon Gets the Proton?

Protonate the end that forms the more stable carbocation:

Kinetic and Thermodynamic Products

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Kinetic and Thermodynamic Products

For the reaction of 1,3-butadiene

If the Reaction is Irreversible,

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the Kinetic Product Predominates

If the Reaction is Reversible, the

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Thermodynamic Product Predominates

Why?

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Why?

Kinetic Control

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Kinetic Control

Thermodynamic Control

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Thermodynamic Control

The 1,2-Product is Always

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DCl was used instead of HCl, so the 1,2- and 1,4-products would be different.

The Kinetic Product

The Proximity Effect

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The Proximity Effect

The proximity effect causes the 1,2-product to be formed faster.

Kinetic and Thermodynamic Products

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Kinetic and Thermodynamic Products

Although the 1,2-product is always the kinetic product,

do not assume that the 1,4-product is always the thermodynamic product.

Kinetic and Thermodynamic Products

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Kinetic and Thermodynamic Products

The DielsAlder Reaction

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Forms a Six-Membered Ring

The Mechanism

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The Mechanism

a pericyclic reaction takes place by a cyclic shift of electrons

a [4+2] cycloaddition reaction

Faster if There is an Electron

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Withdrawing Group on the Dienophile

The Electron Withdrawing Group Makes the

El t hil B tt El t hil

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Electrophile a Better Electrophile

Another DielsAlder Reaction

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Another Diels Alder Reaction

Alkynes Can Also Be Dienophiles

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Alkynes Can Also Be Dienophiles

The cyclic product has two double bonds.

Another DielsAlder Reaction

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Another Diels Alder Reaction

Both Reactants are Not Symmetric

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Both Reactants are Not Symmetric

Two products are possible.

The Reactants Can BeAli d i T W

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Aligned in Two Ways

Which Alignment GivesTh M j P d t?

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The Major Product?

The Answer

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e s e

s-Cis and s-Trans Conformations

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The Diene Must Be inan s Cis Conformation

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an s-Cis Conformation

Locked in an s-Cis Conformation

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Endo and Exo

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Bridged Bicyclic Rings andFused Bicyclic Rings

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Fused Bicyclic Rings

If the DielsAlder Reaction Createsan Asymmetric Center

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an Asymmetric Center

A racemic mixture

Cis Forms CisTrans Forms Trans

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Trans Forms Trans

How to Determine the Reactantsof a Diels Alder Reaction

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of a DielsAlder Reaction

How to Determine the Reactantsof a Diels Alder Reaction

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of a DielsAlder Reaction

Group I Compounds are Nucleophiles;They React with Electrophiles

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They React with Electrophiles