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Organic Chemistry
6th Edition
Paula Yurkanis Bruice
Chapter 7
Delocalized
Electrons and Their
Effect on
Stability, Reactivity,
and pKa
More About Molecular
Orbital Theory
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Localized Versus Delocalized Electrons
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Benzene Has Delocalized Electrons
• A planar molecule
• Has six identical carbon–carbon bonds
• Each p electron is shared by all six carbons
• The p electrons are delocalized
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3
Resonance Contributors and the
Resonance Hybrid
Resonance contributors do not depict any real
electron distribution © 2011 Pearson Education, Inc.
4
p electrons cannot delocalize in
nonplanar molecules
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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. An sp3 carbon cannot accept electrons; it
already has an octet.
5. Do not break sp3() bonds.
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Many Resonance Structures Consist of
Two-Center and/or Three-Center Systems
Two-Center Systems:
Three-Center Systems:
Only p or lone
pair electrons
move!
No  bonds broken!
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Allylic Resonance
Move p electrons toward an sp2 carbon:
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Multi-Center Allylic Resonance
Composed of allylic centers:
Allylic
Allylic
Benzylic cation:
+ CH2
Allylic
CH2
CH2
Allylic
CH2
+
+
+
Allylic
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Three-Center Bonding: Nitro Group Resonance
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Three-Center Bonding: Amide Resonance
Move lone-pair electrons toward the sp2 oxygen:
Electronegative
oxygen
Electropositive
nitrogen
Electronic push-pull
Amide resonance responsible for the strength of amide
bonds found in hair, skin, muscle, Kevlar vests, etc.
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Three-Center Bonding: Carboxylic Acid
Derivatives
(R)
(R)
derivative
B is less stable than A,
no electronic pushpull. Ester bonds not
as robust as amide
bonds.
C and D are equally
stable: carboxylate
resonance responsible
for the acidity of
carboxylic acids.
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,-Unsaturated Carbonyl Compounds
A four-center system composed of two and three
center systems:
Carbonyl
center
Allyl cation
Predicted reactivity:
,-Unsaturated carbonyl
compounds are toxic
because they trap YOUR
nucleophiles.
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Summary of Electron Delocalization Examples
1. Move p electrons toward an sp2 carbon
2. Move lone-pair electrons toward an sp2 carbon
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3. Move lone-pair or p electrons towards an sp carbon:
4. Do not move electrons to an sp3 carbon:
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5. Electrons always move toward the more
electronegative atom:
6. Do not break sp3() bonds :
Alkene cross
conjugated with
carboxylate
No + charge here,
cross conjugated
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Substituent Effects
Resonance release of lone-pair electrons (competes with
inductive withdrawal by the electronegative oxygen):
Isoelectronic with
allyl anion
The methoxy group makes the benzene ring electron-rich:
Electron-releasing groups have a lone pair at the point of attachment.
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Substituent Effects
The nitro group makes the benzene ring electron-deficient
by resonance withdrawal:
Electron-withdrawing groups possess an electron-deficient center at
the point of attachment:
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Features that decrease the predicted stability of a
contributing resonance structure:
1. An atom with an incomplete octet:
More stable
2. A negative charge that is not on the most
electronegative atom:
More stable
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3. A positive charge that is not on the most
electropositive atom:
Not a valid
structure
4. Charge separation:
More stable
charge-separated
structure
More stable
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Delocalization Energy
The extra stability a compound gains from having
delocalized electrons is called the delocalization energy
Electron delocalization is also called resonance
Delocalization energy is also called resonance energy
A resonance hybrid is more stable than any of its
resonance contributors is predicted to be
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Summary
• 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 is the resonance energy
• The more nearly equivalent the resonance contributors,
the greater is the resonance energy
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Using Resonance to Predict Stability
Carbonate is stable and ubiquitous in nature:
Cement, shells, limestone…
The guanidium ion is stabilized by resonance and
deprotonated only in strong base:
Important physiologically:
The amino acid arginine,
guanidium pKa = 12.5, cationic
at physiological pH= 7.4
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Relative Stabilities of Carbocations and Radicals
Carbocations: hyperconjugation more important than
resonance.
Radicals: resonance more important than
hyperconjugation.
H
CH2
H
>
~
3o > 2o > 1o > vinylic ~ methyl
H
H
H
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Cation Stabilization by Resonance and
Hyperconjugation
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Example: Delocalized Electrons Can Affect
the Product of a Reaction
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Delocalized Electrons Affect pKa
Carboxylic acid is a stronger acid because…
Electron withdrawal and electron delocalization
stabilize the conjugate base:
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Phenol is a stronger acid than cyclohexanol because of
phenolate ion delocalization:
Localized
Anion
pKa = 16
pKa = 10
Delocalized
Anion
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Protonated aniline is a stronger acid than protonated
cyclohexylamine because the aniline lone pair is
delocalized:
Localized
lone pair
pKa = 11.2
Delocalized
lone pair
pKa = 4.62
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Connecting Delocalization, Substituent
Effects, and pKa Values
Important in understanding drug design and reaction
mechanisms:
Which phenol is the stronger acid?
First, show the resonance
delocalization of the phenolate
anion:
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Second, pick the structure in which the anion interacts with the nitro
group. This structure represents the more stable anion:
Anion
delocalizes
into the nitro
group
Anion
adjacent to
electronwithdrawing
nitro group
Third, the conjugate acid of the more stable anion is the strongest
acid:
Stronger acid,
pKa = 7.2
Weaker acid, pKa = 8.4.
Nitro only exerts
inductive withdrawal
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Stability of Dienes
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The most stable diene has the lower -DHo value :
Closer in energy to pentane
Why: The hybridization of the orbitals forming the carboncarbon single bonds also causes a conjugated diene to be
more stable than an isolated diene:
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A Molecular Orbital Description
of Stability
• Bonding MO: constructive (in-phase) overlap
• Antibonding MO: destructive (out-of-phase) overlap
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MOs for the Allyl System
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The Molecular Orbitals of
1,3-Butadiene
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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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• 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)
• HOMO = the highest occupied molecular orbital
• LUMO = the lowest unoccupied molecular orbital
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Consider the p molecular orbitals of 1,4-pentadiene:
This compound has four p electrons that are completely
separated from one another
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The Molecular Orbitals of
1,3,5-Hexatriene
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Benzene has six p molecular orbitals
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As the energy of the
p orbitals increase,
the net number of
bonding interactions
decreases
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Benzene is unusually stable because of large
delocalization energies:
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Why isn’t cyclooctatetrene flat?
Pattern of bonding orbitals of
planar cyclic p systems (e.g.,
benzene):
Pattern of bonding orbitals of nonplanar or non-cyclic p systems (e.g.,
1,3-butadiene):
Needs an even number of
electrons to fill orbitals
Needs 4N+2 (N = 0, 1, 2, 3…)
electrons to fill orbitals
Answer: Cyclooctatetrene has
8pelectrons, which cannot fill the
degenerate orbitals of a planar p
system. Therefore it has a tub
shape to remove these degenerate
orbitals.
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Reactions of Isolated Dienes
In the presence of limiting electrophilic reagent, only the
more reactive double bond reacts
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Reactions of Conjugated Dienes
An isolated diene undergoes only 1,2-addition
A conjugated diene undergoes both 1,2- and 1,4-addition
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Mechanism of HBr Addition to a
Conjugated Diene
More stable cation
More stable alkene
Kinetic product
Thermodynamic
product
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If the conjugated diene is not symmetrical…
Protonate to form the more stable carbocation:
More
stable
carbocation
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Thermodynamic Versus Kinetic
Control Reactions
• The thermodynamic product is the most stable product
• The thermodynamic product predominates when the
reaction is reversible (thermodynamic control)
• The kinetic product is the product that is formed most
rapidly
• The kinetic product predominates when the reaction is
irreversible (kinetic control)
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The kinetic product predominates when the reaction is
irreversible (temperature too low):
The thermodynamic product predominates when the reaction is
reversible (higher temperature):
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Consider the reaction coordinate diagram…
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Do not assume that the 1,4-addition product is always
the thermodynamic product…
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How to Identify the Kinetic and
Thermodynamic Products
First, protonate at
the least-substituted
carbon:
Second, show both allylic
resonance forms:
Third, identify the more stable
carbocation. Halide trapping
affords the kinetic product:
Fourth, identify the resonance
form with the more stable
alkene. Halide trapping affords
the thermodynamic product:
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The Diels–Alder Reaction:
A 1,4-Addition Reaction
The Diels–Alder reaction is a pericyclic reaction; a [4+2]
cycloaddition reaction
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The reactivity of the dienophile is increased if one or
more electron-withdrawing groups are attached to its
sp2 carbons
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A Molecular Orbital Description of the
Diels–Alder Reaction
Let’s focus on the HOMO and the LUMO of the
reactants
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Preparation of Cyclic Compounds
Using the Diels–Alder Reaction
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Predicting the Product When Both
Reagents Are Unsymmetrically Substituted
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The charge distribution in each of the reactants
determines the yield of the products
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Major product
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Two Possible Configurations of Bridged
Bicyclic Compounds
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A conjugated diene locked in an s-cis conformation is
highly reactive in a Diels–Alder reaction:
And affords the endo isomer as the major product
(the endo rule).
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The Stereochemisty of the Diels–Alder Reaction
If a Diels–Alder reaction creates a product with an
asymmetric center, identical amounts of the R and S
enantiomers will be formed
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Spanish Fly: Cantharidin
• A potent vesicant that the
insect uses to protect its eggs.
• Originally used as an
aphrodisiac; causes priapism,
but is too toxic.
• Now used to remove warts.
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Failed Attempt Using a Diels–
Alder Approach
Problems:
• Reaction is reversible
• Major product is endo
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The Successful Dauben
Synthesis
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