Organic Chemistry

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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 carboncarbon bonds

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Resonance Contributors and the Resonance Hybrid
Resonance contributors do not depict any real
electron distribution
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• 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
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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
Benzylic cation
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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
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
B is less stable than A, no electronic push-pull.
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
Predicted reactivity
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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 ? 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
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Substituent Effects
Resonance release of lone-pair electrons
(competes with inductive withdrawal by the
electronegative oxygen)
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
2. A negative charge that is not on the most
electronegative atom
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3. A positive charge that is not on the most
electropositive atom
4. Charge separation
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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
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Relative Stabilities of Carbocations and Radicals
Carbocations hyperconjugation more important
than resonance.
Radicals resonance more important than
hyperconjugation.
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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
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Protonated aniline is a stronger acid than
protonated cyclohexylamine because the aniline
lone pair is delocalized
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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
Third, the conjugate acid of the more stable
anion is the strongest acid
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Stability of Dienes
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The most stable diene has the lower -DHo value
Why The hybridization of the orbitals forming
the carbon-carbon 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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• 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 isnt cyclooctatetrene flat?
Pattern of bonding orbitals of planar cyclic ?
systems (e.g., benzene)
Pattern of bonding orbitals of non-planar or
non-cyclic ? systems (e.g., 1,3-butadiene)
Answer Cyclooctatetrene has 8??electrons, which
cannot fill the degenerate orbitals of a planar ?
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
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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
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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
Fourth, identify the resonance form with the more
stable alkene. Halide trapping affords the
thermodynamic product
Third, identify the more stable carbocation.
Halide trapping affords the kinetic product
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The DielsAlder Reaction A 1,4-Addition Reaction
The DielsAlder reaction is a pericyclic
reaction a 42 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
DielsAlder Reaction
Lets focus on the HOMO and the LUMO of the
reactants
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Preparation of Cyclic Compounds Using the
DielsAlder 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 DielsAlder
reaction
And affords the endo isomer as the major product
(the endo rule).
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The Stereochemisty of the DielsAlder Reaction
If a DielsAlder 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