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Electron Delocalization
Organic Chemistry 4th Edition Paula Yurkanis Bruice Chapter 7 Electron Delocalization and Resonance More about Molecular Orbital Theory Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall
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Localized Versus Delocalized Electrons
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Benzene 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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Resonance Contributors and the Resonance Hybrid
Resonance contributors are imaginary, but the resonance hybrid is real
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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 or toward 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: 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 toward
Electrons are neither added to nor removed from the molecule when resonance contributors are drawn Radicals can also have delocalized electrons if the unpaired electron is on a carbon adjacent to an sp2 atom
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The Difference Between Delocalized and Localized Electrons
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delocalized electrons
an sp3 hybridized carbon cannot accept electrons delocalized electrons localized electrons
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Resonance contributors with separated charges are less stable
more stable equally stable
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Electrons always move toward the more electronegative
atom
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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 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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Resonance-Stabilized Cations
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Relative Stabilities of Allylic and Benzylic Cations
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Relative Stabilities of Carbocations
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Relative Stabilities of Radicals
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Some Chemical Consequences of Electron Delocalization
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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
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Account for the Acidity of Phenol by Resonance Stabilization
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Account for the Acidity of Protonated
Aniline by Resonance Stabilization
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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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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 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 the Allyl System
y2 is the nonbonding MO No overlap between the p orbitals: the nonbonding MO
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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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Benzene is unusually stable because of large
delocalization energies
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