Cộng hưởng, FMO

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
Localized Versus Delocalized Electrons
localized
electrons
localized
electrons
Benzene
A planar molecule

Has six identical carbon–carbon bonds
Resonance Contributors and the Resonance Hybrid
Resonance contributors are imaginary, but the resonance hybrid is real
electrons cannot delocalize in
nonplanar molecules
Drawing Resonance Contributors
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
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
Resonance contributors are obtained by moving p
electrons toward a positive charge:
resonance hybrid
resonance hybrid
resonance hybrid
Moving p electrons toward a p bond
Moving a nonbonding pair of electrons toward a p bond
Resonance Structures for the Allylic
Radical and for the Benzyl Radical
Note
Electrons move toward an sp2 carbon but never toward
an sp3 carbon
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
The Difference Between Delocalized
and Localized Electrons
Resonance contributors with separated charges are
less stable
more
stable
equally stable
Electrons always move toward the more electronegative
atom
When there is only one way to move the electrons,

because electron delocalization makes a molecule more
stable
movement of the electrons away from the more
electronegative atom is better than no movement at all
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
Resonance Energy

A measure of the extra stability a compound gains from
having delocalized electrons
Benzene is stabilized by electron delocalization
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
Resonance-Stabilized Cations
Relative Stabilities of Allylic and Benzylic Cations
Relative Stabilities of Carbocations
Relative Stabilities of Radicals

Some Chemical Consequences of
Electron Delocalization
Relative reactivities toward HBr
Delocalized electrons can affect the reactivity of a
compound
Compound A is the most reactive …
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)
Increased resonance stabilization of the conjugated base
Account for the Acidity of Phenol by Resonance Stabilization
Account for the Acidity of Protonated
Aniline by Resonance Stabilization
A Molecular Orbital Description of Stability
Bonding MO: constructive (in-phase) overlap
Antibonding MO: destructive (out-of-phase) overlap
The Molecular Orbitals of
1,3-Butadiene
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
HOMO = the highest occupied molecular orbital

LUMO = the lowest unoccupied orbital
Consider the p molecular orbitals of 1,4-pentadiene:
This compound has four p electrons that are completely
separated from one another
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,
and the allyl anion

The Molecular Orbitals of 1,3,5-Hexatriene
Benzene has six p molecular orbitals
Benzene is unusually stable because of large
delocalization energies