Contributed by:
The highlights are:
1. Localised vs Delocalised electrons
2. Benzene
3. Resonance Contributors and Resonance hybrid
4. Drawing Resonance contributors
5. Resonance Structures for the Allylic
Radical and for the Benzyl Radical
6. Resonance Energy
7. Relative Stabilities of Allylic and Benzylic Cations
8. Relative Stability
1.
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
2.
Localized Versus Delocalized Electrons
CH3 NH2 CH3 CH CH2
localized localized
electrons electrons
O -
delocalized
CH3C electrons
O -
3.
Benzene
• A planar molecule
• Has six identical carbon–carbon bonds
• Each electron is shared by all six carbons
• The electrons are delocalized
4.
Resonance Contributors and the
Resonance Hybrid
Resonance contributors are imaginary, but the
resonance hybrid is real
5.
electrons cannot delocalize in
nonplanar molecules
6.
Drawing Resonance Contributors
7.
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. The numbers of paired and unpaired electrons do
not change
8.
The electrons can be moved in one of the following ways:
1. Move electrons toward a positive charge or
toward a bond
2. Move lone-pair electrons toward a bond
3. Move a single nonbonding electron toward a bond
9.
Resonance contributors are obtained by moving
electrons toward a positive charge:
CH3CH CH CHCH3 CH3CH CH CHCH3
+ +
CH3CH CH CHCH3 resonance hybrid
CH3CH CH CH CH CH2 CH3CH CH CH CH CH2 CH3CH CH CH CH CH2
+ + +
CH3CH CH CH CH CH2 resonance hybrid
CH2 CH2 CH2 CH2 CH2
+
CH2
+ +
resonance hybrid
+
10.
Moving electrons toward a bond
11.
Moving a nonbonding pair of electrons toward a bond
12.
Resonance Structures for the Allylic
Radical and for the Benzyl Radical
13.
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
14.
The Difference Between Delocalized
and Localized Electrons
15.
delocalized electrons
CH2 CH CHCH3 CH2 CH CHCH3
an sp3 hybridized carbon
cannot accept electrons
X
CH2 CH CH2CHCH3
localized electrons
16.
Resonance contributors with separated charges are
less stable
O O-
R C OH R C OH+
more
stable
O O-
R C O- R C O
equally stable
17.
Electrons always move toward the more electronegative
18.
When there is only one way to move the electrons,
CH2 CH OCH3 CH2 CH OCH3
movement of the electrons away from the more
electronegative atom is better than no movement at all
because electron delocalization makes a molecule more
19.
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
20.
Resonance Energy
• A measure of the extra stability a compound gains from
having delocalized electrons
21.
Benzene is stabilized by electron delocalization
22.
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
23.
Resonance-Stabilized Cations
24.
Relative Stabilities of Allylic and
Benzylic Cations
25.
Relative Stabilities of Carbocations
26.
Relative Stabilities of Radicals
27.
Some Chemical Consequences of
Electron Delocalization
28.
Delocalized electrons can affect the reactivity of a
Relative reactivities toward HBr
CH3 CH3 CH3
CH2 C > CH2 C > CH2 C
OCH3 CH3 CH2OCH3
A B C
29.
Compound A is the most reactive …
A. CH3 CH3 CH3
CH2 C HBr CH3 C CH3 C
OCH3 OCH3 OCH3
+ Br -
CH3
CH2 C
OCH3
B. CH3 CH3
CH2 C HBr CH3 C + Br-
CH3 CH3
C. CH3 CH3
HBr
CH2 C CH3 C + Br-
CH2OCH3 CH2OCH3
30.
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)
31.
Increased resonance stabilization of the conjugated base
32.
Account for the Acidity of Phenol by
Resonance Stabilization
33.
Account for the Acidity of Protonated
Aniline by Resonance Stabilization
34.
A Molecular Orbital Description of
Stability
• Bonding MO: constructive (in-phase) overlap
• Antibonding MO: destructive (out-of-phase) overlap
35.
The Molecular Orbitals of
1,3-Butadiene
36.
Symmetry in Molecular Orbitals
1 and 3 in 1,3-butadiene are symmetrical molecular
2 and 4 in 1,3-butadiene are fully asymmetrical orbitals
37.
• The highest-energy molecular orbital of 1,3-butadiene
that contains electrons is 2 (HOMO)
• The lowest-energy molecular orbital of 1,3-butadiene
that does not contain electrons is 3 (LUMO)
• HOMO = the highest occupied molecular orbital
• LUMO = the lowest unoccupied orbital
38.
Consider the molecular orbitals of 1,4-pentadiene:
This compound has four electrons that are completely
separated from one another
39.
The Molecular Orbitals of the Allyl
System
2 is the nonbonding MO
No overlap between the p orbitals: the nonbonding MO
40.
Resonance structures of the allyl cation, the allyl radical,
and the allyl anion
+ +
CH2 CH CH2 CH2 CH CH2 CH2 CH CH2
. .
CH2 CH CH2 CH2 CH CH2 CH2 CH CH2
- -
CH2 CH CH2 CH2 CH CH2 CH2 CH CH2
41.
The Molecular Orbitals of
1,3,5-Hexatriene
42.
Benzene has six molecular orbitals
44.
Benzene is unusually stable because of large
delocalization energies
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