1. 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

2. Localized Versus Delocalized Electrons

3. Benzene A planar molecule Has six identical carbon–carbon bonds
Each p electron is shared by all six carbons
The p 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 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

8. 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

9. Resonance contributors are obtained by moving p
electrons toward a positive charge:
resonance hybrid
resonance hybrid
resonance hybrid

10. Moving p electrons toward a p bond

11. Moving a nonbonding pair of electrons toward a p bond

12. Resonance Structures for the Allylic
Radical and for the Benzyl Radical

13. 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

14. The Difference Between Delocalized and Localized Electrons

15. delocalized electrons
an sp3 hybridized carbon
cannot accept electrons
delocalized electrons
localized electrons

16. Resonance contributors with separated charges are less stable
more
stable
equally stable

17. Electrons always move toward the more electronegative
atom

18. 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

19. Resonance Energy
A measure of the extra stability a compound gains from
having delocalized electrons

20. Benzene is stabilized by electron delocalization

21. 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

22. Resonance-Stabilized Cations

23. Relative Stabilities of Allylic and Benzylic Cations

24. Relative Stabilities of Carbocations

25. Relative Stabilities of Radicals

26. Some Chemical Consequences of Electron Delocalization

27. 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)

28. Increased resonance stabilization of the conjugated base

29. Account for the Acidity of Phenol by Resonance Stabilization

30. Account for the Acidity of Protonated
Aniline by Resonance Stabilization

31. A Molecular Orbital Description of Stability
Bonding MO: constructive (in-phase) overlap
Antibonding MO: destructive (out-of-phase) overlap

32. The Molecular Orbitals of
1,3-Butadiene

33. 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

34. 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

35. Consider the p molecular orbitals of 1,4-pentadiene:
This compound has four p electrons that are completely
separated from one another

36. The Molecular Orbitals of the Allyl System
y2 is the nonbonding MO
No overlap between the p orbitals: the nonbonding MO

37. The Molecular Orbitals of 1,3,5-Hexatriene

38. Benzene has six p molecular orbitals

40. Benzene is unusually stable because of large
delocalization energies