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Chapter 7
Electron Delocalization and Resonance

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07-00CO
Title
Benzene and Cyclohexane
Caption
Potential maps around ball-and-stick models and structures of benzene and cyclohexane.
Notes
Benzene has carbon–carbon sigma bonds with localized electrons and carbon–carbon pi bonds with delocalized electrons, whereas cyclohexane has only carbon–carbon sigma bonds with localized electrons.
Keywords
benzene, cyclohexane, localized, delocalized
07-01a-d
Title
Figure 7.1
Caption
Structure of benzene.
Notes
a. Shows the sigma-bonding framework of benzene b. Shows the p orbitals which form the delocalized pi-bonding system in benzene c. Shape of the pi-electron clouds above and below the plane of the ring in benzene d. Electrostatic potential map of benzene
Keywords
structure, benzene, delocalized, figure, 7.1
07-01-03UN
Title
Mythical Creatures and Resonance Hybrids
Caption
Analogy between a resonance hybrid and a rhinoceros.
Notes
The resonance contributors, like the unicorn and the dragon, are not real. Only the hybrid, like the rhinoceros, describes a real structure.
Keywords
unicorn, dragon, rhinoceros, resonance, hybrid
07-01-04
Title
Cyclooctatetraene
Caption
Structure and potential map of cyclooctatetraene.
Notes
The pi electrons in the double bonds of cyclooctatetraene are not delocalized because the p orbitals do not overlap.
Keywords
cylooctatetraene, pi, electrons, overlap, delocalized
07-06
Title
Resonance Energy
Caption
Hess' Law diagram showing how the resonance energy of benzene is calculated.
Notes
The difference between the heat released by hydrogenating three moles of C-C double bonds ("cyclohexatriene") and that released by hydrogenating one mole of benzene is called the resonance energy of benzene. Since less heat is released by hydrogenating benzene, benzene is more stable than "cyclohexatriene."
Keywords
resonance, energy, benzene, cyclohexatriene
07-07
Title
Resonance Energies of Oxyanions
Caption
Resonance energies of conjugate bases of carboxylic acids are greater than the resonance energies of carboxylic acids.
Notes
Since carboxylic acids and alcohols have similar stabilities whereas carboxylate ions have greater stabilities than alkoxide ions, carboxylic acids are stronger acids than alcohols. The oxyanion of a carboxylic acid is stabilized by resonance energy.
Keywords
resonance, energies, oxyanions, carboxylic, acids, alcohols, carboxylate, alkoxide
07-07-07T01
Title
Table 7.1 Approximate pKa values
Caption
Notes
Keywords
07-08
Title
Figure 7.8
Caption
Pi bonding and antibonding orbitals in ethene.
Notes
In-phase side-to-side overlap of the two p orbitals in the adjacent carbons in ethene results in a bonding pi molecular orbital, and out-of-phase overlap results in an antibonding MO.
Keywords
pi, bonding, antibonding, ethene, figure, 7.8
07-09
Title
Figure 7.9
Caption
Bonding and antibonding orbitals in 1,3-butadiene.
Notes
Four p atomic orbitals on connected carbons overlap in four different ways to give four molecular orbitals. Half of the molecular orbitals are bonding, and the other half are antibonding.
Keywords
bonding, antibonding, butadiene, figure, 7.9
07-09-01UN
Title
Symmetric and Fully Asymmetric Molecular Orbitals
Caption
Symmetric and fully asymmetric (or antisymmetric) molecular orbitals with mirrors situated through their centers.
Notes
Symmetric molecular orbitals are orbitals in which one half of the MO is a mirror image of the other half. Fully asymmetric MOs are orbitals in which the mirror image of one half of the MO has lobes with exactly opposite phases from the other half of the MO.
Keywords
symmetric, asymmetric, MO, mirror
07-09-02UN
Title
1,4-Pentadiene
Caption
Bonding MOs of 1,4-pentadiene.
Notes
1,4-Pentadiene has two independent pi bonds separated from one another by a methylene carbon which does not contribute a p orbital to the system. For this reason neither of the p orbitals which form the one pi system can interact with either of the p orbitals which form the other pi system. The pi electrons cannot delocalize through the methylene carbon.
Keywords
bonding, MO, pentadiene
07-10
Title
Figure 7.10
Caption
MOs of the allyl system and distribution of electrons in the allyl cation, anion, and radical.
Notes
The allyl system contributes three atomic p orbitals to produce three MOs. The most stable bonding MO has to be symmetric, with all lobes interacting constructively with one another. MOs alter between symmetric and antisymmetric as they increase in energy. In order for the nonbonding (intermediate-energy) MO of the allyl system to be antisymmetric, it must have a node through the central carbon.
Keywords
MO, allyl, distribution, electrons, figure, 7.10
07-11
Title
Figure 7.11
Caption
The six MOs of 1,3,5-hexatriene and electron occupancy of the neutral species.
Notes
The six p orbitals on the six connected carbons in 1,3,5-hexatriene all interact, producing six MOs. Three of the MOs are bonding, and three of the MOs are antibonding. The number of nodes in the MOs increases as their energy increases, and MOs alter between being symmetric and antisymmetric as their energy increases.
Keywords
MOs, hexatriene, figure, 7.11
07-12
Title
Figure 7.12
Caption
Lowest energy MO of benzene and energies and electron occupancy of the six MOs in benzene.
Notes
Benzene has MOs built from six p atomic orbitals. Half (three) of its MOs are bonding, and the other half are antibonding. Since benzene has an even number of MOs, it has no nonbonding MO. Systems with odd numbers of MOs have a nonbonding MO (ie., allyl). Since benzene can only accommodate three nodes before all of its contributing p orbitals interact destructively, only four different energy levels exist for benzene's six MOs (zero nodes, one node, two nodes, and three nodes). Therefore, four of benzene's MOs must occupy two energy levels in degenerate pairs.
Keywords
Figure, 7.12, benzene, figure, 7.12
07-13
Title
Figure 7.13
Caption
The six MOs of benzene showing lobe phases and relative energies.
Notes
As the energy of the MOs increases, the number of nodes increases and the net number of bonding interactions decreases. The mirror planes used to establish symmetric MOs and asymmetric MOs are the same as the nodal planes in benzene.
Keywords
benzene, lobe, phases, figure, 7.13
07-14
Title
Figure 7.14
Caption
A comparison of the energy levels of the pi molecular orbitals of ethene, 1,3-butadiene, 1,3,5-hexatriene, and benzene.
Notes
Note that benzene is more stable than 1,3,5-hexatriene. Benzene has more resonance energy than 1,3,5-hexatriene because of the additional connectivity of the pi system in benzene (which has one extra pair of connected carbons). The stabilization of 1,3,5-hexatriene over three isolated double bonds (i.e., three ethenes) is due to its resonance energy (ability of the double bonds to interact by being connected to the same pi system).
Keywords
molecular, orbitals, ethene, butadiene, hexatriene, benzene, figure, 7.14
07-07-07T01
Title
Table 7.1 Approximate pKa values
Caption
Notes
Keywords

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