文档介绍：Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy
Jo Blackburn
Richland College, Dallas, TX
Dallas munity College District
ã 2003, Prentice Hall
Organic Chemistry, 5th Edition L. G. Wade, Jr.
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Chaper 15
Definitions
Conjugated double bonds are separated by one single bond. Example: 1,3-pentadiene.
Isolated double bonds are separated by two or more single bonds. 1,4-pentadiene.
Cumulated double bonds are on adjacent carbons. Example: 1,2-pentadiene. =>
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Chaper 15
Resonance Energy
Heat of hydrogenation for trans-1,3-pentadiene is less than expected.
H for 1-pentene is kcal/mol and for trans-2-pentene is kcal/mol, so expect kcal for trans-1,3-pentadiene.
Actual H is kcal, so the conjugated diene is more stable.
Difference, ( – ) kcal/mol, is the resonance energy. =>
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Chaper 15
Relative Stabilities
twice 1-pentene
more substituted
=>
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Chaper 15
Structure of 1,3-Butadiene
Most stable conformation is planar.
Single bond is shorter than Å.
Electrons are delocalized over molecule.
=>
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Chaper 15
Constructing Molecular Orbitals
Pi molecular orbitals are the sideways overlap of p orbitals.
p orbitals have 2 lobes. Plus (+) and minus (-) indicate the opposite phases of the wave function, not electrical charge.
When lobes overlap constructively, (+ and +, or - and -) a bonding MO is formed.
When + and - lobes overlap, waves cancel out and a node forms; antibonding MO. =>
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Chaper 15
1 MO for 1,3-Butadiene
Lowest energy.
All bonding interactions.
Electrons are delocalized over four nuclei. =>
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Chaper 15
2 MO for 1,3-Butadiene
2 bonding interactions
1 antibonding interaction
A bonding MO =>
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Chaper 15
3* MO for 1,3-Butadiene
Antibonding MO
Empty at ground state
Two nodes =>
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Chaper 15
4* MO for 1,3-Butadiene
All antibonding interactions.
Highest energy.
Vacant at ground state. =>
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Chaper 15