15 Handouts - Organic Chemistry, 5th Edition L. G. Wade,...

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Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy Organic Chemistry , 5 th Edition L. G. Wade, Jr.
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Chaper 15 2 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 (shared by one carbon). Example: 1,2-pentadiene. => (Review)
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Chaper 15 3 Examples
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Chaper 15 4 Resonance Energy • Heat of hydrogenation for trans -1,3- pentadiene is less than expected. Δ H for 1-pentene is 30.0 kcal/mol and for trans -2-pentene is 27.4 kcal/mol, so expect 57.4 kcal for trans -1,3-pentadiene. • Actual Δ H is 53.7 kcal, so the conjugated diene is more stable. • Difference, (57.4 – 53.7) 3.7 kcal/mol, is the resonance energy . =>
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Chaper 15 5 Relative Stabilities twice 1-pentene more substituted =>
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Chaper 15 6 Structure of 1,3-Butadiene • Most stable conformation is planar. • Single bond is shorter than 1.54 Å. • Electrons are delocalized over molecule. =>
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Chaper 15 7 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 8 1,3-Butadiene π MOs • We are going to construct the π molecular orbitals of 1,3-butadiene. • There are four 2p orbitals (one from each carbon. • How many π MOs will result? •A n s w e r : 4
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Chaper 15 9 π 1 MO for 1,3-Butadiene • Lowest energy. • All bonding interactions. • Electrons are delocalized over four nuclei. => There are 3 more MOs for butadiene. Generally, for n p orbitals there are n pi MOs.
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Chaper 15 10 π 2 MO for 1,3-Butadiene • 2 bonding interactions • 1 antibonding interaction • A bonding MO =>
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Chaper 15 11 π 3 * MO for 1,3-Butadiene • Antibonding MO • Empty at ground state • Two nodes =>
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Chaper 15 12 π 4 * MO for 1,3-Butadiene • All antibonding interactions.
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15 Handouts - Organic Chemistry, 5th Edition L. G. Wade,...

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