multiple-center mo theory like all mos, the following general rules apply to multiple-center mos:
TRANSCRIPT
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Multiple-Center MO Theory
• Like all MOs, the following general rules apply to multiple-center MOs:
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Multiple-Center p MOs of the Allyl Cation
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Complete MO Energy Diagram of the Allyl Cation
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Buta-1,3-diene MOs
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Complete MO Energy Diagram of Buta-1,3-diene
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Benzene Structure
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Fig. 21-2, p. 855
Benzene MOs
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Fig. 21-3, p. 856
Benzene MO Diagram
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Heats of Hydrogenation
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Huckel Rules • From his observations on the structural similarities of
compounds in each of these classes, Erich Huckel (1896–1980), a German physicist and physical chemist, proposed what are now known as the Hu ckel rules for aromaticity:
• Alternatively, the Huckel numbers correspond to 4n + 2, where n is any integer >0, and the anti-Huckel numbers correspond to 4n, where n is any integer >1.
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Cyclobutadiene’s π Mos
• The lowest-energy MO (π1) has no nodal planes perpendicular to the bonding axes, and each additional nodal plane raises the energy of the other MOs.
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MO Picture of Cyclobutadiene
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Fig. 21-6, p. 859
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Aromatic Hydrocarbon: A cyclic, planar, fully conjugated hydrocarbon with 4n+2 pi electrons (2, 6, 10, 14, 18, etc). An aromatic hydrocarbon is especially stable relative to an open-
chain fully conjugated hydrocarbon of the same number of carbon atoms.
Nonaromatic Hydrocarbon: A cyclic, non-planar, fully conjugated hydrocarbon with 4n+2 pi electrons. A nonaromatic hydrocarbon has similar stability to its open-chain
fully conjugated hydrocarbon of the same number of carbon atoms.
Antiaromatic hydrocarbon: A monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...). An antiaromatic hydrocarbon is especially unstable relative to an open-
chain fully conjugated hydrocarbon of the same number of carbon atoms.
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Cycloocta-1,3,5,7-tetraene
• According to Huckel’s rules, [8]annulene should be antiaromatic if it is planar.
• Because of the instability associated with antiaromaticity, however, [8]annulene resists planarity.
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Aromaticity and Multiple Rings
• Compounds with two or more rings can also be aromatic.
• These molecules are aromatic, and as a class are called polycyclic aromatic hydrocarbons (PAHs).
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Other Aromatics
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Heterocyclic Aromatic Compounds
• Heterocyclic aromatic compounds include pyridine, pyrrole, and furan.
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Fig. 21-10, p. 864
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Fig. 21-11, p. 864
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p. 864
Biologically Relevant Heterocycles
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Aromaticity and DNA
• Aromaticity affects the structure and properties DNA.