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Chapter 16 Aromatic Compounds
Organic Chemistry II Fall 1999 Chapter 16 Aromatic Compounds benzene: C6H6 (1825, M. Faraday), liquid much less reactive than alkenes: 643 top substitution / reduction: 643 middle / bottom structure: 1866, Kekule 89 Figure 3.20 all the same bond length: 1.40 Å, 644 middle resonance energy: comparison to a reference heat of hydrogenation: 645 & 646 Figure 16.1 practice: Problem OrgChem-Chap16 Chapter 17
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Aromaticity & Antiaromaticity
Organic Chemistry II Fall 1999 Aromaticity & Antiaromaticity very stable (aromatic) very unstable (antiaromatic) molecular orbitals for cyclic conjugated molecules benzene vs cyclohexatriene: 647 Figure 16.2 resonance energy (aromaticity): DE cyclobutadiene: half-filled HOMOs, 649 bottom rectangular geometry & dimerization above 35 K: 650 mid. OrgChem-Chap16 Chapter 17
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Hückel (Aromaticity) Rule
Organic Chemistry II Fall 1999 Hückel (Aromaticity) Rule MOs for other cyclic conjugated molecules odd- & even-numbered rings: 648 Figure 16.3 to be aromatic: (4n+2) p electrons, 651 cyclic & fully conjugated & planar molecules antiaromatic: (4n) p electrons cyclopentadiene / cyclooctatetraene: non-aromatic (non-conjugation / non-planar), 651 middle & 652 bottom practice: Problem 16.3~16.7 OrgChem-Chap16 Chapter 17
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Heterocyclic Aromatic Compounds
Organic Chemistry II Fall 1999 Heterocyclic Aromatic Compounds furan/thiophene: two e- from O/S, 654 top pyrrole: two e- from N, 653 top less basic N & electron-rich ring: 653 bottom pyridine: basic & electron-poor, 654 bottom practice: 654 Practice 16.1 & OrgChem-Chap16 Chapter 17
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Polycyclic Aromatic Hydrocarbons
Organic Chemistry II Fall 1999 Polycyclic Aromatic Hydrocarbons naphthalene: ER = 61 kcal/mol, 656 top less stable than PhH (36) & different bond length anthracene / phenanthrene: 84 / 92 kcal/mol reactive center: 9/10-positions: 656 middle chrysene & benzo[a]pyrene: 656 middle PAH: carcinogens, 657 Focus On fullerene: C60 (one peak in 13C NMR) 663-4 OrgChem-Chap16 Chapter 17
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Aromaticity & Properties
Organic Chemistry II Fall 1999 Aromaticity & Properties NMR: direction of induced magnetic field, 658 anisotropic deshielding / shielding: benzene aromatic molecules: diamagnetic, 658 top antiaromatic molecules: paramagnetic, 658 bottom acidity/reactivity: stability of conjugate ions cyclopentadiene & 7-halocycloheptadiene: 661-2 1-iodocyclopentane vs 5-iodocyclopentadiene: 662 practice: , Problem OrgChem-Chap16 Chapter 17
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Aromaticity of [n]Annulenes & Ions
Organic Chemistry II Fall 1999 Aromaticity of [n]Annulenes & Ions cyclic conjugated olefins: n = No. of C atoms benzene: [6]annulene, cyclooctatetraene: [8]annulene [10], [14] & [18]annulene: aromatic, transannular strain: bridged structure for [10] & [14] cyclic ions: odd-membered rings, 661 cyclopropenyl cation cyclopentadienyl anion cyclopropenyl anion cyclopentadienyl cation aromatic (4n+2) antiaromatic (4n) OrgChem-Chap16 Chapter 17
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