Chapter 16 Aromatic Compounds

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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:  644-6 Problem 16.1-16.2 OrgChem-Chap16 Chapter 17

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

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:  649-652 Problem 16.3~16.7 OrgChem-Chap16 Chapter 17

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 &  655 16.8 OrgChem-Chap16 Chapter 17

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

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:  660-662, Problem 16.11-16.13 OrgChem-Chap16 Chapter 17

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,  659-660 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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