1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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