12.1 AROMATIC COMPOUNDS
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1 12 Arenes and Aromaticity estradiol
2 12.1 AROMATIC COMPOUNDS O O O O C 3 C 3 O O safrole (oil of sassafras) O methyl salicylate (oil of wintergreen) O vanillin (vanilla) C 3 N C 3 C 3 CC 2 ibuprofen C C 3 CO 2 O acetaminophen O CO 2 O C 2 O O O C 3 O 2 N C C N C CCl 2 O chloramphenicol aspirin naphthalene anthracene phenanthrene
3 12.2 TE COVALENT STRUCTURE OF BENZENE or ypothetical bicyclic benzene structure with nonequivalent secondary and tertiary hydrogen atoms. Br Br Br Only one bromobenzene isomer exists. Therefore, all six hydrogen atoms in benzene must be equivalent.
4 Resonance Theory and Benzene equivalent contributing structures for the resonance hybrid of benzene
5 Resonance Energy 3 2 Ni o = -208 kj mole -1 benzene cyclohexane 2 Ni o = -120 kj mole -1 cyclohexene cyclohexane 2 2 Ni o = -231 kj mole -1 1,3-cyclohexadiene cyclohexane
6 Figure 12.1 eats of ydrogenation and the Resonance Stabilization of Benzene The relative energies of cyclohexene, 1,3-cyclohexadiene, 1,3,5-cyclohexatriene, and benzene and their heats of hydrogenation to form cyclohexane in kj mole -1. "cyclohexatriene" "3 2 " kj mole -1 resonance energy 152 kj mole- 1 1,3-cyclohexadiene Energy kj mole -1 cyclohexene kj mole kj mole -1 cyclohexane
7 12.3 TE ÜCKEL RULE 1. To be aromatic, a molecule must be cyclic. 2. The molecule must be planar. 3. The ring must contain only sp2-hybridized atoms that can form a delocalized system of π molecular orbitals. 4. The number of π electrons in the delocalized π system must equal 4n 2, where n is an integer.
8 Nonaromatic and Antiaromatic Cyclic Polyenes cyclobutadiene cyclooctatetraene antiaromatic nonaromatic 4n π electrons tub conformation of cyclooctatetraene
9 Aromatic Ions, I 1,3,5-cycloheptatriene cycloheptatrienyl cation resonance structures of cycloheptatrienyl cation
10 Aromatic Ions, II cyclopentadiene B B cyclopentadiene cyclopentadienyl anion pk a = 36 1,3,5-cycloheptatriene 1,3,5-cycloheptatrienyl anion
11 Aromatic Ions, III 2 K -2 2 K cyclooctatetraenyl dianion Cyclopropene FSO 3 -SbF 5 -SO 2 "Magic acid" Cyclopropenium ion
12 12.4 MOLECULAR ORBITALS OF AROMATIC AND ANTIAROMATIC COMPOUNDS π 6 Energy E = 0 π 4 π 5 π 2 π 3 π 1 Figure 12.2 Relative energies of benzene s MOs and their occupancy.
13 Figure 12.3 Molecular Orbitals in Benzene and Their Relative Energies Relative energies of benzene s MOs and their occupancy. The lowest energy MO, π1 is symmetric, and has no nodal planes. Orbitals π2 and π3, have the same energy. They are antisymmetric, and have one nodal plane. Each of the bonding MOs has two electrons. MOs π4, π5 and π6 have energy greater than zero. They are antibonding, and contain no electrons. π6 π1 π5 π2 E=0 π4 π3
14 Figure 12.4 Energy Levels and Occupancies of the Molecular Orbitals of Cycloheptatrienyl Cation The cylcoheptatrienyl cation contains 4n 2 = 6 π electrons. Therefore, it is aromatic. All three bonding π MOs are fully occupied, and the antibonding orbitals are empty. π 6 π 7 Energy E = 0 π 4 π 2 π 5 π 3 π 1
15 Figure 12.5 Energy Levels and Occupancies of the Molecular Orbitalsof Cyclopentadienyl Anion The cylcopentadienyl anion contains 4n 2 = 6 π electrons. Therefore, it is aromatic. All three bonding π MOs are fully occupied, and the antibonding orbitals are empty. π 4 π 5 Energy E = 0 π 2 π 3 π 1
16 Figure 12.6 Energy Levels and Occupancies of the Molecular Orbitals of Cycloheptatrienyl Anion The cylcoheptatrienyl anion contains 4n = 8 π electrons, where n = 2. Therefore, it is an unstable antiaromatic species. All three bonding π MOs are fully occupied, but the antibonding π4, π5 orbitals each have one electron. π 6 π 7 Energy E = 0 π 4 π 2 π 5 π 3 π 1
17 12.5 ETEROCYCLIC AROMATIC COMPOUNDS N N O S pyridine pyrrole furan thiophene
18 12.7 POLYCYCLIC AROMATIC COMPOUNDS naphthalene anthracene phenanthrene
19 Figure 12.8 Resonance Structures of Naphthalene The ten π electrons of naphthalene are delocalized over both rings. Three resonance forms can be written using localized double bonds. Less stable resonance form 2p orbitals in naphthalene Most stable resonance form
20 Carcinogenic Aromatic Compounds 3,4-benzpyrene 1,2-benzanthracene 1,2,5,6-dibenzanthracene 3,4-benzpyrene
21 Figure 12.7 Relative Energy Levels of the Cyclopropenium Ion The cyclopropenium cation contains 4n 2 = 2 π electrons for n =0. Therefore, it is aromatic. Its two bonding π electrons occupy π1, and the antibonding π2, π3 orbitals are empty. π 2 π 3 Energy E = 0 π 1
22 Figure 12.8 Relative Energy Levels of Cyclobutadiene Cyclobutadiene is a 4n π antiaromatic molecule, for n =1. It has two bonding electrons in π1, and one electron in each of the nonbonding π2, π3 orbitals. It is a very unstable diradical. π 4 Energy E = 0 π 2 π 2 π 1
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