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12 Arenes and Aromaticity
estradiol
12.1 AROMATIC COMPOUNDS
O
O safrole(oil of sassafras)
OH
OCH3
O
HO
CH3OH
O
methyl salicylate(oil of wintergreen)
vanillin (vanilla)
chloramphenicol
C
H
CH3CHCH2
CH3
CO2N C NH
CH2OHOH
C CHCl2
O
CH3
CO2H
ibuprofen
HH
N CH3
O
acetaminophen
CO2H
O CH3
O
aspirin
HO
H
naphthalene anthracene phenanthrene
12.2 THE COVALENT STRUCTURE OF BENZENE
or
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
HH
H
Hypothetical bicyclic benzene structure with nonequivalent secondary and tertiary hydrogen atoms.
Br
H
H
HH
H H
H
Br
HH
H
Only one bromobenzene isomer exists. Therefore, all six hydrogen atoms in benzene must be equivalent.
Br
Resonance Theory and Benzene
equivalent contributing structures for the resonance hybrid of benzene
H
H
H
H
H
H
H
H
H
H
H
H
Resonance Energy
+ 3H2
benzene cyclohexane
∆Ho = -208 kJ mole-1Ni
+ H2
cyclohexene cyclohexane
∆Ho = -120 kJ mole-1Ni
+ 2H2
1,3-cyclohexadiene cyclohexane
∆Ho = -231 kJ mole-1Ni
Figure 12.1 Heats of Hydrogenation and the Resonance Stabilization of Benzene!e 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.
1,3-cyclohexadiene
- 126 kJ mole-1
- 208 kJ mole-13H2
2 H2
- 360 kJ mole-1En
ergy
- 231 kJ mole-1
H2
"3 H2"
cyclohexane
cyclohexene
"cyclohexatriene"
152 kJ mole-1resonance energy
12.3 THE HÜCKEL RULE
1. To be aromatic, a molecule must be cyclic.2. !e molecule must be planar. 3. !e ring must contain only sp2-hybridized atoms that can form a delocalized system of π molecular orbitals. 4. !e number of π electrons in the delocalized π system must equal 4n + 2, where n is an integer.
Nonaromatic and Antiaromatic Cyclic Polyenes
tub conformation of cyclooctatetraene
cyclobutadiene cyclooctatetraeneantiaromatic nonaromatic
4n π electrons
HH
1,3,5-cycloheptatriene
H
cycloheptatrienyl cation
H
resonance structures of cycloheptatrienyl cation
H H H
HHH
Aromatic Ions, I
Aromatic Ions, II
cyclopentadiene
HH
cyclopentadiene
HH
B +
cyclopentadienyl anion
H
BH+
HH
1,3,5-cycloheptatriene
H
1,3,5-cycloheptatrienyl anion
+ H+ pKa = 36
Aromatic Ions, III
+2 K -2 + 2 K+
cyclooctatetraenyl dianion
FSO3H-SbF5-SO2
"Magic acid"H
Cyclopropene Cyclopropenium ion
12.4 MOLECULAR ORBITALS OF AROMATIC AND ANTIAROMATIC COMPOUNDS
Figure 12.2 Relative energies of benzene’s MOs and their occupancy.
π3
π5*π4
*
π2
π6*
π1
Ener
gy
E = 0↑↓
↑↓
↑↓
Figure 12.3 Molecular Orbitals in Benzene and Their Relative EnergiesRelative energies of benzene’s MOs and their occupancy. !e lowest energy MO, π1 is symmetric, and has no nodal planes. Orbitals π2 and π3, have the same energy. !ey 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. !ey are antibonding, and contain no electrons.
π2
+
–
π4
+
–
+
–
+
–
+
–
+
–
π3
π1
+
–
+
–
+
–
+
–
+
– +
–
+
–+
–+
– +
–
+
– +
–
+
–+
–
+
–
+
–
+
–
+
–
+
–
+
–π5
+
– +
–
+
–+
–
+
– +
–
π6
E = 0
Figure 12.4 Energy Levels and Occupancies of the Molecular Orbitals of Cycloheptatrienyl Cation!e cylcoheptatrienyl cation contains 4n + 2 = 6 π electrons. !erefore, it is aromatic. All three bonding π MOs are fully occupied, and the antibonding orbitals are empty.
π3
π5*π4
*
π2
π6*
π1
Ener
gy
E = 0
↑↓↑↓↑↓
π7*
Figure 12.5 Energy Levels and Occupancies of the Molecular Orbitalsof Cyclopentadienyl Anion!e cylcopentadienyl anion contains 4n + 2 = 6 π electrons. !erefore, it is aromatic. All three bonding π MOs are fully occupied, and the antibonding orbitals are empty.
π3
π5*π4
*
π2
π1
Ener
gy
E = 0 ↑↓
↑↓
↑↓
Figure 12.6 Energy Levels and Occupancies of the Molecular Orbitals of Cycloheptatrienyl Anion!e cylcoheptatrienyl anion contains 4n = 8 π electrons, where n = 2. !erefore, it is an unstable antiaromatic species. All three bonding π MOs are fully occupied, but the antibonding π4*, π5* orbitals each have one electron.
π3
π5*π4
*
π2
π6*
π1
Ener
gy
E = 0
↑↓↑↓↑↓
π7*
↑ ↑
12.5 HETEROCYCLIC AROMATIC COMPOUNDS
NN O S
Hpyridine pyrrole furan thiophene
12.7 POLYCYCLIC AROMATIC COMPOUNDS
naphthalene anthracene phenanthrene
Figure 12.8 Resonance Structures of Naphthalene!e ten π electrons of naphthalene are delocalized over both rings. !ree resonance forms can be written using localized double bonds.
2p orbitals in naphthalene
Most stable resonance form
Less stable resonance form
Carcinogenic Aromatic Compounds
1,2-benzanthracene 1,2,5,6-dibenzanthracene
3,4-benzpyrene
3,4-benzpyrene
Figure 12.7 Relative Energy Levels of the Cyclopropenium Ion!e cyclopropenium cation contains 4n + 2 = 2 π electrons for n =0. !erefore, it is aromatic. Its two bonding π electrons occupy π1, and the antibonding π2*, π3* orbitals are empty.
π3*π2
*
π1
Ener
gy E = 0
↑↓
Figure 12.8 Relative Energy Levels of CyclobutadieneCyclobutadiene 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.
π2
π1
Ener
gy
E = 0
↑↓
↑ ↑
π4*
π2
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