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