1. Polycyclic Aromatic Hydrocarbons
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Polycyclic Aromatic Hydrocarbons
Molecules containing several fused benzene rings are called polycyclic benzenoid or polycyclic aromatic hydrocarbons (PAHs).
Common names are used for these systems, since there is no simple naming system for them.
The series of linearly fused benzene rings is called the acenes.
Angular fusion (“annulation”) results in phenanthrene.
Quaternary carbons are numbered using the preceding carbon in the sequence followed by a letter indicating its distance to the preceding carbon.

2. Naphthalene is aromatic: a look at spectra.
Naphthalene is a colorless crystalline material with a melting point of 80o C.
The UV spectrum of naphthalene indicates an extended, conjugated system with peaks at wavelengths as long as 320 nm.

3. The electrons in naphthalene are more delocalized than in benzene and several resonance structures can be drawn:
The overlap of the 10 p orbitals results in a fairly even distribution of electron density.

4. The 1H NMR spectrum of naphthalene shows two symmetric multiples at  = 7.49 and 786 ppm.
These peak positions are characteristic of ring-current deshielded aromatic hydrogens.
Coupling constants in the naphthalene nucleus are similar to those in substituted benzenes:
Jortho = 7.5 Hz
Jmeta = 1.4 Hz
Jpara = 0.7 Hz

5. The 13C NMR spectrum shows three lines with chemical shifts in the range of other benzene derivatives:

6. Most fused benzenoid hydrocarbons are aromatic.
Linear and angular fusion of a third benzene ring onto naphthalene result in anthracene and phenanthrene.
Anthracene is about 6 kcal mol-1 less stable than phenanthrene due to differences in resonance stabilization.

7. Other Cyclic Polyenes: Hückel’s Rule
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Other Cyclic Polyenes: Hückel’s Rule
Cyclic conjugated polyenes can be aromatic as long as they contain 4n+2  electrons (n=0,1,2,…).
Cyclic polyenes containing 4n  electrons may be destabilized by conjugation, or are antiaromtic.
These observations are known as Hückel’s rule.
Nonplanar cyclic systems in which p-orbital overlap is disrupted sufficiently to impart alkenelike properties are classified as nonaromatic.

8. 1,3-Cyclobutadiene, the smallest cyclic polyene, is antiaromatic.
1,3-Cyclobutadiene is a 4n  system and is antiaromatic.
It is air-sensitive and extremely reactive (compared to 1,3-butadiene or cyclobutene).
It is destabilized through  overlap by more than 35 kcal mol-1.
Its structure is rectangular and it exists as two isomers, equilibrating through a symmetrical transition state, rather than resonance forms.

9. Free 1,3-cyclobutadiene can be prepared and observed only at very low temperatures.
Cyclobutadiene can act as either a diene or a dienophile in its rapid Diels-Alder reactions:
Substituted cyclobutadienes are less reactive and have been used to study the spectroscopic features of the 4  electron cyclic system.
In 1,2,3-tris(1,1-dimethylethyl)cyclobutadiene, the ring hydrogen resonates at  = 5.38 ppm, much higher than expected for an aromatic system.

10. 1,3,5,7-Cyclooctatetraene is non-planar and non-aromatic.
1,3,5,7-cyclooctatetraene is a 4n  system and is antiaromatic.
It can be made by a nickel-catalyzed cyclotetramerization of ethyne.
It is a yellow liquid (b.p. 152oC), is stable while cold, but polymerizes when heated.
It is oxidized by air, catalytically hydrogenated to cyclooctane, and subject to electrophilic additions and to cycloadditions.

11. The 1H NMR spectrum shows a sharp singlet at  = 5
The 1H NMR spectrum shows a sharp singlet at  = 5.68 ppm, typical of an alkene.
The molecular structure of cyclooctatetraene is non-planar and tub shaped. The double bonds are nearly orthogonal and are not conjugated.

12. Only cyclic conjugated polyenes containing 4n+2  electrons are aromatic.
An alternative naming system for completely conjugated monocyclic hydrocarbons, (CH)n, is [N]annulene, in which N is the ring size.
The system 1,3,5,7,9,11,13,15,17-cyclooctadecanonaene, or [18]annulene, contains 18  electrons and is aromatic (4n+2, n=4).
[18]annulene is fairly planar with little alternation of single and double bonds. It is relatively stable and undergoes electrophilic aromatic substitution.
It can be described by a set of 2 resonance forms.

13. Hückel’s rule (2n+2) in cyclical conjugated polyenes can be explained by the following:
The p orbitals mix to give an equal number of  molecular orbitals.
All levels are composed of degenerate pairs of orbitals except for the lowest bonding and highest antibonding orbitals.
A closed shell system (aromatic) is possible only if all of the bonding orbitals are fully occupied, 4n+2  electrons.
In the case of 4n  electrons there will always be a pair of singly occupied orbitals, an unfavorable electronic arrangement.

14. Hückel’s Rule and Charged Molecules
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Hückel’s Rule and Charged Molecules
The cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic.
1,3-Cyclopentadiene is unusually acidic because the anion resulting from deprotonation contains a delocalized, aromatic system of six  electrons:
The cyclopentadienyl cation (four  electrons) can only be produced at low temperatures and is highly reactive.

15. If 1,3,5-cycloheptatriene is treated with bromine, a stable salt is formed containing the cycloheptatrienyl cation, an aromatic, 6  electron system.
The cycloheptatrienyl cation is remarkably unreactive for a carbocation, as expected for an aromatic system.
The cycloheptatrienyl anion, on the other hand, is antiaromatic (8  electrons) and has a much lower acidity (pKa = 39) than cyclopentadiene (pKa ~ 16).