Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)


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benzene (Section 15-2 and Figure 15-6). Thus, the added four p electrons enter into 
effi cient overlap with those of the attached benzene ring. In fact, it is possible to draw several 
resonance forms.
Resonance Forms of Naphthalene
Alternatively, the continuous overlap of the 10 p orbitals and the fairly even distribution of 
electron density can be shown as in Figure 15-14.
According to these representations, the structure of naphthalene should be symmetric, with 
planar and almost hexagonal benzene rings and two perpendicular mirror planes bisecting the 
 C h a p t e r 1 5 691
molecule. X-ray crystallographic measurements confi rm this prediction (Figure 15-15). The 
C \u2013 C bonds deviate only slightly in length from those in benzene (1.39 Å), and they are clearly 
different from pure single (1.54 Å) and double bonds (1.33 Å).
Further evidence of aromaticity is found in the 1H NMR spectrum of naphthalene (Fig-
ure 15-16). Two symmetric multiplets can be observed at d 5 7.49 and 7.86 ppm, character-
istic of aromatic hydrogens deshielded by the ring-current effect of the p-electron loop (see 
Section 15-4, Figure 15-9). Coupling in the naphthalene nucleus is very similar to that in sub-
stituted benzenes: Jortho 5 7.5 Hz, Jmeta 5 1.4 Hz, and Jpara 5 0.7 Hz. The 
13C NMR spectrum 
shows three lines with chemical shifts that are in the range of other benzene derivatives 
(see margin). Thus, on the basis of structural and spectral criteria, naphthalene is aromatic.
Figure 15-13 Extended p con-
jugation in naphthalene is manifest 
in its UV spectrum (measured in 
95% ethanol). The complexity and 
location of the absorptions are 
typical of extended p systems.
A B
Figure 15-14 (A) Orbital picture 
of naphthalene, showing its 
 extended overlap of p orbitals. 
(B) Electrostatic potential map, 
 revealing its electron density distri-
bution over the 10 carbon atoms.
1.42 Å
1.37 Å
1.39 Å
121°
1.40 Å
Figure 15-15 The molecular 
structure of naphthalene. The bond 
angles within the rings are 1208.
13C NMR Data of
Naphthalene (ppm)
126.5
128.5
134.4
Exercise 15-10
A substituted naphthalene, C10H8O2, gave the following spectral data: 
1H NMR d 5 5.20 (broad s, 
2 H), 6.92 (dd, J 5 7.5 Hz and 1.4 Hz, 2 H), 7.00 (d, J 5 1.4 Hz, 2 H), and 7.60 (d, J 5 7.5 Hz, 
2 H) ppm; 13C NMR d 5 107.5, 115.3, 123.0, 129.3, 136.8, and 155.8 ppm; IR n\u2dc 5 3300 cm21 
(broad). What is its structure? [Hints: Review the values for Jortho, meta, para in benzene (Section 15-4). 
The number of NMR peaks is less than maximum.]
1 5 - 5 P o l y c y c l i c A r o m a t i c H y d r o c a r b o n s
200 220 240 260 280 300 320 340
Wavelength (nm)
A
bs
or
ba
nc
e 
(lo
g 
) \u3b5
5
4
3
2
1
UV
692 C h a p t e r 1 5 B e n z e n e a n d A r o m a t i c i t y
Most fused benzenoid hydrocarbons are aromatic
These aromatic properties of naphthalene hold for most of the other polycyclic benzenoid 
hydrocarbons. It appears that the cyclic delocalization in the individual benzene rings is not 
signifi cantly perturbed by the fact that they have to share at least one p bond. Linear and 
angular fusion of a third benzene ring onto naphthalene result in the systems anthracene 
and phenanthrene. Although isomeric and seemingly very similar, they have different 
 thermodynamic stabilities: Anthracene is about 6 kcal mol21 (25 kJ mol21) less stable than 
phenanthrene, even though both are aromatic. Enumeration of the various resonance forms 
of the molecules explains why. Anthracene has only four, and only two contain two fully 
aromatic benzene rings (red in the structures shown here). Phenanthrene has fi ve, three of 
which incorporate two aromatic benzenes, one even three benzenes.
Resonance in Anthracene
Resonance in Phenanthrene
Figure 15-16 The 300-MHz 
1H NMR spectrum of naphthalene 
reveals the characteristic 
deshielding due to a p-electronic 
ring current.
Exercise 15-11
Draw all the possible resonance forms of tetracene (naphthacene, Section 15-5). What is the 
maximum number of completely aromatic benzene rings in these structures?
1H NMR
456789 3 2 1 0
ppm ( )\u3b4
4 H 4 H
(CH3)4Si
7.87.9 7.47.6 7.5
H
H
H
H
H
H
H
H
ppm ppm
 C h a p t e r 1 5 693
In Summary The physical properties of naphthalene are typical of an aromatic system. 
Its UV spectrum reveals extensive delocalization of all p electrons, its molecular 
structure shows bond lengths and bond angles very similar to those in benzene, and its 
1H NMR spectrum reveals deshielded ring hydrogens indicative of an aromatic ring cur-
rent. Other polycyclic benzenoid hydrocarbons have similar properties and are consid-
ered aromatic.
15-6 Other Cyclic Polyenes: Hückel\u2019s Rule
The special stability and reactivity associated with cyclic delocalization is not unique to 
benzene and polycyclic benzenoids. Thus, we shall see that other cyclic conjugated polyenes 
can be aromatic, but only if they contain (4n 1 2) p electrons (n 5 0, 1, 2, 3, . . .). In 
contrast, 4n p circuits may be destabilized by conjugation, or are antiaromatic. This pattern 
is known as Hückel\u2019s* rule. Nonplanar systems in which cyclic overlap is disrupted suf-
fi ciently to impart alkenelike properties are classifi ed as nonaromatic. Let us look at some 
members of this series, starting with 1,3-cyclobutadiene.
1,3-Cyclobutadiene, the smallest cyclic polyene, is antiaromatic
1,3-Cyclobutadiene, a 4n p system (n 5 1), is an air-sensitive and extremely reactive 
molecule in comparison to its analogs 1,3-butadiene and cyclobutene. Not only does the 
molecule have none of the attributes of an aromatic molecule like benzene, it is actually 
destabilized through p overlap by more than 35 kcal mol21 (146 kJ mol21) and therefore 
is antiaromatic. As a consequence, its structure is rectangular, and the two diene forms 
represent isomers, equilibrating through a symmetrical transition state, rather than reso-
nance forms.
Transition state
1,3-Cyclobutadiene Is Unsymmetrical
\u2021
Ea 3\u20136 kcal mol\ufffd1
(13\u201326 kJ mol\ufffd1)
~
~ 
Free 1,3-cyclobutadiene can be prepared and observed only at very low temperatures. 
The reactivity of cyclobutadiene can be seen in its rapid Diels-Alder reactions, in which it 
can act as both diene (shown in red) and dienophile (blue).
\ufffd\ufffd
O
O
H
OCH3O
CH3O
CH3O
H
OCH3O
1 5 - 6 O t h e r C y c l i c P o l y e n e s : H ü c k e l \u2019 s R u l e
Cyclobutadiene:
antiaromatic
*Professor Erich Hückel (1896 \u2013 1984), University of Marburg, Germany.
MODEL BUILDING
694 C h a p t e r 1 5 B e n z e n e a n d A r o m a t i c i t y
Substituted cyclobutadienes are less reactive because of steric protection, particularly if 
the substituents are bulky; they have been used to probe the spectroscopic features of the 
cyclic system of four p electrons. For example, in the 1H NMR spectrum of 1,2,3-tris(1,1-
dimethylethyl)cyclobutadiene (1,2,3-tri-tert-butylcyclobutadiene), the ring hydrogen reso-
nates at d 5 5.38 ppm, at much higher fi eld than expected for an aromatic system. This 
and other properties of cyclobutadiene show that it is quite unlike benzene.
CHEMICAL HIGHLIGHT 15-2
Juxtaposing Aromatic and Antiaromatic Rings in Fused Hydrocarbons
In these hydrocarbons, the contribution of antiaroma-
ticity due to the cyclobutadiene rings is reduced by enforcing 
bond alternation in the neighboring benzene rings, thus 
minimizing the relative importance of resonance forms in 
which a double bond is \u201cinside\u201d the four-membered ring. 
This effect is illustrated by the specifi c resonance forms of 
A
Biphenylene
B
Angular
[3]phenylene
C
C3-symmetric
[4]phenylene
Resonance Forms of Biphenylene
Major Intermediate Intermediate Minor
Exercise 15-12
1,3-Cyclobutadiene dimerizes at