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 – 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˜ 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

) ε

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 ( )δ

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’s 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’s* 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

‡
Ea 3–6 kcal mol�1
(13–26 kJ mol�1)

~
~

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).

��

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 ’ s R u l e

Cyclobutadiene:
antiaromatic

*Professor Erich Hückel (1896 – 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 “inside” 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