Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)

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be some symmetry to the molecule.
• The IR spectrum reveals the presence of Caromatic2H units (n˜ 5 3030 cm21), and the signal at
n˜ 5 813 cm21 indicates a para disubstituted benzene.
• Finally, the electronic spectrum shows the presence of a conjugated system, obviously a benzene
ring.
• Putting all of this information together suggests a benzene ring with two substituents, a methyl
and a 1-methylethyl group. The 13C NMR spectrum rules out ortho or meta disubstitution, leaving
only 1-methyl-4-(1-methylethyl)benzene as the solution (see margin). 1-Methyl-4-(1-methylethyl)benzene

(CH3)2CH
A

A
CH3

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

688 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

CHEMICAL HIGHLIGHT 15-1

The Allotropes of Carbon: Graphite, Diamond, and Fullerenes

*Professor Robert F. Curl (b. 1933), Rice University, Houston,
Texas; Professor Harold W. Kroto (b. 1939), University of Sussex,
England; Professor Richard E. Smalley (1943 – 2005), Rice University,
Houston, Texas.
†Richard Buckminster Fuller (1895 – 1983), American architect,
inventor, and philosopher.

Elements can exist in several forms, called allotropes,
depending on conditions and modes of synthesis. Thus,
elemental carbon can arrange in more than 40 confi gura-
tions, most of them amorphous (i.e., noncrystalline), such
as coke (Sections 3-3 and 13-10), soot, carbon black (as
used in printing ink), and activated carbon (as used in air
and water fi lters). You probably know best two crystalline
modifi cations of carbon: graphite and diamond. Graphite,
the most stable carbon allotrope, is a completely fused
polycyclic benzenoid p system, consisting of layers
arranged in an open honeycomb pattern and 3.35 Å apart.
The fully delocalized nature of these sheets (all carbons
are sp2 hybridized) gives rise to their black color and
conductive capability. Graphite’s lubricating property is
the result of the ready mutual sliding of its component
planes. The “lead” of pencils is graphitic carbon, and the

black pencil marks left on a sheet of paper consist of
rubbed-off layers of the element.

In the colorless diamond, the carbon atoms (all sp3
hybridized) form an insulating network of cross-linked
cyclohexane chair conformers. Diamond is the densest and
hardest (least deformable) material known. It is also less
stable than graphite, by 0.45 kcal/g C atom, and transforms
into graphite at high temperatures or when subjected to
high-energy radiation, a little-appreciated fact in the
jewelry business.

A spectacular discovery was made in 1985 by Curl,
Kroto, and Smalley* (for which they received the Nobel
Prize in 1996): buckminsterfullerene, C60, a new, spherical
allotrope of carbon in the shape of a soccer ball. They
found that laser evaporation of graphite generated a
variety of carbon clusters in the gas phase, the most
abundant of which contained 60 carbon atoms. The best
way of assembling such a cluster while satisfying the
tetravalency of carbon is to formally “roll up” 20 fused
benzene rings and to connect the dangling valencies in
such a way as to generate 12 pentagons: a so-called
truncated icosahedron with 60 equivalent vertices — the
shape of a soccer ball. The molecule was named after
Buckminster Fuller† because its shape is reminiscent of
the “geodesic domes” designed by him. It is soluble in
organic solvents, greatly aiding in the proof of its struc-
ture and the exploration of its chemistry. For example, the
13C NMR spectrum shows a single line at d 5 142.7 ppm,
in the expected range (Sections 15-4 and 15-6). Because
of its curvature, the constituent benzene rings in C60 are
strained and the energy content relative to graphite is
10.16 kcal/g C atom. This strain is manifested in a rich
chemistry, including electrophilic, nucleophilic, radical,
and concerted addition reactions (Chapter 14). The enor-
mous interest spurred by the discovery of C60 rapidly
led to a number of exciting developments, such as the
design of multigram synthetic methods (commercial
material sells for as low as $1 per gram); the isolation
of many other larger carbon clusters, dubbed “fullerenes,”

Graphite

3.35��

3.35��

Diamond

 C h a p t e r 1 5 689

Buckminsterfullerene C60 C70 Chiral C84

Carbon nanotube

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

An example of a geodesic dome (of the type whose design
was pioneered by Buckminster Fuller) forms part of the
entrance to EPCOT Center, Disney World, Florida. It is
180 feet high with a diameter of 165 feet.

such as the rugby-ball – shaped C70; chiral systems (e.g.,
as in C84); isomeric forms; fullerenes encaging host atoms,
such as He and metal nuclei (“endohedral fullerenes”);
and the synthesis of conducting salts (e.g., Cs3C60, which
becomes superconducting at 40 K). Moreover, reexamina-
tion of the older literature and newer studies have revealed
that C60 and other fullerenes are produced simply upon
incomplete combustion of organic matter under certain
conditions or by varied heat treatments of soot and there-
fore have probably been “natural products” on our planet
since early in its formation.

From a materials point of view, perhaps most useful
has been the synthesis of graphitic tubules, so-called nano-
tubes, based on the fullerene motif. Nanotubes are even
harder than diamond, yet elastic, and show unusual mag-
netic and electrical (metallic) properties. There is the real
prospect that nanotubes may replace the computer chip as
we currently know it in the manufacture of a new genera-
tion of faster and smaller computers (see also Chemical
Highlight 14-2). Nanotubes also function as a molecular
“packaging material” for other structures, such as metal
catalysts and even biomolecules. Thus, carbon in the
 fullerene modifi cation has taken center stage in the new
fi eld of nanotechnology, aimed at the construction of
devices at the molecular level.

690 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

a series called the acenes. Angular fusion (“annulation”) results in phenanthrene, which
can be further annulated to a variety of other benzenoid polycycles.

Naphthalene Anthracene Tetracene
(Naphthacene)

Phenanthrene

1 8
8a

4a
4

2

3

6

5

4b
7

6
5

4
4a

10a
1

3

2

8a

9
10

8
8a

10a

7

6
5

10
10a

6a

9

8

7

8

7

1 9
9a

4a
4

2

3
10

11
11a

5a
6

1 12
12a

4a
4

2

3
5

Each structure has its own numbering system around the periphery. A quaternary carbon is
given the number of the preceding carbon in the sequence followed by the letters a, b, and
so on, depending on how close it is to that carbon.

MODEL BUILDING

Exercise 15-9

Name the following compounds or draw their structures.

(a) 2,6-Dimethylnaphthalene (b) 1-Bromo-6-nitrophenanthrene (c) 9,10-Diphenylanthracene

(d)

Br

(e)

NO2

HO3S

Naphthalene is aromatic: a look at spectra
In contrast with benzene, which is a liquid, naphthalene is a colorless crystalline material
with a melting point of 80 8C. It is probably best known as a moth repellent and insecticide,
although in these capacities it has been partly replaced by chlorinated compounds such as
1,4-dichlorobenzene (p-dichlorobenzene).

The spectral properties of naphthalene strongly suggest that it shares benzene’s delocal-
ized electronic structure and thermodynamic stability. The ultraviolet and NMR spectra are
particularly revealing. The ultraviolet spectrum of naphthalene (Figure 15-13) shows a pattern
typical of an extended conjugated system, with peaks at wavelengths as long as 320 nm. On
the basis of this observation, we conclude that the electrons are delocalized more extensively
than in