Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)


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be some symmetry to the molecule.
\u2022 The IR spectrum reveals the presence of Caromatic2H units (n\u2dc 5 3030 cm21), and the signal at 
n\u2dc 5 813 cm21 indicates a para disubstituted benzene.
\u2022 Finally, the electronic spectrum shows the presence of a conjugated system, obviously a benzene 
ring.
\u2022 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 \u2013 2005), Rice University, 
Houston, Texas.
\u2020Richard Buckminster Fuller (1895 \u2013 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\u2019s lubricating property is 
the result of the ready mutual sliding of its component 
planes. The \u201clead\u201d 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 \u201croll up\u201d 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 \u2014 the 
shape of a soccer ball. The molecule was named after 
Buckminster Fuller\u2020 because its shape is reminiscent of 
the \u201cgeodesic domes\u201d 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 \u201cfullerenes,\u201d 
Graphite
3.35\ufffd\ufffd
3.35\ufffd\ufffd
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 \u2013 shaped C70; chiral systems (e.g., 
as in C84); isomeric forms; fullerenes encaging host atoms, 
such as He and metal nuclei (\u201cendohedral fullerenes\u201d); 
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 \u201cnatural products\u201d 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 
\u201cpackaging material\u201d 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 (\u201cannulation\u201d) 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\u2019s 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