Chemistry Reference
In-Depth Information
Fig. 1.6
Hydrogen-depleted structures of two catafusenes (isomeric anthracene and phenanthrene
C
14
H
10
), one perifusene (pyrene, C
16
H
10
), and one coronafusene (kekulene, C
48
H
24
).
1.6
A Soft Carbon Allotrope, Graphite; Graphene;
and Benzenoid Hydrocarbon Relatives
Naturally-occurring graphite is a soft, black carbon allotrope in which carbon atoms
have sp
2
hybridization with interatomic distances of 1.41 Å, and form huge planar
sheets stacked upon one another with inter-sheet distances of 3.35 Å, which are rigid
in two directions but may glide in the third spatial direction, explaining thereby the
low hardness (see Table
1.1
). Geim's and Novoselov's discovery that one can isolate
such one-atom thick sheets was honored with the 2010 Nobel Prize in Physics. Again,
now for the different sp
2
hybridization, the graphene sheet is the perfect lattice, both
as far as bond angles are concerned, and taking aromaticity into account. Graphene
properties, especially the electrical conductivity, are astounding (Geim
2009
).
Aromaticity started with August Kekulé, and was subsequently developed by
well-known chemists, too many to mention (Balaban
1980
; Randic
2003
; Balaban
et al.
2005
). There are two ways of defining benzenoid hydrocarbons (or benzenoids,
for short): (i) a restrictive definition, as planar fragments of the graphene lattice, and
(ii) a more relaxed definition, which includes non-planar helicenes, and considers
just the presence of benzenoid rings sharing CC bonds. The traditional classifica-
tion into cata-condensed and peri-condensed benzenoids (or for short catafusenes
and perifusenes, respectively) was based on the absence or presence of internal
carbon atoms, respectively. A simpler and more comprehensive classification was
advanced (Balaban and Harary
1968
; Balaban
1969
) on the basis of dualists (cen-
ters of hexagons, connected by an edge for adjacent hexagons sharing a CC bond):
catafusenes, perifusenes, and coronafusenes have dualists that are acyclic, contain
triangles, or larger rings that are not perimeters of triangle aggregates, respectively.
The examples for all these types of benzenoids presented in Fig.
1.6
are all crystalline
colorless compounds.
According to Erich Hückel, on the basis of quantum-chemical arguments, when-
ever the perimeter of a monocyclic or polycyclic benzenoid catafusene has an odd
number of double bonds (i. e. 4
n
+
2
π
-electrons with
n
=
0, 1, 2, etc.), enhanced
