Carbon has electronic configuration [He]2s22p2,
and main formal oxidation state +4 (there are other oxidation
states, but all use all of carbon's valence electrons in bonding).
Alongside the central role of Carbon in Organic Chemistry,
it forms numerous compounds, both inorganic and organometallic.
There are two main isotopes, with relative abundances: 12C
(98.9%, I=0), and 13C
(1.1%, I=0.5). I is the nuclear spin, and the half-integer value
of the nuclear spin for 13C gives it its usefulness
in structure determination by NMR.
Elemental carbon occurs in several different forms, ie. it
displays a complex allotropy. The
main forms are diamond and graphite,
and they exhibit markedly different properties due to the very
different structures they adopt.
Structures and Descriptions
An electrical insulator,
and 3D-lattice crystal structure. This is the hardest known
substance (this is because it is made up of very strong C-C
covalent bonds). Each C atom forms four bonds, tetrahedrally
arranged, to other C atoms, resulting in an open, but strongly
tetrahedral carbon coordination.
An electrical conductor,
and layered lattice crystal structure. This is slippery and
used as a lubricant (this is a property of its layered structure,
with the lubricating effect coming from the ability of the
layers to slide over one another, as they are only weakly
held together by van der Waals
forces). Here, each C atom forms three covalent σ-bonds
to further C atoms. These σ-bonds
are made up of sp2 hybrid orbitals. The remaining
p-orbitals, which are perpendicular to the plane of the σ-bonds,
overlap to form a delocalized π-system.
The planes are widely separated as they are held together
only by the weak van der Waals forces.
planar carbon coordination. the lines show that
the carbon atoms in every other layer are
in line, and not those in adjacent
Diamond does not convert to graphite under standard conditions,
even though it is spontaneous (ΔGo
= -2.90 kJmol-1). This is a kinetic phenomenon,
and diamond is thus described as metastable.
The electrical conductivity of graphite is direction-dependent:
the π-system of delocalized electrons
allows metallic conduction parallel to the planes, while the
much lower conductivity perpendicular to the planes, which
nevertheless increases with temperature, suggests semiconductor
behavior in that direction.
The directionality of the conductivity suggests a band structure
of graphite which has a fully filled valence band with a small
separation to the empty conduction band (the overlapping p-orbitals,
with one electron from each C atom, form a π-system
with the bonding orbitals fully occupied and the anti-bonding
orbitals fully unoccupied. Hence, graphite may form intercalation
compounds with species which act either as electron
donors (where graphite acts as an electron acceptor, incorporating
the donated electrons into the vacant conduction band), or
as electron acceptors (where graphite now donates electrons
from the full valence band).
Reactions of Graphite
|Reduction by K (extra electrons
from the K enter the graphite conduction band, and therefore
|Oxidation by Br (electrons from
the graphite valence band are lost to form Br anions,
leaving holes in the graphite band which therefore increase
|With F; produces (CF)n,
an electrical insulator, which has a structure resembling
continuous fused cyclohexane rings, containing sp3
hybridised C atoms.
If sheets of Graphite were bent, which in practice
is achieved by replacing some of the six-membered rings by five-membered
rings, then other forms of carbon may be formed. These are known
as fullerenes (so named
after the inventor of the geodesic dome, Buckminster Fuller;
the domes have the similar shapes to these compounds), or Bucky-balls.
In practice, they are formed when an electric arc is struck
across graphite electrodes in an inert atmosphere.
The most important of these is C60,
often referred to as Buckminster-Fullerene but others such as
C70, C76 and C84 occur in smaller
- C60: an individual C60 molecule
has the shape of a soccer-ball, and Ih (icosahedral)
symmetry. Each pentagon is surrounded by hexagons, and each
hexagon is surrounded by three pentagons and three hexagons.
It crystallizes to give a magenta solid, and dissolves in
benzene to give a magenta solution. The 13C NMR
spectrum shows one signal, ie. all the C atoms in C60
- C70: this has the structure of C60
with an extra strip of five hexagons around the center of
the soccer-ball. It crystallizes to give a red-brown solid,
and dissolves in benzene to give a red solution. The 13C
NMR spectrum shows five signals, so C70 contains
C atoms in five different environments.
|Plan view, looking down
the C5 axis.
||The bonds between sp2
hybridised carbon atoms make up the familiar shape of
a football, and the pentagons and hexagons can be seen.
Reactions of Fullerenes:
The reactivity of fullerenes is somewhere between
that of an arene (with an extended
graphite like p-system) and an alkene
(with an isolated C=C double bond).
The addition product with K is a superconductor
below 18K, whose structure is an face centered cubic array
of C60 molecules, with K+ ions occupying
all the octahedral and tetrahedral holes; the product with
OsO4 is a standard alkene-like addition, where
the OsO4 adds across a C=C bond. The reaction with
Na and NH3 is known as the Birch
If a C60 molecule
is split in half, the two hemispheres can be placed on the
ends of a rolled-up sheet of graphite to produce what is known
as a nanotube. Many different sizes of tube may be formed,
capped or uncapped by bucky-balls of different sizes, and
these may have huge future technological use as molecular
wires, as they retain some of the conductivity of the parent