Fullerenes
Fullerenes
Fullerenes are made by the heating of graphite in an
electric arc in the presence of inert gases such as helium or argon. The sooty material formed by condensation of
vaporized Cn small molecules consists of mainly C60 with smaller
quantity of C70 and traces of fullerenes consisting of even number
of carbon atoms up to 350 or above. Fullerenes are the only pure form of carbon
because they have smooth structure without having ‘dangling’
bonds. Fullerenes are cage like molecules. C60 molecule has a shape
like soccer ball and called Buckminsterfullerene.
It contains twenty six-membered rings and twelve five
membered rings. A six membered ring is fused with six or five membered rings
but a five membered rings. All the carbon atoms are equal and they undergo sp2
hybridization. Each carbon atom forms three sigma bonds with other three
carbon atoms. The remaining electron at each carbon is delocalized in molecular
orbitals, which in turn give aromatic character to molecule. This ball shaped
molecule has 60 vertices and each one is occupied by one carbon atom and it
also contains both single and double bonds with C-C distances of 143.5 pm and
138.3 pm respectively. Spherical fullerenes are also called bucky balls
in short.
It is very important to know that graphite is
thermodynamically most stable allotrope of carbon and, therefore, △∫H- of graphite is taken as zero. △∫H- values of diamond and
fullerene, C60 are 1.90 and 38.1 kJ mol-1, respectively.
Other forms of elemental carbon like carbon black, coke and
charcoal are all impure forms of graphite or fullerenes. Carbon black is
obtained by burning hydrocarbons in a limited supply of air. Charcoal and coke
are obtained by heating wood or coal respectively at high temperatures in the
absence of air.
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