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.