143.

What are fullerenes? How are they prepared?
Or
Discuss the structure of fullerenes.

The newly discovered molecular forms of carbon are known as fullerenes. The structure has been explained and understood by extensive-bonding between carbon atoms. One example of fullerenes is C₆₀ known as Buck minister fullerene. Fullerenes are the only purest form of carbon.

Preparation: Fullerenes are made by heating of graphite in an electric are in the presence of inert gases such as helium or argan. The sooty material formed by condensation of vaporised C small molecules consists of mainly C₆₀ with a smaller quantity of C₇₀ and traces of other similar fullerenes C₃₂, C₅₀, C₇₆ and C₈₄. Thus fullerenes can be easily extracted from the soot by dissolving in benzene or hydrocarbon solvents, giving a wine-red solution and finally mustard coloured crystals. The different fullerenes are separated by chromatography. Out of all the fullerenes, C₆₀ isotope is the most stable.

Structure: Fullerenes are cage-like molecules. 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 ring can only fuse with six-membered rings. Fullerene molecule has a spherical shape and contains 60 vertices-with sp² hybridised carbon at each vertex and are called buckyballs.

In this structure, each carbon atom forms three sigma bonds with other three carbon atoms. The remaining electron at each carbon is delocalised in molecular orbital, which in turn gives aromatic character to the molecule.