10.5 Allotropes of Carbon

Carbon exhibits many allotropic forms; both crystalline as well as amorphous. Diamond and graphite are two well-known crystalline forms of carbon. In 1985, third form of carbon known as fullerenes was discovered by H.W.Kroto, E.Smalley and R.F.Curl. For this discovery they were awarded the Nobel Prize in 1996.

Diamond

It has a crystalline lattice. In diamond each carbon atom undergoes \(s p^3\) hybridisation and linked to four other carbon atoms by using hybridised orbitals in tetrahedral fashion. The \(\mathrm{C}-\mathrm{C}\) bond length is 154 pm . The structure extends in space and produces a rigid three-dimensional network of carbon atoms. In this structure (Fig. 10.3) directional covalent bonds are present throughout the lattice.

It is very difficult to break extended covalent bonding and, therefore, diamond is a hardest substance on the earth. It is used as an abrasive for sharpening hard tools, in making dyes and in the manufacture of tungsten filaments for electric light bulbs.

Example 10.6: Diamond is covalent, yet it has high melting point. Why?

Solution: Diamond has a three-dimensional network involving strong \(\mathrm{C}-\mathrm{C}\) bonds, which are very difficult to break and, in turn has high melting point.

Graphite

Graphite has layered structure (Fig.10.4). Layers are held by van der Waals forces and distance between two layers is 340 pm. Each layer is composed of planar hexagonal rings of carbon atoms. C-C bond length within the layer is 141.5 pm . Each carbon atom in hexagonal ring undergoes \(s p^2\) hybridisation and makes three sigma bonds with three neighbouring carbon atoms. Fourth electron forms a \(\pi\) bond. The electrons are delocalised over the whole sheet. Electrons are mobile and, therefore, graphite conducts electricity along the sheet. Graphite cleaves easily between the layers and, therefore, it is very soft and slippery. For this reason graphite is used as a dry lubricant in machines running at high temperature, where oil cannot be used as a lubricant.

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 vapourised \(\mathrm{C}^{\mathrm{n}}\) small molecules consists of mainly \(\mathrm{C}_{60}\) with smaller quantity of \(\mathrm{C}_{70}\) 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. \(\mathrm{C}_{60}\) molecule has a shape like soccer ball and called Buckminsterfullerene (Fig. 10.5).

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. All the carbon atoms are equal and they undergo \(s p^2\) hybridisation. Each carbon atom forms three sigma bonds with other three carbon atoms. The remaining electron at each carbon is delocalised 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 \(\mathrm{C}-\mathrm{C}\) 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, \(\Delta_f H^{\ominus}\) of graphite is taken as zero. \(\Delta_f H^{\ominus}\) values of diamond and fullerene, \(\mathrm{C}_{60}\) are 1.90 and \(38.1 \mathrm{~kJ} \mathrm{~mol}^{-1}\), 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.

Uses of Carbon

Graphite fibres embedded in plastic material form high strength, lightweight composites. The composites are used in products such as tennis rackets, fishing rods, aircrafts and canoes. Being good conductor, graphite is used for electrodes in batteries and industrial electrolysis. Crucibles made from graphite are inert to dilute acids and alkalies. Being highly porous, activated charcoal is used in adsorbing poisonous gases; also used in water filters to remove organic contaminators and in airconditioning system to control odour. Carbon black is used as black pigment in black ink and as filler in automobile tyres. Coke is used as a fuel and largely as a reducing agent in metallurgy. Diamond is a precious stone and used in jewellery. It is measured in carats (1 carat \(=200 \mathrm{mg}\) ).