Fullerenes

Structure:

Fullerene is naturally occurring allotrope of carbon in which 60 carbon atoms are linked to form a stable structure. Previously, only two forms of carbon (diamond and graphite) were known. The third allotrope of carbon, called fullerene, was discovered in 1985 by Robert Curl, Herald Kroto and Richard Smalley.

They correctly suggested the cage structure as shown in the figure and named the molecule Buckminster fullerene after the architect Buckminster Fuller, the inventor of the Geodesic dome, which resembles the molecular structure of C60. Molecules of C60 have a highly symmetrical structure in which 60 carbon atoms are arranged in a closed net with 20 hexagonal faces and 12 pentagonal faces. The pattern is exactly like the design on the surface of a soccer ball. C60 has been found to form in sooting flames when hydrocarbons are burned.

All the fullerenes have even number of atoms, with formulae ranging upto C400 and higher. These materials offer exciting prospects for technical application. For example, because C60 readily accepts and donates electrons, it has possible application in batteries.

Uses of Fullerenes:

It is hoped fullerenes or their compounds may find uses as-

• Superconductors
• Semiconductors
• Lubricants
• Catalysts
• As highly tensile fibres for construction industry.
• Inhibiting agents in the activity of the AIDS virus.

Catenation

The property of self linking of atoms of an element through covalent bonds to form straight or branched chains and rings of different sizes is called catenation. The tendency of an element to form chains depends upon the strength of the element−element bond. C −C bond strength (355 kJ mol−1) is maximum, therefore, carbon shows maximum tendency for catenation.

Multiple bonding

Carbon shows a pronounced ability to form multiple bonds with itself (e.g. in graphite) and with other non−metals especially nitrogen and oxygen.

Formation of hydrides

Carbon forms a large number of hydrides called hydrocarbons (alkanes, alkenes, alkynes and arenes). This is mainly due to maximum tendency of carbon for catenation.

Formation of halides

Tetrahalides are covalent compounds and have tetrahedral structures.

For example, CCl₄, CF₄.

Formation of oxides

Carbon forms mainly two types of oxides.

(i) Carbon monoxide (CO): It is a neutral oxide of carbon and is prepared in the laboratory by heating oxalic acid with conc. H₂SO₄. It is a poisonous gas, burns with a blue flame and when inhaled produces suffocation and finally may lead to death. The poisoning effect of CO is due to the fact that it combines with haemoglobin (red colouring matter of the blood) to form carboxyhaemoglobin which destroys its capacity to supply oxygen to the body. The antidote for CO poisoning is carbogen (mixture of 95% O₂ and 5% CO₂). Carbon monoxide acts as a reducing agent and is thus used to reduce heated metal oxides to metals.

Fe₂O₃ + 3CO → 2Fe + 3CO2 CuO + CO → Cu + CO₂

Because of the presence of a lone pair of electrons, CO acts as a Lewis base and thus combines with many metals such as Fe, Cr, Ni etc. forming their respective metal carbonyls.

Fe + 5CO → Fe(CO)₅ Ni + 4CO →  Ni(CO)₄

Iron pentacarbonyl Nickel tetracarbonyl

Carbon monoxide is used in mond’s process for purification of nickel via its nickel tetracarbonyl.

(ii) Carbon dioxide (CO₂): it is prepared in the laboratory by the action of dil. HCl on calcium carbonate. It is a linear molecule in which carbon is sp hybridized. It is a resonance hybrid of the following structures

O = C = O ↔ O−− C ≡ O+ ↔  O+≡ C − O−

Due to resonance, carbon−oxygen double bond acquires some triple bond character and hence its bond length decreases from 1.22 Å to 1.15 Å.

It is used in fire extinguishers and in the manufacture of aerated waters.

Dry ice: Solid carbon dioxide is called dry ice since it does not wet the surface on which it melts. Dry ice is used as a covalent for preserving pershable articles in food industry and for making cold baths in the laboratory.

 Anomalous behavior of carbon

Carbon differs from rest of the elements of group 14 due to (i) smaller size and high electronegativity (ii) catenation and (iii) non−availability of d−orbitals. Some important points of difference are

•  Because of smallest atomic radius and lowest atomic volume, carbon is the hardest element of group 14. It has the highest melting and boiling points, has the highest ionization energy and is the most electronegative element of this group.
• Due to the absence of d−orbitals, it has no tendency to form complex ions. As a result, its maximum covalency remains 4.
• Carbon has strong tendency to form multiple bonds, i.e., double and triple bonds. For example alkenes, alkynes etc. The most remarkable property of carbon is catenation by virtue of which it forms thousands of compounds in which carbon is linked to other carbon atoms.