Introduction to Organic Chemistry
Carbon: the Exceptional Chemical Element
Organic chemistry is the chemistry of carbon and carbon compounds. The number of carbon compounds exceeds the number of all compounds of all other elements put together. The outstanding position of carbon in comparison with all other chemical elements should be explained. Why do so many carbon compounds exist that a separate category of chemistry deals with them?
The chemical element carbon appears in three modifications, which differ from each other considerably:
- diamond, which propably represents the most popular modification of carbon,
- graphite, which everybody knows from pencils, and
- fullerenes, which belong to a family of football-shaped compounds, can be synthesized in an electric arc in the lab.
To understand which properties are responsible for the specific features of carbon, look at the periodic system:
- As an element of the second period carbon shows a small atomic diameter and a tendency to form multiple bonds to minimize the steric compression by reducing the number of ligands.
- Carbon shows a medium electronegativity. Therefore, carbon forms stable covalent bonds.
- Additionally, the electronegativity difference between carbon and hydrogen is very small, whereby the carbon-hydrogen bond is also very stable and hardly polarized.
Because of the medium electronegativity and the small atomic diameter, carbon atoms can form very stable bonds with other carbon atoms. This means long carbon chains, which may be branched, or even carbon rings, which also might contain multiple bonds, can be formed. This behaviour results in a huge number of possible combinations of carbon atoms and therefore in a nearly infinite number of organic compounds.
The ratio of atomic diameters of carbon and hydrogen results in a nearly complete shielding of the carbon atom by the hydrogen atoms. This is the reason for the higher stability of hydrocarbons (alkanes) in comparison with the corresponding silicon hydrides (silanes). Alkanes, for example, are stable against an attack of water, while silanes are decomposed even by atmospheric humidity.
The ability of carbon atoms to form chains and rings by bonding to other carbon atoms results in an immense number of organic compounds. No other chemical element shows this special property. Only the combinations of carbon atoms with hydrogen, oxygen and nitrogen lead to more than 12 million organic compounds known today. All living organisms on earth are based on such organic compounds.