Monday, August 18, 2008


Organic Chemistry is the chemistry of hydrocarbons and their derivatives. Since carbon is an essential constituent of all organic compounds, organic chemistry is defined as the chemistry of carbon compounds. But, carbon compounds such as CO, CO2, carbonates, bicarbonates, metal carbides, metal cyanide, etc. are considered as inorganic because of their properties.

The reason for the existence of large number of organic compounds are:-

  • Catenation - the self linking property of carbon atoms.

  • Isomerism - the existence of different compounds having the same molecular formula.

Hybridisation in carbon.

In the excited state, carbon atom has four unpaired electrons to form four covalent bonds. The four valence orbitals are different, one being an s-orbital and the other three p-orbitals. These four orbitals (2s and three 2p) mix up to produce equivalent orbitals.

The process of mixing up of atomic orbitals of almost equal energies to get an equal number of orbitals of identical shapes and equal energies is known as hybridisation.

Carbon atom can have 3 types of hybridisation : - sp3, sp2 and sp.

sp3 hybridisation.

This is known as tetrahedral hybridisation. The four sp3 hybridised orbitals of carbon atom are directed to the four corners of a regular tetrahedron with an angle of 109*18' between two hybridised orbitals. The orbital has 25% s- character. eg:- methane, ethane.

sp2 hybridisation.

This is known as trigonal hybridisation. The three sp2 hybridised orbitals of carbon atom are directed to the corners of an equilateral triangle with an angle of 120*. The hybrid orbitals has 33.3% s- character. eg:- ethylene.

sp hybridisation.

This is known as diagonal hybridisation. The two sp hybridised orbitals of carbon atom are directed along a line with an angle of 180* to each other. The orbitals has 50% s- character. eg:- acetylene.

Bond length and bond energy are influenced by the type of hybridisation. As the s-character of the hybrid orbitals increases the electronegativity of the atom increases and hence the bond length decreases. Hence the bond energy increases. Therefore, sp hybrid orbitals has the shortest bond length with the highest bond energy.

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