STRENTH OF BONDS WITH OTHER ELEMENTS

The C-C and C-H bonds in organic compounds are very strong.These bonds generally do not break easily during the reactions.However;other atoms or groups such as Cl,OH,etc.,attached to the carbon atom may get replaced easily.For example,aqueous KOH reacts with haloalkanes to give the corresponding alcohol                   CH3-CH2-Cl +KOH(aq)  = CH3-CH2-OH + KCl                                                                                   chloroethane                        ethanol

CATENATION

Carbon has a unique propery of liniking itself to other carbon atoms to give open chain or/and cyclic structures.This property of self-linking is termed catenation.Catenation is favoured by the atoms where atom to atom covalent bond is quite strong.In carbon,C-C bond enthlpy is very large(347.3kJ/mol).So,carbon shows catenation.
   Carbon atom due to its tetravalency can be bonded to two,three or four carbon atom s by forming single and multiple bonds.Therefore,chains of carbon atoms may be linear,branched or cyclic.For example,

 Catenation is responsible for the existence of such a large number of organic compounds.

IONIGATION ENERGY AND ELECTRONEGATIVITY

The first ionisation energy(now ionisation enthalpy) of carbon atom is 1085kJ/mol.This value is very high .The electronegativity of carbon is 2.5,which is neither too low nor too high.Thus,carbon has no tendency to lose or gain electrons.As a result,it forms bonds only by mutual sharing of electrons and thus forms only covalent bonds.

CHARACTERISTICS OF CARBON ATOM

Carbon forms a very large number of compounds because of special characteristics of carbon atom such as:

• Tetravalency of Carbon

 

Tetravalency of Carbon

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Carbon has in all, six electrons, which are distributed in first two shells, i.e., K-shell has two and L-shell accommodates four electrons viz.,

This distribution indicates the use of two half-filled orbitals and therefore makes carbon to act as divalent. But, experimental observations indicate carbon to show tetravalency in the combined state. To account for tetravalency of carbon, it has been suggested that one of the 2s electrons unpairs and gets promoted to the vacant 2p_z orbital. This leads to the electronic configuration given below for the excited state.

Thus, the excited state has four unpaired electrons occupying four different orbitals opening the possibility for the formation of four bonds. This accounts for the tetravalency of carbon. The energy required for promoting 2s electron to 2p_z orbital is compensated by the energy released during bond formation (as in CH₄).

ORGANIC CHEMISTRY

The number of organic compounds known today is about four million .Thousands of new ones are being synthesised and described daily.The ezistenc of such a large number of compounds of carbon is made possible by the unique bonding characteristics of the element.The three exceptional properties of carbon atom are:
1)Strong carbon-carbon bonds:Carbon is tetracovalent.It can link with other carbon atoms by strong carbon-carbon bonds.Thus carbon atoms can join together into stable chains and rings.

2)Covalent bonds with other elements:Carbon atoms linked as above can further be linked to atoms of other elements such as hydrogen ,oxygen,nitrogen and halogen.

3)Multiple bonds:Double and triple bonds between carbon atoms,and between carbon and other atoms are easily formed.

 These multiple covalent bonds permit the formation of a still greater number of organic structures.