Bond Cleavage

Bond Cleavage

Organic reactions take place through the formation of reactive intermediates. These intermediates are formed due to cleavage of covalent bonds. These intermediates can be

(i) free radicals like

(ii) carbocation like

(iii) carbanion like

Homolytic (symmetrical) cleavage

In which the two electrons shared in a σ bond become unpaired as the bond is broken.

The species formed are called free radicals

(i) They are electrically neutral.

(ii) They are extremely reactive.

Their stability is in the order of

Benzylic and allylic free radicals are resonance stabilized hence are more stable than alkyl free radicals.

Thus greater the stability easier will be formation of the species. (Methyl radical) is
sp2 – hydridized (bearing three σ C⎯H bonds and singly occupied p – orbital) with HCH angle 1200 and three C ⎯ H bonds coplanar. Thus when a methyl radical is formed in the homolytic cleavage of CH3 ⎯ X bond, the carbon undergoes a geometric change from tetrahedral to planar and rehybridisation from sp3 to sp2.

Heterolytic (unsymmetrical) cleavage

When a covalent bond joining two atoms A and B breaks in such a way that both the electrons of the covalent bond (i.e. shared pair) are taken away by one of the bonded atoms, the mode of bond cleavage is called heterolytic cleavage. Heterolytic cleavage is usually indicated by a curved arrow which denotes a two electron displacement. For example

As shown above heterolytic fission results in the formation of charged species, i.e. cations and anions. It usually occurs in polar covalent bonds and is favoured by polar solvents.

In the formation of carbocation, we also find that sp3 hybridised carbon (in CH₃ ⎯X) changes to sp² hybridised carbon.

An organic ion with a pair of available electrons and a negative charge on the central carbon atom is called carbanion and stability is in order

Electron attracting group (⎯CN, > C = O) increases stability and electron – releasing group

(⎯ CH₃ etc) decreases stability of carbanion.

Benzyl carbanion is again stabilized by resonance.