Fission of Covalent Bond
We know that a covalent bond is formed by sharing of electrons.
This bond can be broken either by :
- Homolytic fission (homolysis)
- Heterolytic fission (heterolysis)
If the bond breaks in such a fashion that each atom gets one electron of the shared pair, it is called homolytic fission. The single electron movement is shown by half headed arrow ( ).
Homolytic fission results in the formation of neutral species that contains an unpaired electron. These species are called free radicals. Some examples of free radicals are :
Cl●, Li●, CH3●, CH3CH2●
Free radicals are highly reactive in nature.
Reactions that involve homolytic cleavage are called free radical reactions. Homolytic fission usually occurs in non-polar bonds. Conditions that generally favour homolytic fission are :
- High temperature
- Ultraviolet light
- Presence of peroxides
Alkyl Free Radicals
Free radicals of carbon such as CH3● are known as alkyl free radicals. They can be primary, secondary or tertiary.The stability of alkyl free radicals increases in the order : primary < secondary < tertiary. The main reason for this order is hyperconjugation.
When a covalent bond breaks in such a way that both the electrons of shared pair remain with one part, then the cleavage is termed as heterolytic fission.
Heterolytic fission results in the formation of charged species which is evident from the above example. Since a pair of electrons is moved, the arrows used are full headed.
CH3+ (methyl cation) in the above reaction is a carbocation.
A positively charged carbon ion is called carbocation. Carbocations are highly unstable and reactive species. Carbocation can be primary, secondary or tertiary depending upon how many alkyl groups are attached to the positively charged carbon. The stability of carbocations follows the order : tertiary > secondary > primary due to +I effect and hyperconjugation.
Stability of carbocation : (CH3)3C+ > (CH3)2CH+ > CH3CH2+ > CH3+
If its carbon that gets away with shared pair of electrons, then the carbon is termed as carbanion.
The order of stability of carbanion is exact opposite of that of carbocation due to inductive effect.
Stability of carbanion : (CH3)3C− < (CH3)2CH− < CH3CH2− < CH3−