(N/A) In organic compounds,covalent bonds are present between atoms. During organic reactions,the fission of a covalent bond takes place. Depending on the mode of electron displacement,three main types of reactive intermediates are formed:
$(a)$ In heterolytic fission,a carbocation or carbanion is formed.
$(b)$ In homolytic fission,a free radical is formed.
$(a)$ $(i)$ Carbocation: $A$ species in which a positive charge is present on the carbon atom. Examples: $H_3C^{+}$,$CH_3CH_2^+$,$(CH_3)_2CH^{+}$,$(CH_3)_3C^{+}$. The stability order is: $CH_3^+ < CH_3CH_2^+ < (CH_3)_2CH^{+} < (CH_3)_3C^{+}$ (i.e.,$\text{Methyl} < 1^{\circ} < 2^{\circ} < 3^{\circ}$).
(ii) Carbanion: $A$ species bearing a negative charge on the carbon atom. Examples: $H_3C^{-}$,$H_3CCH_2^-$,$(H_3C)_2CH^{-}$,$(CH_3)_3C^{-}$. The stability order is: $\text{Methyl} > 1^{\circ} > 2^{\circ} > 3^{\circ}$.
$(b)$ Free radical: These are produced by homolytic cleavage of covalent bonds. $A$ free radical contains one unpaired electron on the carbon atom. Examples: $\dot{C}H_3$,$CH_3\dot{C}H_2$,$(CH_3)_2\dot{C}H$,$(CH_3)_3\dot{C}$. The stability order is: $\dot{C}H_3 < CH_3\dot{C}H_2 < (CH_3)_2\dot{C}H < (CH_3)_3\dot{C}$ (i.e.,$1^{\circ} < 2^{\circ} < 3^{\circ}$).