Breaking of Covalent Bond
Chemical reactions involve breaking of one or more of the existing chemical bonds in reactant molecules and formation of new bonds leading to products. The breaking of a covalent bond is known as bond fission.
A covalent bond is formed by the sharing of two electrons from two atoms. During bond breaking or bond fission, the two shared electrons can be distributed equally or unequally between the two bonded atoms. There are two types of bond fission:
- Homolytic Fission
- Heterolytic Fission
Homolytic Fission
The fission of a covalent bond with equal sharing of bonding electrons is known as homolytic fission.
A–B → A· + ·B
The neutral species formed after homolytic fission are known as free radicals.
Free radicals are neutral but reactive species having an unpaired electron and these can also initiate a chemical reaction.
Heterolytic Fission
The fission of a covalent bond involving unequal sharing of bonding electrons is known as heterolytic fission.
A–B → A+ + B-
Heterolytic bond fission results in the formation of ions. The ion which has a positive charge on the carbon atom, is known as the carbonium ion or a carbocation. On the other hand, an ion with a negative charge on the carbon atom is known as the carbanion.
The charged species obtained by the heterolytic fission initiate chemical reactions and they are classified as electrophiles and nucleophiles.
Electrophiles
An electrophile is an electron deficient species and it may be positively charged or neutral. Examples are H+, NO2+, Br+, Cl+, Ag+, BF3
An electrophile is an electron seeking species and hence, it attacks at a position of high electron density.
Nucleophiles
A nucleophile is negatively charged or electron rich neutral species. Examples of nucleophiles are OH–, NO2–, H2O, :NH3
Nucleophiles attack a position of low electron density.