Bond Fission: Homolysis and Heterolysis
Bond Fission
A covalent bond is formed by sharing of electrons between two atoms. A single bond (σ bond) is made up of two electrons. A chemical reaction takes place when old bonds are broken and new ones are formed. Old bond can be broken by two ways-
- Homolytic Bond Fission (Homolysis)
- Heterolytic Bond Fission (Heterolysis)
Homolytic Bond Fission (Homolysis)
Breaking of bonds takes place in such a way that both the bonded atoms take their shared electron. This process generates two free radicals, which are neutral species with an unpaired electron each. Homolytic fission is sometimes called homolytic cleavage or bond homolysis, derived from the Greek word "homo," meaning equal.
Conditions Favoring Homolytic Fission:
- The electronegativity difference between the bonded atoms is very low (nonpolar bonds).
- Occurs typically in nonpolar solvents.
- Reactants are usually in the gaseous state.
- Requires high energy input such as heat, ultraviolet (UV) radiation, or initiators like peroxides.
- Common in radical reactions like combustion, photochemical reactions, and polymerizations.
Heterolytic Bond Fission (Heterolysis)
Breaking of bonds takes place in such a way that both the bonded electrons are taken by the more electronegative bonded atom. This creates two ions: a cation (positive ion) and an anion (negative ion). The term "heterolysis" originates from Greek, meaning unequal breaking.
Conditions Favoring Heterolytic Fission:
- Large electronegativity difference between bonded atoms (polar covalent bonds).
- Occurs in polar solvents that stabilize the ions formed.
- May occur at lower energy or temperature compared to homolytic fission.
- Common in ionic reactions like nucleophilic substitutions and acid-base reactions.
| Comparison: Homolytic and Heterolytic Bond Fission | ||
|---|---|---|
| Aspect | Homolytic Fission | Heterolytic Fission |
| Electron distribution | One electron per atom (equal split) | Both electrons to one atom (unequal) |
| Species formed | Two free radicals (neutral) | One cation and one anion |
| Bond type | Nonpolar covalent | Polar covalent or ionic |
| Energy requirement | High (UV, heat) | Lower energy, favored by polar solvents |
| Common conditions | Nonpolar molecules, high temp/UV | Polar molecules, polar solvents |
| Examples | Cl₂, peroxide cleavage | HCl, H₂O ionization |
| Applications | Radical chain reactions, polymerization, combustion | Ionic reactions, nucleophilic substitutions, acid-base |