20.4:
Radical Formation: Homolysis
When sufficient energy breaks a bond, and one atom gets both the electrons, leading to the formation of ions, it is known as heterolysis.
Alternatively, when the electrons are evenly distributed as an unpaired bonding electron on both atoms, generating radicals, it is known as homolysis.
Energy supplied as heat from high temperatures can homolyze most bonds, while heat a little above room temperature can homolyze some weak bonds. For instance, the weak O–O bond in peroxides undergoes homolysis on heating to form alkoxyl radicals.
Alternatively, light of an appropriate wavelength can homolyze specific bonds.
The relatively weak bonds in halogen molecules easily homolyze with heat or light, resulting in halide radicals.
20.4:
Radical Formation: Homolysis
A bond is formed between two atoms by sharing two electrons. When this bond is broken by supplying sufficient energy, either two electrons can be taken up by one atom forming ions by the cleavage called heterolysis, or the two electrons are shared by two atoms, with one each creating radicals by the cleavage called homolysis.
For example, HCl in solution cleaves into H+ and Cl− ions, where the chlorine atom takes both bonding electrons with it, leaving a naked proton. However, at about 200 °C in the gas phase, the electron pair forming the H–Cl bond is shared between the two atoms.
Some weak bonds undergo homolysis at around room temperature. In such cases, light is the best energy source for the homolysis of bonds. Peroxides and halogens are quite readily homolysed by heat and light. Dibenzoyl peroxide and azobisisobutyronitrile (AIBN) are often used as initiators of radical reactions because they can easily homolyse to form radicals.