Radicals are produced when a covalent bond undergoes homolytic fission: the bonding electron pair splits equally, so each atom receives one electron. Ultraviolet (UV) light can supply the energy required to break the bond.
A covalent bond contains a shared pair of electrons. When a molecule absorbs a UV photon, the photon may provide enough energy to break that bond symmetrically.
For example, chlorine undergoes photochemical homolysis:
Each chlorine atom takes one electron from the original CltoCl bond. Each product therefore has an unpaired electron, represented by the dot, and is called a chlorine radical.
| Type of bond fission | Distribution of bonding electrons | Products |
|---|---|---|
| Homolytic fission | One electron goes to each atom | Two radicals |
| Heterolytic fission | Both electrons go to one atom | Ions |
The formation of radicals is the initiation step in the radical substitution of an alkane. The chlorine radical can then remove a hydrogen atom from methane:
This forms a methyl radical, which continues the chain reaction. Radicals are highly reactive because their unpaired electron makes them unstable. Heat can also provide the energy for homolytic fission, although UV light is the usual condition specified for halogens reacting with alkanes.
A common misconception is that UV light causes chlorine to form ions. It does not: homolytic fission produces neutral radicals, while heterolytic fission produces ions.
This is assessed at both SL and HL in Reactivity 3.3: Electron sharing reactions. For an explain question, state that UV supplies energy, the bond breaks homolytically, and one bonding electron goes to each atom. Include radical dots and identify the process as initiation.
