A radical is a molecular entity containing an unpaired electron. Radicals are usually highly reactive because the unpaired electron makes their electron arrangement unstable, so they readily react to form a paired-electron covalent bond.
Why Are Radicals So Reactive?
Most stable molecules have electrons paired in orbitals. A radical has an unpaired electron, represented by a dot, such as the chlorine radical . This species can form a covalent bond by interacting with an electron from another particle.
Radicals are commonly formed through homolytic fission, in which a covalent bond breaks and each atom receives one bonding electron. For example, ultraviolet light supplies the energy needed to break the chlorine bond:
The chlorine radicals can then react with an alkane through radical substitution. For methane, the key stages are:
| Stage | What happens |
|---|---|
| Initiation | Radicals form by homolytic fission of . |
| Propagation | A radical reacts and another radical is produced, continuing the chain. For example, . |
Because propagation regenerates radicals, one initiation event can cause many reactions. Further substitution may also occur, so reactions between alkanes and halogens usually produce a mixture of products.
A common misconception is that radicals are ions. Radicals are defined by an unpaired electron, not by electrical charge; they may be neutral or charged.
IB Exam Technique
This is SL and HL content in Reactivity 3.3. In an exam, define a radical precisely, show the dot representing the unpaired electron, and distinguish homolytic fission from heterolytic fission. If asked to explain reactivity, connect the unpaired electron directly to rapid bond formation rather than merely stating that radicals are “unstable.”
