CFCs release chlorine radicals because high-energy ultraviolet radiation causes homolytic fission of their carbontochlorine bonds in the stratosphere. The chlorine radicals then catalyse the conversion of ozone into oxygen, so one radical can destroy many ozone molecules.
CFCs are very stable in the lower atmosphere because they are largely unreactive and do not readily dissolve in rainwater. This long atmospheric lifetime allows them to reach the stratosphere, where they encounter higher-energy UV radiation.
The absorbed photon supplies enough energy to break a carbontochlorine bond. Homolytic rather than heterolytic fission forms two neutral radicals: each fragment takes one electron from the original bonding pair. The dot must be shown because it represents the unpaired electron responsible for the radical's high reactivity.
During homolytic fission, the two electrons in a covalent bond divide equally between the resulting fragments. For example:
The species is a radical because it contains an unpaired electron. It reacts rapidly with ozone through a radical chain mechanism:
| Step | Reaction and significance |
|---|---|
| Ozone attack |
These two equations are propagation steps. Adding them cancels species formed and then consumed, including the chlorine monoxide radical, while the chlorine radical appears on both sides and therefore cancels. This gives the overall equation and demonstrates its catalytic role.
Because the chlorine radical is regenerated, it acts as a catalyst and continues the chain. Ozone absorbs harmful UV radiation, so reducing its concentration allows more UV to reach Earth’s surface.
A common misconception is that each CFC molecule destroys only one ozone molecule. In fact, chlorine radicals are not consumed overall and can participate repeatedly until removed by a termination reaction.
Exam technique: This is SL and HL content in Reactivity 3.3. For an “explain” question, state UV-induced homolytic fission, show the unpaired-electron radicals, write both propagation equations, and explicitly identify the regenerated chlorine radical.