4.5.4Core Organic Chemistry
Environmental Concerns of Organohalogen Use
5 min read 5 parts Lesson 23 of 29 Virtual lab
Part 1 of 5Uses and Benefits
The stability that made CFCs so useful also let them drift unchanged up to the stratosphere.
Solvents
- Organohalogens such as dichloromethane dissolve many organic substances, so they are used in cleaning and industry.
Refrigerants
- CFCs were widely used in refrigerators and air conditioners because they are stable and easily liquefied.
Propellants and foams
- CFCs were also used in aerosols and in blowing foams, because they are unreactive and non-flammable.
Polymers and pharmaceuticals
- Many plastics, such as PVC, and many medicines contain carbon-halogen bonds.
Part 2 of 5Ozone Depletion by CFCs
1
Ozone in the stratosphere absorbs harmful ultraviolet radiation and protects living things at the surface.
2
UV light breaks the C–Cl bond in CFCs by homolytic fission, forming a chlorine radical: CCl₂F₂ → •CClF₂ + Cl•.
3
The chlorine radical removes an oxygen atom from ozone: Cl• + O₃ → ClO• + O₂.
4
The ClO• radical then reacts with an oxygen atom to regenerate the chlorine radical: ClO• + O → Cl• + O₂.
5
The overall change is O₃ + O → 2O₂, and Cl• acts as a catalyst, so one radical can destroy many ozone molecules.
Part 3 of 5Other Radicals
Other Radicals
Nitrogen oxides
- NO• radicals, formed from lightning and aircraft engines, also catalyse ozone breakdown.
First step
- NO• + O₃ → NO₂ + O₂ removes an ozone molecule and forms nitrogen dioxide.
Second step
- NO₂ + O → NO• + O₂ regenerates the NO• radical, which is a catalytic cycle.
Comparison
- Both Cl• and NO• are catalysts, because they are used up in one step and re-formed in the next.
Part 4 of 5Solutions and Responses
International action
- The Montreal Protocol agreed to phase out the production and use of CFCs worldwide.
Replacements
- HFCs contain no chlorine, so they do not destroy ozone, but they are greenhouse gases.
Alternatives
- Hydrocarbons such as propane and butane, and carbon dioxide, can also be used as refrigerants and propellants.
Monitoring
- Atmospheric ozone and CFC levels are monitored, showing that the ozone layer is slowly recovering.
| Radical | Source | Step 1 |
|---|---|---|
| Cl• | UV breakdown of CFCs | Cl• + O₃ → ClO• + O₂ |
| NO• | Lightning and aircraft engines | NO• + O₃ → NO₂ + O₂ |
Part 5 of 5Exam Checklist & Tips
4.5.4 – Key Points for the Exam
- Ozone layer A region of the stratosphere containing ozone that absorbs harmful UV radiation.
- CFC A chlorofluorocarbon, an organic compound containing carbon, chlorine and fluorine.
- Catalyst A substance that speeds up a reaction and is regenerated at the end.
- Radical A species with an unpaired electron, which is highly reactive.
Common Mistakes
- Saying CFCs are broken down by oxygen in the lower atmosphere;
- they are stable until reaching the stratosphere.
- Forgetting that chlorine radicals are regenerated and so act as catalysts.
- Confusing ozone depletion with the greenhouse effect.
- Writing the radical step without the dot, or showing the C–F bond breaking instead of the weaker C–Cl bond.