Environmental Concerns of Organohalogen Use (OCR A Level Chemistry A): Revision Note

Breakdown of the Ozone Layer

Fluorohalogenoalkanes

• The most common halogenoalkanes are chlorofluorocarbons (CFCs)

• These compounds contain carbon atoms with chlorine and fluorine atoms attached to them

  • E.g. CCl₃F and CCl₂F₂

• CFCs have many uses due to their chemical inertness as they are non-flammable and non-toxic

  • They are used as refrigerators

  • Propellants for aerosols

  • As solvents for dry cleaning

Hydrofluorocarbons

• Hydrofluorocarbons (HFCs) are compounds that contain a carbon attached to hydrogen and fluorine atoms only

  • E.g. CH₂F-CF₃

• As with CFCs, HFCs are chemically inert and are therefore used for a wide range of purposes

• Though chlorofluorocarbons (CFCs) have many beneficial uses, they also have devastating effects on the ozone layer:

  • CFCs absorb a lot of UV radiation in the upper atmosphere

  • The CFCs are then broken down by the UV light causing the formation of chlorine radicals

  • The chlorine radical acts as a catalyst in a chain reaction:

Propagation 1: Cl• + O₃ → ClO• + O₂

Propagation 2: ClO• + O → Cl• + O₂

Overall: O₃ + O → 2O₂

• The Cl• radical is regenerated in Propagation 2, so it is free to react with another ozone molecule, continuing the chain

• This is why hydrofluorocarbons (HFCs) are often used as an alternative to CFCs

• As these compounds don’t contain any chlorine atoms, they will not have adverse effects on the ozone layer

Other radicals

• CFCs are not the only radicals that catalyse the breakdown of the ozone

• One of the other sets of radicals linked to ozone breakdown are the nitrogen oxide radicals which can be formed:

  • Naturally during lightning strikes

  • As a result of air travel in the stratosphere

• Nitrogen monoxide is formed by the direct combination of nitrogen and oxygen at high temperatures inside a combustion engine

  • The nitrogen monoxide formed is a free radical as it has an odd number of electrons / unpaired electron 

• The nitrogen monoxide radical reacts with one molecule of ozone to form the nitrogen dioxide molecule

  • NO^• + O₃ → NO₂^• + O₂ 

• The nitrogen monoxide radical is then reformed 

  • NO₂^• + O → NO^• + O₂ 

• Overall, the breakdown equation is O₃ + O → 2O₂