Haloalkanes and the environment

Chlorofluorocarbons

• Chlorofluorocarbons (CFCs) are a type of haloalkane containing only carbon, chlorine and fluorine

• They contain no C-H bonds as all H atoms are substituted by Cl or F

• E.G CCl₃F

• CFCs were widely used as refrigerants and solvents until late 80s

• They were used because of there high stability, low toxicity and non-flammability

How CFCs reach the ozone layer

• O3 in the upper atmosphere acts as a protective barrier by absorbing harmful UV radiation from the sun. This acts as a shield preventing damage to humans and ecosystems

• In the upper atmospheres, high energy UV radiation breaks the C-Cl bond releasing chlorine radicals

• CF₂CL₂ → CF₂CL· + Cl·

Chlorine radicals destroy ozone

• Chlorine radicals initiate a chain reaction that depletes ozone through a series of propagation reaction

• UV radiation forms chlorine radicals which react with ozone molecules

  • Cl· + O₃ → ClO· + O₂

• The chlorine radical is regenerated

  • ClO· + O₃ → Cl· + 2O₂

• Overall reaction

  • 2O₃ → 3O₂

• This allows a single Cl radical to destroy more than 10k ozone molecules. This results in depletion of the ozone layer and holes in the ozone layer, increasing UV penetration

Alternatives to CFCs

• Global ban on CFCs in 1989

• Safer alternatives

  • Hydrofluorocarbons - These don’t contain Cl so they don’t harm the ozone layer

  • Hydrocarbons - These substances decompose rapidly but are flammable and contribute to greenhouse gases

• These alternatives are not perfect, but they are less harmful to the environment tham CFCs.