15.2 Organohalogen Compounds In The Environment
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What are organohalogen compounds?
Molecules that contain at least one halogen atom joined to a Carbon chain
Why are organohalogens concerning?
They are not broken down naturally in the environment
Where is the ozone layer found?
At the outer edge of the stratosphere
What is the job of the ozone layer?
Absorb most of the biologically damaging UV radiation from the Sun’s rays
Why is UV-B radiation bad for living organisms?
Commonly linked to sunburn Increased genetic damage and a greater risk of skin cancer to humans
How is ozone formed in the stratosphere?
High energy UV breaks down Oxygen into O radicals O2 -> 2O O2 + O >< O3
CFCs common uses
Refrigerants, air conditioning units and aerosol propellants
Why are CFCs very stable?
Due to the strength of the Carbon Halogen bond within molecules
Why are CFCs so harmful?
CFCs remain stable until they reach the stratosphere They start to break down here, forming chlorine radicals, which catalyse breakdown of the ozone layer
Why do CFCs break down in the stratosphere?
The UV radiation provides enough energy to break a Carbon - Halogen bond by homolytic fission to form radicals
Photodissociation of CF2Cl2
CFCl2 -> CF2Cl. + Cl. Propagation step 1- Cl. + O3 -> ClO. + O2 Step 2 - ClO. + O -> Cl. + O2 Overall equation O3 + O -> 2O2
What other radicals catalyse breakdown of ozone?
Nitrogen oxide radicals