4.2.2 (e) (e) Environmental concerns from use of Organohalogen compounds
Syllabus
(e) production of Halogen radicals by the action of Ultraviolet radiation on CFCs in the upper atmosphere & the resulting catalysed breakdown of the Earth’s protective Ozone Layer, including equations to represent:
(i) the production of Halogen radicals
(ii) the catalysed breakdown of Ozone by Cl• and other radicals e.g. •NO.
{Simple equations of the breakdown process are required:
C2F2Cl2 → C2F2Cl • + •Cl
•Cl + O3 → •Cl O + O2
•Cl O + O → •Cl + O2 }
{Learners could be expected to construct similar equations for other stated radicals.}
What does this mean?
We've already seen that Halogen radicals (generally Cl• but also Br•) can react randomly with Alkanes.
CFCs - Chlorofluoroalkanes - we once used as refrigerant gases, propellants in aerosols and in insulation.
This was because they were non-flammable and inert - so therefore not toxic.
As a result for decades they were vented into the atmosphere and drifted slowly upwards.
No one thought that this was a problem due to their inert nature and - without the action of UV - the CFCs wouldn't have reacted with anything.
Unfortunately, UV has the right frequency to break C-Cl (and C-Br bonds) and create radicals in the Upper Atmosphere.
But not C-F bonds which are too strong.
On the old syllabus it was occasionally necessary to know that a CFC would contain only Carbon, Fluorine and Chlorine atoms.
But that Hydrochlorofluorocarbons - HCFC's - (which are only now being phased out) did not count as CFC's despite containing Chlorine, Fluorine and Carbon.
Who knows if this will still be important?
You'll be familiar with skin cancer and cataracts as possible problems with Ozone depletion.
You should probably know one or two others too.
With Chlorine radicals.
Take any CFC specified by the examiner and break one of the C-Cl bonds to produce two radicals, one of which is Cl•.
The first propagation step involves the Chlorine radical first colliding with Ozone (O3) to form Oxygen (O2) and a ClO• radical.
The second propagation step involves the ClO• radical colliding with an Oxygen atom to form more Oxygen and reform the Cl• radical.
Weirdly, although Oxygen atoms have two unpaired electrons, making them di-radicals, they are not shown with a dot because this signifies one unpaired electron.
The reason being that a di-radical colliding with a radical will still leave a radical rather than being a termination step.
Oxygen atoms are fomed when Oxygen molecules are broken up by UV
O2 → 2 O
Overall,
With Nitrogen Oxide radicals.
Not all Ozone depletion is down to CFCs since any radical can catalyse the destruction of Ozone in the same way.
Nitrogen Oxide formation in the atmosphere would be rare if not for the action of jet engines.
NO• + O3 → NO2•+ O2
NO2• + O → NO• + O2
Which overall is, again;
O + O3 → 2 O2
You should be able to reproduce the above too.
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