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Part 3.
The chemistry of HALOGENOALKANES
Doc Brown's
Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK
KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry
Part 3.9
OZONE and halogenoalkanes/haloalkanes
The chemistry of ozone
depletion - cause and prevention
HALOGENOALKANES chemistry notes INDEX
All Advanced A Level Organic
Chemistry Notes
Index of basic Oil and Organic
Chemistry Revision Notes
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An introduction and general description of
the problem and solution Abbreviations used:
CFC/CFCs = chlorofluorocarbon; HCFC/HCFCs
= hydrochlorofluorocarbon;
HFC/HFCs =
hydrofluorocarbon
is a typical CFC molecule CCl2F2,
dichlorodifluoromethane, known commercially as CFC-12.
These kinds of molecules 'where' used in aerosol
sprays as propellant gases and refrigerant gases.
Unfortunately, when CFCs get into the atmosphere
at ground level, because they are chemically inert,
they diffuse up into the upper atmosphere where they are decomposed by
ultraviolet (uv) radiation producing chlorine radicals.
(chlorine atoms are free
radicals with an unpaired electron)
The chlorine radicals decompose (destroy) ozone via free radical chain reactions
and it is estimated that one chlorine atom can lead to the
decomposition of 100, 000 ozone molecules in a catalytic cycle.
In the Antarctic winter the chlorine radicals build up on
small
ice crystals in the air high up in the upper atmosphere.
When these ice crystals melt in the spring sun, the
chlorine radicals
are released causing massive depletion of the ozone layer - in fact they 'did'
cause a
large hole in the (uv protecting) polar ozone layer of the southern hemisphere.
Fortunately, CFCs are now banned and new refrigerant
coolants and aerosol propellant gases have been developed that contain hydrogen atoms - known as HCFCs
and HFCs Examples: a
HCFC CHClF2
chlorodifluoromethane and a
HFC difluoromethane CH2F2
HCFCs and HFCs are
more reactive and are decomposed at lower altitudes, before they can diffuse up
into the ozone layer and HFCs obviously cannot generate chlorine radicals.
However, they
still have the disadvantage of being powerful greenhouse gases!
Other ozone and CFC notes on the
detailed chemistry have been already written on
The CFC chemistry of ozone depletion and how this environmental problem
was solved
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HALOGENOALKANES chemistry notes INDEX
All Advanced A Level Organic
Chemistry Notes
Index of basic Oil and Organic
Chemistry Revision Notes
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