9.10
Ozone, CFC's and halogen
organic chemistry links
Abbreviations used:
CFC =
chlorofluorocarbon;
HCFC = hydrochlorofluorocarbon;
HFC
= hydrofluorocarbon
CFCs, Ozone and Free
Radicals
Since these
notes where written I've done a more detailed page on the free radical chemistry
of ozone formation and destruction in catalytic cycles
-
CFCs –
what is so good about them? (before we get into the problems
they cause!)
-
If enough energy
is
supplied by heat or by visible/uv electromagnetic radiation, or the is weak
enough, a covalent bond can break in two ways. This illustrated with
the molecule chloromethane CH3Cl.
-
The
bond breaks unevenly
where the electron bond pair can stick with one fragment and a positive
and negative ion form.
-
The bond
breaks evenly, where the
bonding pair of electrons are equally divided between two highly
reactive fragments called free radicals.
-
Free radicals are
characterised by having an unpaired electron not involved in
a chemical bond.
-
The
. means the 'lone' electron on the free radical, which is not part of
a bond anymore, and wants to pair up with another electron to form a
stable bond – that's why free radicals are so reactive!
-
e.g.
CH3Cl
==> CH3. + .Cl
(called homolytic bond fission)
-
shows
what happens to the molecule.
-
Homolytic bond
fission can occur by molecules hit by uv photons i.e.
ultraviolet electromagnetic radiation of quite high energy –
great enough to cause homolytic bond fission.
-
The chemistry of free radicals
is important in the current environmental issue of ozone
layer depletion.
-
Chlorofluorocarbons
(CFC's for shorthand) are organic
molecules containing carbon, fluorine and chlorine
-
e.g. dichlorodifluoromethane
has the formula CCl2F2 (shown above).
-
They are very useful low boiling
organic liquids or gases, until recently, extensively used in refrigerators and
aerosol sprays e.g. repellents.
-
They are
relatively
unreactive, non–toxic and have low flammability, so in many ways they
are 'ideal' for the job they do.
-
However it is their
chemical stability in the environment that eventually causes the ozone
problem but first we need to look at how ozone is formed and
destroyed in a 'natural cycle'. This presumably has been in
balance for millions of years and explains the uv ozone protection in
the upper atmosphere – the stratosphere.
-
Ozone is formed in the
stratosphere by free radical reactions.
-
'ordinary' stable
oxygen
O2 (dioxygen) is split (dissociates) into two
by high energy ultraviolet electromagnetic radiation (uv photon energy
'wave packets from Planck's Equation E = uv) into two oxygen atoms (which are themselves
radicals) and then a 'free' oxygen atom combines with an oxygen
molecule (dioxygen) to form ozone (trioxygen).
-
The ozone is a highly
reactive and unstable molecule and decomposes into dioxygen when hit
by other uv light photons. The oxygen atom radical can do several
things including ...
-
This last reaction is
the main uv screening effect of the upper atmosphere and the
ozone absorbs a lot of the harmful incoming uv radiation from the
Sun.
-
If the ozone levels
are reduced more harmful uv radiation reaches the Earth's surface
and can lead to medical problems such as increased risk of sunburn
and skin cancer and it
also accelerates skin aging processes.
-
There
is strong evidence to show there are 'holes' in the ozone layer with
potentially harmful effects, so back to the CFC problem for some
explanations and solutions!
-
The chemically
very stable CFCs diffuse up
into the stratosphere and decompose when hit by ultraviolet light
(uv) to produce free radicals, including free chlorine atoms, which
themselves are highly reactive free radicals.
-
The formation
of chlorine atom radicals is the root of the problem because
they readily react with ozone and change it back to much more stable
ordinary oxygen.
-
(i)
O3 + Cl.
==>
O2 + ClO.
bye bye ozone! and no uv
removed in the process!
-
and then (ii)
ClO
+ O ==> Cl + O2 , which means the 'destructive' Cl
atom free radical is still around!
-
The two
reactions above involving chlorine atoms are known as a catalytic
cycle because the chlorine atoms from CFC's etc. act as a
catalyst in the destruction of ozone.
-
So, if you add up (i) +
(ii) you get
-
O3 + O ==>
2O2
and is an example of the catalytic cycle
-
(i)
X▪ + O3 ===>
XO▪ + O2 |
(ii)
XO▪ + O ===> X▪
+ O2 |
(iii)
O + O3 ===> 2O2 |
-
X can be Cl,
NO or even OH, e.g. Cl from CFCs, NO from road
vehicles, OH from atmospheric water.
-
See extra note
on NO cycle
-
Therefore
many
countries are banning the use of CFCs, but not all despite
the fact that scientists predict it will take many years for the
depleted ozone layer to return to its 'original' O3
concentration and alternatives to CFC's are already being marketed.
-
Alternatives to
CFCs i.e. HFCs and HCFCs
-
The idea is to use
replacement compounds that are less harmful to the ozone layer.
-
The molecules listed
below contain C–H bonds and are broken down in the lower
troposphere before they reach the ozone layer in the
stratosphere.
-
Hydrochlorofluorohydrocarbons (a HCFC is composed of hydrogen,
chlorine, fluorine and carbon atoms)
-
e.g.
CH3CFCl2
1,1–dichloro–1–fluoroethane
-
HCFCs
break down more easily than
CFCs in the atmosphere, so are less destructive towards
ozone.
-
Hydrofluorocarbons (a
HFC is composed of hydrogen, fluorine
and carbon atoms)
-
e.g.
CH2FCF3
1,1,1,4–tetrafluoroethane
-
HFCs are much better to use than CFCs
because they do NOT contain chlorine atoms.
-
Alkanes
(composed of hydrogen and carbon atoms)
-
e.g. butane
CH3CH2CH2CH3
-
but they are
flammable!
-
However, all of
these molecules are greenhouse gases and will contribute to
global warming!
-
Index of all my notes
on halogenoalkanes
CHLOROALKANES
(halogenoalkanes)
- Alkanes are
usually not very reactive unless burned!
BUT they will react with reactive chemicals like chlorine
when heated or subjected to uv light to form chlorinated hydrocarbons.
- Despite the reactivity of chlorine you
still need something extra to initiate the
reaction.
- A substitution reaction occurs
and a chloro–alkane is formed e.g.
- a hydrogen is swapped for a chlorine
and the hydrogen combines with a chlorine atom
- ethane + chlorine ==> chloroethane +
hydrogen chloride
- C2H6 + Cl2
===> C2H5Cl + HCl
-
+ Cl2 ==>
+ HCl
- Chloro–alkanes are useful
solvents in the laboratory or industry but though their vapours can be
harmful.
-
Index of all my notes on
halogenoalkanes
Halogen – alkene addition reaction
|
Used as a test for alkenes: Hydrocarbons are colourless.
Bromine
dissolved in water or trichloroethane solvent forms an orange
(yellow/brown) solution. When bromine solution is added to both an alkane or an alkene the
result is quite different. The alkane solution remains orange – no
reaction. However, the alkene decolourises the bromine as it forms
a colourless dibromo–alkane compound – see equations below. |
Ex 1.
.... or
ethene + bromine ==> 1,2–dibromoethane
Ex 2.
....
or
propene + bromine ==> 1,2–dibromopropane
Ex 3.
cyclohexene + bromine ==> 1,2–dibromocyclohexane
Index of all my notes on
halogenoalkanes
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Advanced
A
level chemistry: Organic Halogen Compound
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