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1.6 Chlorination and bromination of
alkanes, reaction mechanisms and the structure and uses of products
Part 1.
ALKANES and the PETROCHEMICAL INDUSTRY - Doc Brown's Advanced A Level
Organic Chemistry Revision Notes
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The reactions of alkanes with
halogens are important processes in the chemical industry for the
production of a variety of useful products.
The reaction between
alkanes and halogens (chlorine and bromine) is described by reaction
conditions, equations and the detailed mechanisms of halogenation,
namely chlorination and bromination.
Sub-index for this page on the
halogenation of alkanes
1.6.1
Comments on lack of
alkane reactivity
1.6.2
The reaction
between chlorine and alkanes - uses of chloroalkanes
1.6.3
The reaction
between bromine and alkanes - uses of bromoalkanes
1.6.4
Some footnotes on
other examples of free radical chemistry
See also
Organic reaction mechanism index
and terms defined and explained.
A basic introduction to
the chemistry of
alkanes
1.6.1 The reactivity of alkanes - or rather the lack of it!
Alkanes
are not very reactive molecules. Most
reactions of alkanes require some 'energetic' input to initiate a reaction e.g.
high temperature and catalyst for cracking, uv light for chlorination or a spark
to ignite them in combustion. In many cases this involves initiating free radical reactions.
A combination
of two
reasons account for the lack of reactivity of alkanes compared to most
other homologous groups of organic molecules.
-
Bond
Strength: Strong sigma (σ) bonds
in alkane molecules
-
The
single covalent
C-C bonds (bond enthalpy 348 kJ mol-1) and
the C-H bonds (bond enthalpy 412 kJ mol-1) in alkanes are very strong so bond fission does not
readily happen.
-
The carbon and hydrogen atomic radii
are small, giving a short and
strong bonds.
-
Therefore, in terms of C-C and C-H
bond fission, alkane reactions
will tend to have high activation energies resulting
in slow/no reaction.
-
Nature of
bonding:
-
Carbon
and hydrogen have similar electronegativities (C = 2.5, H =
2.1, Δelec = 0.4), so there is effectively no polar bond
(non-polar bond) giving a slightly positive carbon (Cδ+) which
can be attacked by electron pair donating nucleophiles.
[e.g. contrasting with halogenoalkanes (δ+C-Clδ-)
or aldehydes/ketones
(δ+C=Oδ-)]
-
All the
C-C and C-H bonds in alkanes are single covalent (saturated
hydrocarbons) with no region of
particularly high electron density like a pi (π) bond
electron cloud
susceptible to attack by electron pair accepting
electrophiles. [e.g. like in the double C=C bond in
unsaturated
alkenes]
-
Free radicals are highly reactive
species with an unpaired electron
-
e.g. methyl radical CH3▪,
ethyl radical CH3CH2▪., the latter better
shown as ▪CH3 and ▪CH2CH3
, because the ▪ dot represents
the unpaired electron, which is in an orbital of a carbon atom.
-
These free radicals facilitate (propagate) rapid
chain reactions e.g. in combustion!
-
Heat or uv light can generate
free radicals by homolytically splitting halogen molecules such as chlorine and bromine
into atoms that can then propagate a chain reaction to form substituted
products known as halogenoalkanes (haloalkanes).
-
Although Cl▪ and Br▪ are individual atoms, they are highly reactive free radicals because
they have an unpaired electron and seek another electron on another atom to
'pair up' and form a stable bond with this other atom.
TOP OF PAGE
and sub-index
1.6.2 The reaction of alkanes with chlorine
As pointed out already, alkanes are not very reactive unless burned and they will react with reactive chemicals like chlorine
and bromine
when heated or subjected to uv light to form chlorinated alkane hydrocarbons.
The butane - chlorine substitution reaction
butane + chlorine ===>
{1-chlorobutane or 2-chlorobutane} + hydrogen chloride
to give two possible isomeric (C4H9Cl)
monosubstitution products.
+ Cl2
{
or
 }
+ HCl
+ Cl2
 
or
 
+ HCl
Diagram mechanism 50: Sketch of the mechanism for the
monosubstitution of butane with chlorine. Note the possibility of different
abstractions at different positions on the carbon chain of butane ultimately giving a mixture of
two structural isomer products - the structural isomers
1-chlorobutane and 2-chlorobutane.
The big dots represent the
unpaired electron of the halogen atom or alkyl radical.
Technically, you can expect traces of octane,
3,4-dimethylhexane and 3-methyheptane in the final reaction mixture.
The presence of CH3(CH2)6CH3,
CH3CH2CH(CH3)CH(CH3)CH2CH3
and (CH3)2(CH2)5CH3
from the termination steps support the mechanism described
above. The other two
possible termination steps will give 1-chlorobutane and
2-chlorobutane.
The cyclohexane - chlorine substitution reaction
cyclohexane + chlorine ===>
chlorocyclohexane + hydrogen chloride
+ Cl2
 + HCl
In all these examples you can substitute
bromine for chlorine to predict the mechanism steps and the bromoalkane
products (see
section 1.6.3 below).
The uses of chloroalkanes
Chloromethane and chloroethane are gases
at room temperature, but bigger chloro–alkane molecules are liquids and
useful
solvents in the laboratory or industry. However, they are still quite volatile
and chlorohydrocarbon vapours can be
harmful if breathed in. Halogenoalkanes are used as refrigerants.
Long-chain chloroalkanes have a low solubility in
water and are used as flame retardants in rubber, paint, leathers and
sealing compound formulations.
TOP OF PAGE
and sub-index
1.6.3 The reaction of alkanes with bromine
The bromine - ethane reaction
e.g. monosubstitution: ethane
+ bromine ===> bromoethane + hydrogen
bromide
Initiation step - homolytic
bond fission of the bromine molecule
Br2
===> 2Br•
Propagation steps - two needed
to form the product
CH3CH3
+ Br• ===> •CH2CH3 +
HBr
•CH2CH3
+ Br2 ===> CH3CH2Br
+ Br•
Termination steps - the
meeting of any two radicals
•Br
+ •Br ===> Br2
•CH2CH3
+ •Br ===> CH3CH2Br
•CH2CH3
+ •CH2CH3 ===> CH3CH2CH2CH3
You should find a trace of butane in the final
reaction mixture of products - evidence supporting this mechanism.
The propane - bromine substitution reaction
propane + bromine
===> {1-bromopropane or 2-bromopropane} +
hydrogen bromide word equation followed by displayed formula
equation, abbreviated structural formula equation and skeletal formula equation
+ Br2
or
+ HBr
+ Br2
{
or
 }
+ HBr
+ Br2
or
+ HBr
Note the formation of two isomeric
(C4H9Br) monosubstituted products, 1-bromopropane and 2-bromopropane.
Diagram mechanism 51 (above): The mechanism for the monosubstitution
of propane with bromine.
The big dots represent the unpaired
electron of the atom or alkyl radical on a chlorine atom (Cl•) or a carbon
atom of the carbon based free radical (e.g. •CH2CH2CH3).
Again, note the possibility of two structural
positional isomers (C3H7Br), 1-bromopropane and 2-bromopropane.
The mechanism is
identical to that for chlorination of alkanes.
The cyclohexane - bromine substitution reaction
cyclohexane + bromine ===>
bromocyclohexane C6H11Br
+ Br2
 + HBr
There is only one possible monosubstitution product.
If excess bromine present, then other disubstituted
products can form
Further substitution can take place to give three
isomeric disubstitution products of
C6H10Br2 e.g.
+ Br2
 ,
 ,
 + HBr
overall ...
cyclohexane + bromine ===>
{1,2- & 1,3- &
1,4-dibromocyclohexane} + hydrogen bromide
+ 2Br2
,
,
+
2HBr
Another example of a multiple substitution of an alkane by
a halogen
Note: I don't think iodine is reactive enough to produce
iodoalkanes by this method.
Uses of bromoalkanes
Bromine compounds are used as flame
retardant additives in polymer compositions and fire extinguishers.
Due to their toxicity, bromine compounds are also used as
fumigants e.g. bromomethane has been used to control insects, weeds and
rodents, but this application is controversial and is being phased out.
TOP OF PAGE
and sub-index
1.6.4 Some footnotes
on free radical chemistry
(a)
First encounters with free radical
In your advanced level chemistry
course you usually first come across free radical
substitution reactions when looking at the reaction between hydrogen and
chlorine (one of the simplest free radical mechanisms, below) or the methane -
chlorine reaction (already described in detail above on this page).
(i) initiation step:
Cl2
===> 2Cl.
Homolytic bond fission by
heat or light to give two chlorine free radicals.
(ii) propagation steps:
Cl.
+ H2 ===> HCl + H. followed by
H. + Cl2
===> HCl + Cl.
Two steps giving the product and a
free
radical to continue the chain reaction
(iii) termination steps:
H. + Cl. ===> HCl or
2H. ===> H2
or 2Cl.
===> Cl2
The three possible ways of
ending a free radical reaction chain sequence.
You can see it is very similar to the
methane - chlorine reaction, a H radical instead of CH3 radical, but no
complications due to further substitution.
(b)
Use of CFCs and destruction of the
ozone layer
The chemistry of ozone
formation and its destruction by CFCs involves free radical chemical
reactions.
Ozone, CFC's and halogen organic chemistry
links
(c)
Polymerisation
Ethene can be polymerised to
poly(ethene) using oxygen or organic peroxide catalysts which generate
free radicals which facilitate polymerisation.
Free radical polymerisation
to give poly(alkene) polymers e.g. ethene ==> poly(ethene)
(d)
Biochemistry
Many reactions in living
cells quite naturally generate free radicals as a by-product, which, because of their high reactivity,
need to be removed before causing unwanted chemical reactions and cell
damage.
Free radicals are formed in the
body's normal metabolic activity e.g. respiration and they are important
in the production of enzymes and hormones.
Free radicals can damage cell
membranes and the genetic molecules of DNA.
Excess of free radicals are linked to
premature aging and various chronic illnesses.
Smoking, sunbathing and air pollution
from road vehicles also exposes the body to free radicals.
Note that uv radiation in bright
sunlight can directly produce free radicals in skin cells!
However, the body has a natural
defence mechanism in the form of molecules called anti-oxidants
which counteract excess free radicals and prevent them harming cell
function.
Two vitamins are known to be part of
this defence mechanism. Vitamin C in fresh fruit and vegetables
and Vitamin E in fats, oils, nuts, egg yolk, liver and green vegetables.
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+ Cl2
+ HCl
+ Cl2 ===>
+ HCl
