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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
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.
Alkanes and Petrochemical Industry INDEX
All Advanced Organic
Chemistry Notes
Organic reaction mechanism index
and terms defined and explained.
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
A basic introduction to
the chemistry of
alkanes
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The reactivity of alkanes - or 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 the 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. 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 ▪ dot represents
the unpaired electron).
-
These free radicals initiate and propagate a rapid
chain reaction e.g. in combustion!
-
Similarly, heat or uv light can generate
free radicals by 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.
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.
- Despite the reactivity of chlorine you
still need something extra to initiate the
reaction like uv light at room temperature or higher temperature (250oC
- 400oC) in this case.
- 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 2nd chlorine atom e.g.
- (i) methane + chlorine ===> chloromethane +
hydrogen chloride
- CH4 + Cl2
===> CH3Cl + HCl
-
 + Cl2
===>
+ HCl
-
If other hydrogen
atoms are available on the original hydrocarbon then polysubstituted
chloroalkanes will be formed
- The basic schematic mechanism for the free
radical chain reaction between an alkane and chlorine is given in
the mechanism 6 diagram below.
-
- Mechanism 6. R = H or alkyl in various combinations, when
all the R groups s are H, CR3H = methane
-
Step
(1)
is the initiation
step when the chlorine molecule is split into two chlorine atoms/radicals
by homolytic bond fission by the impact-absorption of the
ultraviolet photon. Its quantum of energy, E=hv, must
be great enough to break the Cl-Cl bond.
-
Homolytic
bond fission
means the original
pair of (Cl-Cl) bonding electrons is split between the
two radicals formed - hence each radical has an unpaired
electron.
-
Step (1)
illustrates how to use
half-arrows to show a homolytic bond fission step
-
The red dots represent
the unpaired electron on the
free radical and the half-arrows show the individual electron
'shifts'.
-
The
breaking of the Cl-Cl bond in the chlorine molecules begins the
reaction because it is the weakest of the bonds of any reactant
molecule involved.
-
Bond enthalpies/kJmol-1:
Cl-Cl = 242, C-C = 348, C-H = 412, and even the new bond
formed, C-Cl, is 338.
-
As already stated, free
radicals are highly reactive species with an unpaired electron
and tend to form a new bond as soon as is
possible by e.g. in this case by ...
-
Steps
(2) and (3)
are chain propagation steps, because as well as producing one
of the reaction products, a new free radical is also produced to
continue the reaction, which is why such reactions are sometimes
referred to as free radical 'chain reactions'.
-
Steps
(4) to (6)
are three possible chain
termination steps which remove the highly reactive free radicals
as two unpaired electrons form a new bond, in this case single C-C
covalent bonds.
-
Termination steps break the
chain i.e. they prohibit a possible chain propagation step.
-
Step (5)
illustrates how to use
half-arrows to indicate a termination step where the unpaired
electrons of the two radicals pair up to form a new bond, in
this case a C-Cl bond.
-
When the
alkane is methane, traces of ethane are found in the final
mixture of products.
-
This provides evidence for a mechanism
involving a methyl radical.
-
It would
be formed from combining two methyl radicals:
H3C.
+ .CH3 ===> H3C-CH3
-
Similarly if the alkane was
ethane, traces of butane are formed, similarly, for other
alkanes you will traces of an alkane with twice as many carbon
atoms as the original reactant alkane.
-
- Diagram mechanism 48: I've sketched out the
mechanism for chlorine reacting with methane to give chloromethane
(the monosubstituted product).
- Chlorine atoms produced in the initiation
step (via uv or heat).
- In the two propagation steps (a) a chlorine
atom abstracts hydrogen atom from methane to form HCl. (b) The
resulting methyl radical abstracts a chlorine atom from a chlorine
molecule to form the product. BUT, you are left with a chlorine
atom/radical that continues the chain reaction via (a).
- Termination occurs when two radicals meet and
combine. This has a very low probability because the free radical
concentrations are very small in the first place. This allows
propagation steps (a) and (b) to occur several hundred times before
termination occurs.
-
- Diagram mechanism 52: I've sketched out the
propagation steps for the polysubstitution of methane.
-
to redo
- All the carbon based free radicals involved
in the chlorination of methane are pyramidal in shape. The
pyramid shape arises from the mutual repulsion of four lots of
electrons (best appreciated in the dot & cross diagrams), three
bonding pairs and one lone electron. The bond angles would be around
109o (as in methane).
- (ii) ethane + chlorine ==> chloroethane +
hydrogen chloride
-
CH3CH3 + Cl2
===> CH3CH2Cl + HCl
-
+ Cl2 ===>
+ HCl
-
- Diagram mechanism 49: The mechanism for
the monosubstitution of ethane with chlorine.
(iii) butane + chlorine ===>
{1-chlorobutane or 2-chlorobutane} + hydrogen chloride
+ 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 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 atom or alkyl radical.
(iv) cyclohexane + chlorine ===>
chlorocyclohexane + hydrogen chloride
+ Cl2
 + HCl
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.
The reaction of alkanes with bromine
(i) 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
to redo
Diagram mechanism 51: The mechanism for the monosubstitution
of propane with bromine.
The big dots represent the unpaired
electron of the atom or alkyl radical.
Again, note the possibility of two structural
positional isomers, 1-bromopropane and 2-bromopropane. The mechanism is
identical to that for chlorination of alkanes. (ii) 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.
Bromine compounds are used as flame
retardant additives in polymer compositions and fire extinguishers.
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Alkanes and Petrochemical Industry INDEX
All Advanced Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
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