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15.1 Alkanes
a
Compounds containing only carbon and hydrogen are called hydrocarbons. This
class of compound can be sub-divided into alkanes, alkenes and arenes.
b
understand the general unreactivity of alkanes, including towards polar reagents
c
describe the chemistry of alkanes as exemplified by the following reactions of
ethane:
(i)
combustion
(ii)
substitution by chlorine and by bromine
d
describe the mechanism of free-radical substitution at methyl groups with particular
reference to the initiation, propagation and termination reactions
e
explain the use of crude oil as a source of both aliphatic and aromatic hydrocarbons
f
suggest how cracking can be used to obtain more useful alkanes and alkenes of
lower Mr from larger hydrocarbon molecules
ALKANES
CEDAR COLLEGE
89
ALKANES
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15 Hydrocarbons
Compounds containing only carbon and hydrogen are called hydrocarbons. This class of compound can
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be sub-divided into alkanes, alkenes and arenes.
Learning outcomes
Candidates should be able to:
15.1 Alkanes
a) understand the general unreactivity of alkanes, including towards polar
reagents
b) describe the chemistry of alkanes as exemplified by the following
reactions of ethane:
(i) combustion
(ii) substitution by chlorine and by bromine
c) describe the mechanism of free-radical substitution at methyl groups
with particular reference to the initiation, propagation and termination
reactions
d) explain the use of crude oil as a source of both aliphatic and aromatic
hydrocarbons
e) suggest how cracking can be used to obtain more useful alkanes and
alkenes of lower Mr from larger hydrocarbon molecules
15.2 Alkenes
a) describe the chemistry of alkenes as exemplified, where relevant, by the
following reactions of ethene and propene (including the Markovnikov
addition of asymmetric electrophiles to alkenes using propene as an
example):
(i) addition of hydrogen, steam, hydrogen halides and halogens
(ii) oxidation by cold, dilute, acidified manganate(VII) ions to form the
diol
(iii) oxidation by hot, concentrated, acidified manganate(VII) ions
leading to the rupture of the carbon–carbon double bond in order to
determine the position of alkene linkages in larger molecules
(iv) polymerisation (see also Section 21)
b) describe the mechanism of electrophilic addition in alkenes, using
bromine/ethene and hydrogen bromide/propene as examples
c) describe and explain the inductive effects of alkyl groups on the stability
of cations formed during electrophilic addition
d) describe the characteristics of addition polymerisation as exemplified by
poly(ethene) and PVC
e) deduce the repeat unit of an addition polymer obtained from a given
monomer
f)
identify the monomer(s) present in a given section of an addition
polymer molecule
g) recognise the difficulty of the disposal of poly(alkene)s, i.e. nonbiodegradability and harmful combustion products
Back to contents page
CEDAR COLLEGE
www.cie.org.uk/alevel
90
ALKANES
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37
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ALKANES
Alkanes are the hydrocarbons that make up most of crude oil and natural gas.
In alkane molecules, every carbon atom forms four single covalent bonds.The
electrons in these four bonds repel each other to tetrahedral positions
around each carbon atom.
structure of propane showing the tetrahedral shape around each carbon atom.
1
PROPERTIES
The alkanes are non-polar molecules with only van der Waals’ forces between molecules.
This means that they are volatile (evaporate easily), with the first four members being
gases at room temperature. Due to their non-polar nature, they are also insoluble in water.
As the molecules get longer and larger, the intermolecular forces increase, so the melting
and boiling points rise as the number of carbon atoms per molecule increases.
Because of the weak intermolecular forces they are highly volatile. As the series is
ascended due to the increase in the number of electrons the strength of the van der
Waals’ forces increases.
Consequently the melting and boiling point, density and viscosity increases, while
volatility and vapour pressure decreases.
2
CEDAR COLLEGE
91
ALKANES
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PROPERTIES
For any one set of isomers the boiling point decreases as branching
increases.
This is because as branching increases the molecules become more
nearly spherical and expose less surface area.
Hence less intermolecular forces are exerted. The melting point also
depends on the packing of the molecules.
Closer packing leads to higher melting point.
3
STRUCTURES
c-
c- m
H
1
A-
t
H
H
it
H
H
It
1
I
C
-
I
C
-
I
I
I
H
It
't
H
I
-
cI
tl
displayed formula
displayed formula
H
I
C
-
I
C
I
1
'+
H
of
hexane
-11
N
skeletal formula
othexane
of pentane
A
,
qpentane
|
F
of pentane
t
,
HYYCWH
It
skeletal formula
Czltg
It
CIA
|\H
com
Hlc
structural formula
-
It
H
displayed formula
502
302
92
M
y
skeletal formula
qcydohexane
ofcydohexane
4
CEDAR COLLEGE
It
A
H
1
H
CHZCHZCHZCHZCHZ
~
It
A
c-c11Cut 't
1
It
+
41+20
ALKANES
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REACTIVITY
The bond enthalpies of C–C and C–H bonds are relatively high, so the
bonds in alkanes are difficult to break.
c-
In addition, these bonds in alkanes are non-It polar.
ThisIt means
H
H that alkanes
A
H
I
I
I
It
H
H alkalis,
It H such
ACI
are very unreactive with ionic reagents Itin water,
as
acids,
C
C -11
C
I
oxidising agents and reducing agents.
c-c11Cut
cm
cc11Cut
c- m
H
1
A-
1
H
H
1
1
it
H
It
A
1
-
I
H
t
I
C
I
IC
-
-
'tI
ItI
cI
c+
tlI
I
-
-
CI
-
'
H
1
I
it
1
A
pentaneformula
ofdisplayed
1
'+
tl
't
It
H
displayed formula of hexane
It
H
H
H
~
of
skeletal formula
othexane
skeletal
formula
othexane
hexane
H
It
H
A It
A , ,It
structural formula
CHZCHZCHZCHZCHZ
CHZCHZCHZCHZCHZ
~
displayed formula
N
N
I
C1 -11
displayed
ThereH are, however,
threeformula
important reactions of alkanes involving
't
H
H
t
I
1
HI
't
combustion, halogenation
and cracking.
of pentane
A-
I
-
-
structural formula
qpentane
qpentane
5
CIA
|\H
com
CIA
|\H
com
Hlc
Hlc
,
,
M
M
yy
HYYCWH
HYYCWH
It
It
skeletal formula
formula
pentane
ofskeletal
|
|
F
F
of pentane
-
-
It
H
It
H
displayed formula
displayed formula
skeletal formula
skeletal formula
qcydohexane
qcydohexane
ofcydohexane
ofcydohexane
COMBUSTION
In air or oxygen, alkanes are completely oxidised to carbon dioxide and water, and the
reaction is highly exothermic. e.g.
+
t
302
502
41+20
Czltg
+
t
302
502
41+20
Czltg
In a limited supply of air or oxygen, alkanes burn incompletely forming highly toxic
carbon monoxide, as well as carbon dioxide and water.
If the supply of air or oxygen is even more restricted, the major products will be soot
(carbon) and water.
CZHS
CZHS
CzH8
CzH8
++
++
3.502
3.502
->3CO
->3CO
202
202
+
+
3C
3C
+
+
4h20
4h20
4h20
4h20
6
CEDAR COLLEGE
Catty
Catty
,g,g
,,
++
02cg,
n+%) ) 02cg
n+%
,
((
93
nC02(g,,
+
+
ALKANES
9124204
9124204) )
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/
th
x
Onlineisomerisation
Classes : Megalecture@gmail.com
CZHS
Nn
3.502
+
CzH8
neptune
2,4 dimethyl pentane
.
->3CO
202
+
4h20
+
3C
4h20
+
⇐
COMBUSTION
tHz
/
The exothermic nature of the combustion of alkanes has led to their widespread use as
fuels.
methylwgdohexane
Combustion is an exothermic reaction. The greater the number of carbon atoms, the more
energy produced and the greater the amount of oxygen needed for complete combustion.
Catty
CH4μ,
t
,g ,
+
202$ )
(
n+% )
COqg,
Czottkcyt 30.502cg,
ZCO
nC02(g,
02cg,
+
DHOI
+24204 )
2002$
ZNO
9124204 )
+
+21429,
BH=
,
ZCOZ
7
Nz
NO
+
+
-890
-
KJMOT
13368
'
KJMOTI
Nz
Oz
POLLUTION
Motor vehicles are generally the main source of air pollutants. Engines that
burn petrol or diesel fuels can pollute the air for three main reasons:
they do not burn the fuel completely
the fuel contains impurities
they run at such a high temperature that nitrogen and oxygen in the
air can react.
8
CEDAR COLLEGE
94
ALKANES
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c-
It
A
CIA
|\H
com
structural
formula
Online
Classes
: Megalecture@gmail.com
, ,
CHZCHZCHZCHZCHZ
~
Hlc
qpentane
skeletal formula
of pentane
M
y
HYYCWH
It
|
POLLUTION F
-
It
skeletal formula
H
qcydohexane
displayed formula
POLLUTANT
PROBLEMS
Carbon monoxide
A toxic gas that combines strongly with haemoglobin, which
means that the blood can carry less oxygen. In low doses, this can
put a strain on the heart. In higher doses, it kills
Oxides of nitrogen:
NO & NOCzltg
2
Reacts with moist air to make nitric acid in acid rain. Affects the
lungs
and502
can cause breathing
+
t
302problems
41+20
Unburned
hydrocarbons
Some of the hydrocarbons, such as benzene, are carcinogenic.
Others react with ozone to form harmful pollutants
ofcydohexane
9
CZHS
3.502
+
CzH8
->3CO
202
+
4h20
+
3C
4h20
+
CATALYTIC CONVERTERS
Catalytic converters have done a great deal to improve air quality.
+
( up reactions
TheCatty
catalyst speeds
from motor
vehicle )
+
9124204
n+% ) 02cg, that remove pollutants
nC02(g
,g ,
,
exhausts.
The reactions convert oxides of nitrogen to nitrogen and oxygen.
They also convert carbon monoxide to carbon dioxide and unburned
hydrocarbons to carbon dioxide and steam.
ZCO
+
ZNO
ZCOZ
Nz
NO
+
+
Nz
Oz
10
CEDAR COLLEGE
95
ALKANES
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Evidence fo
GREENHOUSE
EFFECT
Test yourself
Without2the
atmosphere,
the whole
wouldice
besamples
colder than
Antarctica.
The and
Two
of the techniques
usedEarth
to analyse
are mass
spectrometry
greenhouse
effectspectroscopy
keeps the surface
of the
Earth about 30 °C warmer than it would
infrared
(see Topic
15).
Which
technique is suitable
measuring the
ratiothere
of oxygen
in ice?
be if therea)were
no atmosphere.
With nofor
greenhouse
effect,
wouldisotopes
be no life
b) Which technique is suitable for measuring the concentration of carbon
dioxide in air samples from ice bubbles?
3 Ice samples contain only tiny traces of some pollutants. Suggest two precautions
When thenecessary
Sun’s radiation
reaches
the Earth’s
about
30%toislaboratories
reflected for
to achieve
accurate
resultsatmosphere,
when samples
are sent
analysis.
into space,
20% is absorbed by gases in the air and about 50% reaches the surface
4 What can you conclude about changes in the atmosphere of CO2 from the
of the Earth.
information in Figure 18.4?
5
Analysis
ice inisGreenland
shown
that
level
of lead
pollution
1969
The surface
of theofEarth
very muchhas
cooler
than
thethe
Sun.
This
means
that itinradiates
was 200 times the level in 1700. In 1990, the level of lead was just 10 times its
most of itsnatural
energy
back
into space.
Some offor
this
infrared
radiation is absorbed by
level.
Suggest
explanations
these
changes.
on Earth.
molecules in the air and warms up the atmosphere. This is the greenhouse effect.
18.3 Greenhouse gases and climate change
11
Without the atmosphere, the whole Earth would be colder than Antarctica.
The greenhouse effect keeps the surface of the Earth about 30 °C warmer than
it would be if there were no atmosphere. With no greenhouse effect, there
would be no life on Earth.
Most of the energy in the Sun’s radiation is concentrated in the visible and
ultraviolet regions of the spectrum. When this radiation reaches the Earth’s
atmosphere, about 30% is reflected into space, 20% is absorbed by gases in the
air and about 50% reaches the surface of the Earth.
GREENHOUSE EFFECT
Visible
from
thethe
Visibleand
andUV
UVradiation
radiation
from
Sun
Sun warms
warms the
thesurface
surfaceofofthe
theEarth.
Earth.
Figure 18.5 !
Greenhouse e
Some
Some of
of the
theSun’s
Sun’s
radiation
radiationisisreflected.
reflected.
Greenhouse
Greenhousegases
gasesininthe
the
atmosphere
atmosphereabsorb
absorbsome
someofofthe
outgoing
infrared
radiation.
the outgoing
infrared
radiation.
The warm
warm surface
surface of
the
Earth
radiates
of the Earth radiates
infrared radiation.
infrared
radiation.
12
The surface of the Earth is very much cooler than the Sun. This means that it
radiates most of its energy back into space at longer infrared wavelengths.
Some of this infrared radiation is absorbed by molecules in the air and warms
up the atmosphere. This is the greenhouse effect.
The Earth’s average temperature stays the same if the energy input from the
Sun is balanced by the re-radiation
that
CEDAR COLLEGE
96of energy back into space. AnythingALKANES
changes this steady state gradually leads
to
global
warming
or
global
cooling.
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Nitrogen and oxygen make up most of the air, but they do not absorb
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ENHANCED GREENHOUSE EFFECT
The Earth’s average temperature stays the same if the energy input from the Sun is
balanced by the re-radiation of energy back into space. Anything that changes this
steady state gradually leads to global warming or global cooling.
The gases in the air that do absorb infrared radiation are called greenhouse gases.
The natural greenhouse gases that help keep the Earth warm are water vapour,
carbon dioxide and methane. Water vapour makes the biggest contribution.
Since the Industrial Revolution, human activity has led to a marked increase in the
concentrations of greenhouse gases in the atmosphere. This enhances the
greenhouse effect. As a result, the Earth warms up more than it would naturally,
and as the Earth warms up, its climate changes.
13
GREENHOUSE GASES
This increase in temperature could cause rising sea levels, resulting in the
flooding of low-lying areas of land. The climate will also change around the
world, with more extreme weather predicted.
Unfortunately, catalytic converters can do nothing to reduce the amount of
carbon dioxide (a greenhouse gas) given off in the exhaust gases of cars.
Carbon dioxide is not a toxic gas but it is still considered a pollutant
because of its contribution to enhanced global warming.
14
CEDAR COLLEGE
97
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FRACTIONAL DISTILLATION OF CRUDE OIL
B.P / oC
# of C atoms
Name of fraction
Uses
< 25The exothermic
1 - 4nature of the combustion
LPG of alkanes has led to
Camping
their Gas
widespread use as fuels in industry, homes and vehicles.
The combustion of alkanes involves a radical mechanism which occurs
40-100
- 12
GASOLINE
Petrol
rapidly in the6gas
phase. This means
that liquid and solid alkanes must
vaporise
before they burn
less volatile alkanes burnPetrochemicals
less easily.
100-150
7 - 14and it explains why
NAPHTHA
The burning of alkanes is immensely important in any advanced
150-200
- 15
Fuelin
technological11 society.
It is used to KEROSINE
generate energy of one kindAviation
or another
power stations,
furnaces, domestic
heaters, cookers, candlesCentral
and vehicles.
220-350
15 - 19
GAS OIL (DIESEL)
Heating Fuel
Unfortunately, this mass burning of alkanes is now accepted to be the major
>350cause of global
20 -warming
30
LUBRICATING
OIL
Lubrication
Oil
and the
increased greenhouse
effect (Section
18.3).
In a limited supply of air or oxygen, alkanes burn incompletely forming
> 400
30 - 40
FUEL OIL
Power Station Fuel
1 !
highly toxic carbon monoxide, as well as carbon dioxide and water. If the
s cylinders contain propane
or-oxygen
is even more
restricted,
will be soot
> 400supply of air40
50
WAX,
GREASE the major products
Candles
uel.
(carbon) and water.
> 400
> 50
BITUMEN
Road surfaces
C4H10(g) + 2 12 O2(g) → 4C(s) + 5H2O(g)
15
C4H10(g) + 4 12 O2(g) → 4CO(g) + 5H2O(g)
Carbon monoxide is colourless, it has no smell and it is very poisonous.
It reacts irreversibly with haemoglobin in the blood to form
carboxyhaemoglobin, and this reduces the capacity of the blood to carry
oxygen. Because of this, it is dangerous to burn hydrocarbon fuels in a poor
supply of air or in faulty gas appliances.
Reactions with chlorine and bromine (halogenation)
Alkanes react with chlorine and bromine on exposure to ultraviolet light.
2 !
During the reactions, hydrogen atoms in the alkane molecules are replaced
s with the engine of this lorry
(substituted) by halogen atoms. These are described as substitution reactions.
is emitting dangerous and
Any or all of the hydrogen atoms in an alkane may be replaced and the
of carbon (soot) and toxic
reaction
continue
until
all of the
atoms
have been substituted
Alkanes react withcan
chlorine
and
bromine
onhydrogen
exposure
to ultraviolet
light.
oxide from its exhaust
REACTION WITH HALOGENS
by halogen atoms. Consequently, the product is a mixture of similar
are difficult to separate and purify. Because of this, the
During the compounds
reactions, that
hydrogen
atoms in the alkane molecules are replaced
reactions of alkanes with halogens are limited as methods of synthesising
halogenoalkanes,
even though
canas
be substitution
used in variousreactions.
ways.
(substituted)
by halogen atoms.
Thesethese
are products
described
ion reaction is one in
Alternative
ways
of
synthesising
different
halogenoalkanes
and
the
uses
of
om or group of atoms is
halogenoalkanes
are
discussed
in
Topic
14.
Any
or
all
of
the
hydrogen
atoms
in
an
alkane
may
be
replaced
and
the
reaction
ubstituted) by another
In strong sunlight, methane and chlorine react explosively. The first
oup of atoms.
can continue
until all
the hydrogen
atoms
have been substituted by halogen
products
areof
chloromethane,
CH
3Cl, and hydrogen chloride (Figure 11.13).
atoms.
3"
and models representing
stitution reaction of methane
sunlight
+
+
sunlight
CH4(g)
+
Cl2(g)
CH3Cl(g)
+
HCl(g)
16
The reaction involves breaking certain bonds for which energy must be
supplied, and then making new bonds, when energy is released.
The reaction between methane and chlorine will not occur in the dark
because the molecules do not have enough energy for bonds to break when
they collide. The energy provided by photons in ultraviolet (UV) light is
sufficient to cause homolytic fission of chlorine molecules. This produces free
CEDAR COLLEGEchlorine atoms (highly reactive radicals)
98
to start the reaction, and this step inALKANES
the reaction mechanism is described www.youtube.com/megalecture
as initiation.
Online Classes : Megalecture@gmail.com
FREE RADICAL FORMATION
In the presence of u.v light chlorine undergoes homolysis to give Cl free radicals.
:&
'x¥u÷ :
EEIY
FREE RADICALS are reactive species (atoms or groups) which possess
an unpaired electron. However, they are neutral.
t.E.io
t.CI
Their reactivity is due to them wanting to pair up the single electron.
Formed by homolytic fission (homolysis) of covalent bonds.
17
city
+
Ha
->
ANAIC
at ;
+
H
H
I
it
a
H
FREE RADICAL
FORMATION
-
-
H
During initiation, the WEAKEST BOND
IS BROKEN as it requires less energy.
H
There are three possible bonds in a mixture of alkanes and chlorine.
C—H
Ci
410
+
C—C
350 city
Cl — Cl
->
242
HCI
CHZ
+
The Cl-Cl bond is broken in preference to the others as it is the weakest and
requires requires less energy to separate the atoms.
:&
'x¥u÷H :
a
.
H
~
-
-
C
EEIY
-
~~1
I
It
-
It
-
a
413
=
=
KJMOT
Cl
+
AH
431 KJMOH
338
C
-
H
I
H
413-431=-18
=
99
=
I
-
'
CEDAR COLLEGE
Ctl
.
18
H
C
H
H
H
KJMOH
KJMO
ALKANES
www.youtube.com/megalecture
AH
=
413
-
338=+75
KJM 's
Online Classes : Megalecture@gmail.com
t.E.io
t.CI
PROPAGATION
:&
'x¥u÷ :
EEIY
The free radicals then attack the methane
molecules
abstracting a
->
Ha
at
city
+
;
+
hydrogen atom to form hydrogen chloride and a methyl CH3! free radical.
ANAIC
H
H
t.E.io
t.CI
t.E.io
~~1
t.CI
t.E.io
~~1
t.CI
I
:&
'x¥u÷ :
it
-
H
EEIY
a
-
H
Ci
city:
:&
'x¥u÷
+
H
->
city
+
HCI
Ha
->
EEIY
ANAIC
CHZ
at ;
+
+
H
H
I
it
-
H
a
.
H
~
-
H
C
-
city
+
H
-
H
19
->
H
.
+
H+
H city
It
It
a
-
ANAIC
ANAIC
'
KJMOT
413
=
it
-
431 KJMOH
=
Cl
atI ;
+
C
-
city
AH
+
H
H
H
->=
CHZ
Ha
at ;
+
413-431=-18
KJMOY
H
H H
H
Ci338 HKJMOH
CcityH
+
~
H
H
a
-
PROPAGATION
->
I
-
I
H +
H
HCI
->
I
-
Ha
H
I
Ci
C
a
-
"
I
+C
CHZ KJM 's
+
H
H413
a Cl 338=+75
H HCI
I
Reaction with methane could occur in one of two ways. In theIprevious reaction, a C—
H H
H H
a
Ctl
.
=
-
-
-
-
AH
it
.
=
-
-
-
H bond has been broken, and a H—Cl bond has been formed, an exothermic reaction.
C
It
-
It
Ci
a
-
a
=
=
.
H
KJMOT
413
'
H
431 +KJMOH
city
~
-
-
C
-
AH
413-431=-18
HCI
CHZ
H +
~~1 Eu
~~1
->
H
H
=
.
Cl
+
I
C
-
KJMOY
H
-
IF the Cl were
the C atom, a C—H bond
wouldI be broken, and a C—Cl
I
H
H to combine with
"
AH reaction.
Ctlwould338
Hin an endothermic
413
bond form and
resultKJMOH
A
•u#a H '
#
C
-
338=+75
H
ta
H
-
-
I It
.
-
H
431
=
Cl
-
=
C Ctl
aIt =
C
H
a
It
-
=
-
#
-
KJMOH
C
~
-
-
-
I
H
242
338 KJMOT
338 KJMOH
413
=
=
431
H
=
338
DH=
= .
H It
'
AH
AH
A
KJMOH
AH
#
CHZ
CEDAR COLLEGE
I
H
+42
->
•u#a
CHI
-
.
100
A
-
Eu
338=+75
I
KJM 's
"
KJMOY
KJM 's
KJMOT
ta
'
.
242-338=-96
H
a
-
It
413
KJMOT
ta
I
=
DH=
+44
+
-
KJMOY
-
I
H
Eu
=
20
-
.
I
-
338=+75
413-431=-18
-
KJM 's
.
413-431=-18
Cl
H
C
413 H
=
-
=
+
242-338=-96
KJMOH
242 CHE
Cl
CI
Q•+CH4
H
=
338
C
a
-
I
AH
H
•u#a
I
Ctl
KJMOT
413
=
It H a
CI
=
=
.
"
'
ALKANES
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It
a
.
H
-
~
C
-
-
~~1
H
H
.
Cl
+
I
C
-
-
H
I
Online Classes
: Megalecture@gmail.comI
C
It
It
-
-
a
KJMOT
H
a413
.
=
'
~
431 KJMOH
=
~~1
H
H
-
C
-
H
AH
-
I
H
H
H
I
ClKJMOY
C
.
+
It
C 338
-
=
413
=
reactive as a chlorine radical.
a
It
431
=
-
KJMOTAH
413
=
KJMOH
-
338=+75
AH
H
I
H
PROPAGATION
H
The methyl radical formed
in the first step' of propagation is equally
as
KJM 's
KJMOH
Ctl
-
-
413-431=-18
=
"
413-431=-18
=
KJMOY
It is likely to react with the first molecule it collides with. If it collides with a
chlorine molecule, the following highly exothermic reaction occurs:
AH
Ctl
338=+75
338 KJMOH
413
=
=
#
Eu
•u#a
I
A
-
-
C
-
ta
I
.
It
Cl
-
a
DH=
242
338
=
=
#
H
KJMOT
242-338=-96
•u#a
I
A
-
-
CHZ
.
CI
C
Cl
+42
-
Eu
I
ta
.
It
Q•+CH4
-
'
H
21
H
a
CHE
=
->
=
242
338
CHI
+44
a
+
DH=
.
242-338=-96
KJMOT
PROPAGATION SO FAR
In the initiation reaction, we had produced two reactive chlorine atoms
HCI
CHZCIoccur.
Cltytclz
and the following
two reactions
+
Q•+CH4
CHZ
.
+42
+44
CHE
->
CHI
+
a
.
After these two reactions have taken place we have:
• used up one molecule of methane, and one molecule of chlorine
• produced one molecule of chloromethane, and one molecule of hydrogen chloride
but regenerated another one.
• used up a chlorine atom,
HCI
CHZCI
Cltytclz
+
22
CEDAR COLLEGE
KJM 's
H
H
H
CI
-
101
ALKANES
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'
C
a
-
=
338
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TERMINATION
Q•+CH4
Q•+CH4
CHE
+44
CHZ
+42
.
+44
CHE
CHI
->
a
+
.
.
a
The above
reactions
a never-ending chain reaction. They
+
-> together
CHZ two+42
CHI constitute
.
could, in theory, continue until all the methane and chlorine had been converted into
chloromethane and hydrogen chloride.
This situation does not occur in practice, however. UV light initiates the production of
CHZCI + HCI
Cltytclz
many thousands of molecules of chloromethane.
Cl
Cl
Eventually the chain reaction
the
+ HCI
CHZCI comprising
Cltytclz
.
CH
above two reactions stops, because there is a
;
Clz
•
+
Cl
+
za
.
CH
;
CH3cHz
(small) chance that two radicals could collide
CH
CHz•+
with each other, to form a stable molecule.
23
Clz
Cl
.
Cl
+
CH
;
+
initiation
Clz
•
Cl
OVERALL
CH
CHz•+
•
29
a CH + CH4
za
CHF
+
CHZCI
.
.
CHF
;
Ck
HCI
+
Cf
+
Clz
CHZA
.
the chain reaction.
CHI + Cl
CH ; + CH ;
Chsatg
It would require another chlorine molecule to slit in order
to restart the chain.
.
Overall, the chlorination of methane is
propagation
}
termination
CH3cHz
• and CH• , and so stop
Any of these reactions would use up the free radicals Cl
Cl + Ct 3
Clz
}
29
•
initiation
an example of a radical substitution
reaction.
The different stages of the chain
reaction are named as initiation,
propagation and termination.
a
.
CHF
Cl
.
Ck
+
+
Cf
CHZCI
Ct + CH 34
.
CH ;
+
CHZCI
Clz
CHZA
•
Cl
HCI
+
+
Ct
+
CHI
CH ;
CHF
CH4
+
t
}
propagation
Clz
}
Chsatg
HCI
+
•
CHZCI
CHZCIZ
+
Cf
termination
24
Ct
•
CEDAR COLLEGE
+
CHZCI
102
+
+
CHZCI
34 Cl HCI
CHZCHZ
CH
.
t
•
Clz CHZCHE
CHZCIZ+
+
Cf
CK
HCI
+
CH3CHz•
CHBCHZCI
+
a
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.
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CHLORINATION OF METHANE: ENERGY CHANGES
^
J
±
¥
^
~⇐Et*
'
PROGRESS
±¥e÷÷μI
#CltHC1
Cl
.
Cl
+
CH
;
+
Clz
•
Cl
CH
.
za
CH
CHz•+
;
.
-
-
CH3cHz
Reaction
OF
)
25
Clz
a
29
.
initiation
CHF
CH4
+
CHF
•
Ck
+
HCI
+
CHZCI
}
Cf
+
propagation
FURTHER SUBSTITUTION
Cl
.
+
Clz
Ct
}
As the reaction takes place,
concentration
is being reduced, and
Cl
+
CHZA of methanetermination
CHIthe
.
CH ; + CH ; is increasing.
the concentration of chloromethane
Chsatg
Chlorine atoms are therefore increasingly likely to collide with molecules of
chloromethane, causing the following reactions to take place:
Ct
•
+
CHZCI
HCI
34
CH
Clz
t
+
•
CHZCI
CHZCIZ
+
Cf
26
Cl
.
+
CHZCHZ
CHZCHE
CEDAR COLLEGE
+
HCI
CK
103
+
CH3CHz•
CHBCHZCI
+
a
.
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FURTHER SUBSTITUTION
Further substitution gives dichloromethane, trichloromethane and
eventually tetrachloromethane. A mixture is formed.
dichloromethane
Cl
trichloromemanl
.
t
CHZCI
Hz
t
CHZCI
u•
t
Methane
CP
+
U•
CHCIE
+
Ha
CHCIG
+
cl•
CHCIZ
Caz•
+
HCI
cafe
cay
CHAE
+
Ckt
HCI
+
CHZCK
"
CHIK
Ckt
tutrachloro
CHZCF
+
a
.
27
CHY
+
Uz
Ha
Ha
CHZCI
+
Clz
CHZCK
+
Ck
CHCIZ
+
Uz
chloromethane
+CHμ
CHIK
+
dichloromethane
Ha
+
CHCB
trickoromemane
Ha
+
Ccly
tetrachloometnane
BY PRODUCTS
A set of by-products are chlorinated ethanes.
These arise from the ethane produced in termination) undergoing a similar
set of substitution reactions, or chloromethyl radicals combining at
termination stages.
Ck
a
29
TCHY
Azt CHz•
Chai
.
H•
CHZCI
Ci
+
tree radicals created
.
Ck
Cl•tCAz•
28
CH ;
Cttzatz
CEDAR COLLEGE
+
+
Ha
+
C1•
free radicals
removed
CHI
City
Bvzcg )
-7
104
2Br°
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radicals
29
Ck
tree+ radicals
29treeChai
Ck
Ha
TCHY
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Classes : Megalecture@gmail.com
C1•
+Chai
CHZCI
Ha
Chai
a
CHz•
+ + Ha
Azt
TCHY
TCHY
.
a
created created
.
.
Azt
H•
.
.
a
CHz•
Azt
H•
CHz•
Ci
+
Ci
H•
Ci
+
Cl•tCAz•
+
CHZCI
C1•
+CHZCI
Ck
removed removed
free radicals
radicals
Ck
CHI free
Ck
+
C1•
removed
free radicals
BROMINATION OF ETHANE
CHI
Cl•tCAz•
CHI
Cl•tCAz•
CH
Cttzatz
; + City
Cttzatz
City
; +CH
+ City to produce
BromineCH
reacts
with
bromoethane,
C2H5Br. The reaction is a
Cttzatz
; ethane
free radical substitution reaction.
Initiation: Bromine splits by homolytic fission using UV light.
)
Bvzcg ) Bvzcg-7
)
Bvzcg
2Br°
-7
2Br°
2Br°
-7
Propagation: The Br• radical removes a hydrogen atom from ethane.
Bro
Bro
+
C2H6
+
C2H6
Bro
+
CzHg• CzHg•
CzHg•
CzHg•
HBV t
t
Bsi
+
CZHSBV CZHSBV
CZHSBV
Brz
+
Brz
+
CzHg•
t
CzHg•
C2H6
+
Brz
+
+
HBV
HBV
Bsi
Bsi
Termination: Two radicals join to form a molecule.
++
BfBf Cuts
Bf
+
Goltz
Ciottzz
Cuts
Cuts
Cceltk
Goltz
Goltz
Czltg
Ciottzz
Ciottzz
29
CzHsB✓ CzHsB✓
CzHsB✓
Cults
Cceltk ++ Cults
Cceltk
Cults
+
t
Czltytcglho
t
Czltytcglho
t
Czltytcglho
Czltg
Czltg
HALOGENATION OF HIGHER ALKANES
There are two monochloropropane isomers: 1-chloropropane, CH3CH2CH2Cl, and 2chloropropane, CH3CHClCH3.
When propane reacts with chlorine, a mixture of the two monochloro- compounds is
produced.
This is because the chlorine atom can abstract either one of the end-carbon
hydrogens or one of the middle-carbon hydrogens, the former resulting in
CH3CH2CH2Cl and the latter CH3CHClCH3.
a
•
C1•
+
+
CHZ
CHZ
-
-
CHZ
CHZ
-
-
CHZ
CH3-CHz-CH2•
->
CHZ
CHZ
-
EH
-
CH3
+
+
H
H
-
-
Cl
4
30
CEDAR COLLEGE
105
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SKILL CHECK
Draw the structures of all possible free radicals in the mono-chloro substitution of:
CH3CHz
C.
-
C
-
: Hz
CH3
31
HALOGENATION OF HIGHER ALKANES
The above reaction can produce organic by-products, in addition to
C2H5Cl. Draw the structural formulae of four possible organic by-products.
Two of your by-products should contain 4 carbon atoms per molecule and
write the equation of the formation of each of your four products.
32
CEDAR COLLEGE
106
ALKANES
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CHCIZ
tetrachloometnane
Ha
+
Ccly
Online
Uz Classes : Megalecture@gmail.com
+
SKILL CHECK
Predict the major organic products formed during the monobromination
of:
(a) CH3CH2CH2CH3
a
Ck
29
TCHY
Azt CHz•
Chai
.
H•
Cl•tCAz•
+
C1•
free radicals
removed
CHI
Cttzatz
City
+
Ha
Ck
(b) (CH3)2CH—CH3
CH ;
+
CHZCI
Ci
+
tree radicals created
.
33
Bvzcg )
Bro
CzHg•
C2H6
+
+
Brz
2Br°
-7
CzHg•
t
CZHSBV
+
CRACKING
HBV
Bsi
Cracking is the decomposition of hydrocarbons by heat and pressure with or without
a catalyst to produce lighter hydrocarbons, specially in oil refining. It is the breakdown
of a large alkane into smaller, more useful alkenes and an alkane.
+
Bf
The larger hydrocarbon
molecules
chamber which contains no
Cuts are fed into a steel CzHsB✓
oxygen, so combustion does not take place. The larger hydrocarbon molecules are
heated to a high temperature and are passed over a catalyst.
When large alkane molecules are cracked they form smaller alkane molecules and
alkene molecules. Possible examples of a cracking reaction are:
Goltz
Ciottzz
Cceltk
Czltg
Cults
+
t
Czltytcglho
34
CEDAR COLLEGE
107
ALKANES
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443,3
tetra
methyl pentane
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3,3 diethyl hexane
-
OTHER REACTIONS
/
th
isomerisation
Nn
neptune
x
2,4 dimethyl pentane
.
⇐
/
tHz
methylwgdohexane
35
CH4μ,
t
202$ )
COqg,
Czottkcyt 30.502cg,
CEDAR COLLEGE
DHOI
+24204 )
2002$
108
+21429,
,
BH=
-890
-
13368
KJMOT
'
KJMOT
ALKANES
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