Title: Lesson 3 Alkanes
Learning Objectives:
– Explain the stability of the alkanes
– Observe the combustion of alkanes
– Describe the free-radical substitution reactions of alkanes and its
mechanism
– Observe the free-radical substitution of hexane
Reviewing Your Notes
You should spend 60
seconds reviewing your
notes from last lesson
before attempting this.
Refresh

a)
b)
The following is a computer-generated
representation of the molecule, methyl 2hydroxy benzoate, better known as oil of
wintergreen.
Deduce the empirical formula of
methyl 2-hydroxy benzoate and draw
the full structural formula, including any
H
multiple bonds that may be
present…The computer-generated
representation shown does not
distinguish between single and multiple
bonds.
Name all the functional groups present H
in the molecule.
Your notes and mind-map
must be ready for me to
inspect.
H
C
H
C
C
C
C
C
O
H H
C
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H
O
C
O
H


Empirical Formula: C8H8O3
Full Structural Formula:
H
C
H
H
C
C
C
C
H
C
O
H
H H
C
O


Functional Groups present:
Hydroxyl, Ester, Phenyl
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C
O
H
Combustion of Alkanes (5 mins)

The alkanes really don’t do much


Complete combustion:


alkane + oxygen  carbon dioxide + water
Incomplete combustion:



Combustion is of one of two notable reactions (this is why we use them for fuels)
Alkane + oxygen  carbon + carbon monoxide + carbon dioxide + water
The amounts of C, CO and CO2 will vary depending on conditions
Task: Observe the combustion of the gas from the gas taps (propane/butane mix) and of a small
amount hexane (in spirit burners). Hold the end of a clean boiling tube just over the flame for 15
seconds, this will collect soot from the flame.


Record all observations clearly and try to account for them
Include balanced equations to describe the (complete) combustion
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Alkanes


General structure CnH2n+2
Saturated hydrocarbons

The term hydrocarbons refer to compounds containing carbon and hydrogen
only

Chemically, alkanes are very unreactive

Weird, when you think that most alkanes are used for quick burning (propane,
butane, etc.)

We use alkanes to store reactive metals such as sodium

Because alkanes have strong carbon to carbon and carbon to hydrogen bonds,
they require a high activation energy. (348 kJmol-1 and 412 kJmol-1respecively)

C-C and C-H bonds are non-polar, so not susceptible to attack by most
reactants
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Combustion: Alkanes as fuels

When activation energy is provided – highly exothermic reaction

Large amount of energy release in forming the double bonds of CO2 and the
bonds in H2O

Other hydrocarbons (alkenes, alkynes and arenes) undergo similar complete
or incomplete reactions

As C:H ratio increases with unsaturation  increase of smokiness due to
unburned carbon
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Complete combustion
In excess oxygen, short chain alkanes can undergo
complete combustion:
alkane + oxygen → carbon dioxide + water
For example:
propane + oxygen → carbon dioxide + water
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
The combustion of alkanes is a highly exothermic process.
This makes them good fuels because they release a
relatively large amount of energy per gram of fuel.
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© Boardworks Ltd 2009
Incomplete combustion
If oxygen is limited then incomplete combustion will occur:
alkane + oxygen → carbon monoxide + water
alkane + oxygen → carbon + water
For example:
propane + oxygen → carbon monoxide + water
C3H8(g) + 3½O2(g) → 3CO(g) + 4H2O(g)
For even more limited oxygen:
propane + oxygen → carbon + water
C3H8(g) + 2O2(g) → 3C(s) + 4H2O(g)
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© Boardworks Ltd 2009
Impact on environment

Carbon dioxide and water are greenhouse gases. The
absorb infrared radiation and contribute to global warming
and climate change.

Carbon monoxide is a toxin. It combines irreversibly with
haemoglobin and the blood and prevents uptake of oxygen.
Idling of engines in heavy traffic produces high
concentrations.

Unburned carbon is released into the air as particulates.
Causes detrimental effects to the respiratory system and
smog in the air which leads to global dimming.
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Substitution reactions of alkanes Halogenation

Alkanes are saturated, so the main type of reaction they undergo is substitution.

This occurs when another reactant, e.g. a halogen, takes the place of a hydrogen atom in
the alkane

Alkanes will undergo halogenation if reacted with a halide in the presence of u.v. light (to
break bond in chlorine molecule)

For example:

CH4(g) + Cl2(g)
methane
u.v.
CH3Cl(g) + HCl(g)
chloromethane

This reaction is an example of free radical substitution.

The radicals start a chain reaction, which are shown by a sequence of steps (reaction
mechanism)
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Initiation - Radicals

Radicals are species with unpaired electrons
 They are crazy reactive

Halogens form radicals when hit by uv light of the right frequency:

u.v.

Cl2
2 Cl•

The dot after the Cl represents the unpaired electron and tells us we have a radical

This process is called homolytic fission – the bond breaks equally with one electron
going to each chlorine
Task: draw Lewis structures for the Cl2 molecule and each of the Cl• radicals
Curly arrows (fish hooks) show
movement of a single electron
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Reaction Mechanism: Free Radical Substitution

Cl2
u.v.
2 Cl•

Initiation

Radicals formed by homolytic fission
Propagation

Cl• + CH4  CH3• + HCl

CH3• + Cl2  CH3Cl + Cl•

CH3Cl + Cl•  CH2Cl• + HCl

CH2Cl• + Cl2  CH2Cl2 + Cl•

Cl• + Cl•  Cl2

Cl• + CH3•  CH3Cl

CH3• + CH3•  C2H6

A single radical can cause thousands of cycles of the propagation stage before it reaches
termination

This same mechanism applies to all of the halogens

The alkane can be substituted multiple times, until every H has been replaced



The use and produce free radicals and
allow the reaction to continue
Termination

Any two radicals can combine to
terminate the reaction

Concentration of radicals is low so this is
a rare event
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Initiation
• Light is used to temporarily break the bond
between either Bromine or Chlorine to form
the free radical.
Oh great!!! He`s
Fine!! I`m taking mine
then too!
Cl
so
when
Ya,unstable
well im taking
my
electron
andthis!!
going
he`s like
home
Cl
Hey Ma!! I`m a free
radical now!!!
Propagation
• Two possibilities:
– 1) Cl radical reacts with an alkane
– 2) Radical alkane reacts with Cl2(g)
H
Cl
C
H
H
H
H
H
Cl
H
Cl
Termination
• Three possibilities:
– 1) Cl radical reacts with another Cl radical
– 2) Cl radicalwe`ll
reactsbewith an alkane radical
Sigh...Guess
– 3) Alkane
radical react
hanging
out together
for awith another alkane radical
Just about to ask
you the same thing!!
Let`s Do This!!!!
Your electron
or
while eh?
your life mate!
Cl
Cl
Cl
Cl
NOPE!
Tired yet?
Bring it!!
Ahhhhhhhhh!!!
Termination
• Three possibilities:
– 1) Cl radical reacts with another Cl radical
– 2) Cl radical reacts with an alkane radical
– 3) Alkane radical react with another alkane radical
H
C
H
H
C
H
H
H
H
H
H
H
H
H
C
H
Cl
H
Cl Cl
H
Cl
Alkanes and Bromine

A similar chain reaction can be seen with the reaction of bromine
Initiation stage
Methane
CH4
Propagation stage
Test can be used to distinguish between
alkanes and alkenes
Termination stage
Br• + Br•  Br2
Br• + CH3•  CH3Br
CH3• + CH3•  C2H6
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Try it Yourself

Place approx 1 cm3 of hexane into two test tubes

Add roughly 1 cm3 of bromine water to each and stopper them, then give them a good
shake.

Leave one test-tube in the classroom but take the other outside and shake it in direct
sunlight.

Record and explain all observations

Write equations showing the mechanism of the reaction

Draw full structural and skeletal formulas of at least 6 possible products, and name each one.
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Extension:

Research the role of free radical reactions in the depletion of the ozone layer
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Key Points

Alkanes are unreactive

They release a lot of energy on combustion, and are easy to handle which
makes them good fuels

Undergo free radical substitution to form halogenoalkanes and a hydrogen
halide in the presence of UV light
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