1. Alkanes and their Reactions
13-Apr-15
Objective:
Be able to write equations for the combustion of alkanes
Be able to describe the free radical substitution of Alkanes
3. Reactions of Alkanes
At room temperature the alkanes are very
unreactive.
Why?
Hint:
Electronegativity and
enthalpy
4. Alkanes are unreactive because:
The C-C and C-H bonds involve an equal sharing of
electrons. The electronegativities are similar and therefore
the bonds are not polar. This makes them less susceptible
to attack from many organic molecules.
1. Electronegativity
2. Bond Enthalpy
Alkanes are relatively simple molecules that contain only
single C-C and C-H bonds. These bonds have a relatively
high bond enthalpy. So they only react in the presence of a
strong energy source.
5. Heating Alkanes
In the absence of air
In a plentiful supply
of air
Cracking Combustion
A form of thermal
decomposition where large
molecules are broken down
into smaller more useful.
The burning of a
fuel to create
thermal energy.
6. Why is the combustion of alkanes so
important to our life?
1. Generating electricity
2. Fuel fires
3. Central heating
4. Cooking
5. transport
It is an exothermic reactions.
Energy is transferred from chemical energy
to thermal energy that we can use.
7. Alkanes are often used as fuels because they
release large amounts of energy on combustion.
• The products of the combustion of alkanes
contain relatively strong bonds (C=O and O-H)
that release a lot of energy on their formation.
Combustion
8. Which alkanes do we use for combustion?
Methane Propane Butane
• Methane 🡪 butane are colourless gases
• Propane and butane can be condensed under
pressure
• Alkanes containing 5-19 carbons are colourless
liquids
• Alkanes with 20+ carbons are colourless waxy
solids
1. Why do we use these alkanes for combustion?
2. Why is propane and butane used more than methane?
3. Suggest why we do not use ethane for combustion
9. Candles are made from alkanes that generally
contain 20-25 carbons
if this is true
why does the
whole candle
not combust?
10. complete Incomplete
Burns in a plentiful supply
of oxygen
Burns in a limited supply
of oxygen
Creates carbon dioxide (a
greenhouse gas)
Creates carbon monoxide
Very energy efficient Low energy efficiency
Creates soot (carbon)
CH4
+ 2O2
🡪 CO2
+ 2H2
O CH4
+ 1.5O2
🡪 CO + 2H2
O
CH4
+ O2
🡪 C + 2H2
O
11. Substitution Reaction
CH4
+ Br2
🡪 CH3
Br + HBr
1. UV light is needed to start a substitution
reaction
2. The bromine atom actually takes the place of
a Hydrogen atom.
3. The molecule formed is called
bromomethane
Why is this called an substitution reaction?
Recap from GCSE:
12. Bond fission: breaking bonds
When you break bonds there are 2 ways the
electrons can be shared out
1. Homolytic fission
2. Heterolytic fission
Same/destruction
different/destruction
13. Homolytic fission
• When the breaking of a covalent bond leads to
both atoms having an equal share of
electrons.
• Formation of atoms with unbonded electons=
free radicals
• Draw dot and cross diagrams to represent
what is happening
Cl-Cl 🡪 Cl◦ +Cl◦
14. Free radicals produced from homolytic
fission
1. They are not charged
2. But extremely reactive
15. Heterolytic fission
• Unequal sharing of electrons
• Results in two charged particles
• Covalent bond will already have a degree of
polarity
H-Cl 🡪 H+
+ Cl-
16. Summary Halogenation of Alkanes
• Requires ultraviolet (UV) radiation
• Requires 300o
C
• Bonds break by homolytic fission, which forms
free radicals – an atom with an unpaired
electron
• The radicals substitute
• There is a three stage mechanism for the
reaction
17. Step 1: Initiation
• A chlorine molecule, Cl-Cl is broken by
homolytic fission.
– UV radiation provides the energy for this to
happen
Cl-Cl 🡪 Cl
.
+ Cl
.
• The rest of the reaction requires no more
energy to be added, as the chlorine radical
attack the methane (or other alkane)
18. Step 2: Propagation
• Methane reacts with the chlorine radical
– A single C-H bond is broken by homolytic fission,
forming a
.
CH3
radical
– HCl is also formed
CH4
+ Cl
.
🡪 .
CH3
+ HCl
• The methyl radical then reacts with a chlorine
molecule, forming chloromethane and
another chlorine radical.
.
CH3
+ Cl2
🡪 CH3
Cl + Cl
.
19. • In halogenation of alkanes, the halogen undergoes
homolytic fission to produce two free radical chlorines
• These free radicals attack the C-H bonds in the alkane
• This produces a methyl free radical group which can
continue to react with chlorine
• Which produces another free radical. This recycling of free
radicals produces a self-propogating chain reaction
Step 2: Propagation
20. • Propagation reactions are rapid.
• This is a chain reaction, as each time a
chloromethane molecule is formed, another
chlorine radical is formed to react with
another methane molecule
• Therefore the reaction will continue until no
chlorine is left
21. Step 3: Termination
• Two radicals combine to form a molecule
• There are three possibilities for the reaction
between chlorine and methane:
Cl
.
+ Cl
.
🡪 Cl2
.
CH3
+ Cl
.
🡪 CH3
Cl
.
CH3
+ .
CH3
🡪 C2
H6
• This stage removes radicals, thus terminating the
reaction.
• The propagation step would have happened
22. Further Reactions of Chloromethane
• Chloromethane can undergo further reactions
with chlorine radicals, forming:
–
–
–
24. Task
• Show the mechanism for the bromination of
methane.
– Include all three steps!
25. Which stage of substitution reactions
do the following describe?
Two free radicals collide and produce
a molecule
UV light provides the energy needed for
homolytic fission of a Chlorine molecule
A free radical reacts with molecule to
crate another free radical
What order do the stages of the
mechanism?
26.
27. Which stage of substitution reactions
do the following describe?
Two free radicals collide and produce
a molecule
UV light provides the energy needed for
homolytic fission of a Chlorine molecule
A free radical reacts with molecule to
crate another free radical
What order do the stages of the
mechanism?
28. Draw curly arrow and dot and cross
diagrams for the following reactions
H-Br 🡪 hydrogen and bromine ions
Br-Br 🡪 bromine free radicals
