http://chemdemos.uoregon.edu/demos/Bromination-of-Pringles
Organic Chemistry 3
Reactions of Hydrocarbons
IB Topics 10.2 and 10.3
Reference: Higher Level Chemistry, p. 382-389
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Organic Chemistry - Types of Reactions
combustion reactions
substitution
reactions
addition reactions
oxidation reactions
condensation
reactions
organic
compounds
polymerization reactions
elimination reactions
and more!
2
Reactions of Alkanes
Alkanes are generally stable compounds and have a low reactivity.
Reasons for the low reactivity of hydrocarbons include:
1. high bond enthalpy values
of the C-C and C-H bonds
C-C
347 kJ mol-1
C-H
413 kJ mol-1
Alkanes will only react when there is a strong source of energy to break these
bonds. A common example is a combustion reaction.
2. non-polar nature of the bonds and molecules
The lack of a polar region on the molecule makes
it less likely to react with most common reactants.
X
In organic chemistry language, we say that alkanes are not “susceptible to
attack” by the common attacking agents (e.g. nucleophiles and electrophiles).
http://donyaquick.deviantart.com/art/Nucleophilic-Attack-41688417
3
Reactions of Alkanes
Alkanes are SATURATED compounds.
butane - C4H10
octane - C8H18
Saturated hydrocarbons only have single C-C bonds.
Every carbon atom is bonded to 4 other atoms
(the maximum number of atoms that can bond to a carbon atom).
4
Reactions of Alkanes
Alkanes undergo 2 main types of reactions.
http://www.trunity.net/envsci/articles/view/171985/
http://www.sciencephoto.com/media/4209/enlarge
1. combustion reactions
2. substitution reactions
with halogens
5
Reactions of Alkenes
Alkenes are UNSATURATED Compounds
Unsaturated hydrocarbons
have at least one double
carbon-carbon bond or a
triple carbon-carbon bond.
The carbon atoms in the double bond are only
bonded to 3 other atoms, which is less than the
capacity for carbon (able to bond to 4 atoms).
6
Reactions of Alkenes
Alkenes are more reactive than alkanes.
The double C=C bond
is a site of reactivity in an
alkene.
The double bond is made of one sigma bond and one pi bond.
The pi bond is weaker than the sigma bond and is easily broken.
When the pi bond breaks, it is possible to ADD one more atom
to each carbon atom.
7
Reactions of Alkenes
Alkenes undergo ADDITION reactions.
An addition reaction is an atom or a group of atoms is added
to each of the carbon atoms in a C=C double bond.
Only the pi bond of the double bond breaks.
+ H2
ethane
(4 atoms bonded to
each C atom)
ethene
(3 atoms bonded to
each C atom)
+ H2O
ethanol
(4 atoms bonded to
each C atom)
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Hydrocarbon Reaction Task
Create a narrated step by step model of your assigned reaction.
Work in a group of 2-3.
Upload your amazing work to Moodle.
Your “model” should include:
- the type of reaction and the type of hydrocarbon involved
-the balanced overall chemical equation for the reaction
- the structure, name and type of reactants
- step by step changes in the reactants (showing how bonds change)
- the structure, name and type of products formed
- annotations to accompany the model
- narration to identify the type of reaction, to describe the steps in the process
and to explain why or how each step happens
... it can be a ...
video / animation / narrated slide show / other format upon approval
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Hydrocarbon Reaction Task
REACTION OPTIONS:
A. complete and incomplete combustion
B. ethane + bromine
C. propane + chlorine
D. propene + hydrogen bromide
E. but-2-ene + hydrogen bromide
F. hex-3-ene + water
G. pent-2-ene + hydrogen
H. propene + iodine
I. 2-methylpropene + bromine
J. propene + water
K. tests for saturated vs unsaturated compounds
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Hydrocarbon Reaction Task
PLANNING:
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Reactions of Alkanes - Combustion
Alkanes are widely used as fuels in household heating and
internal combustion engines in cars and airplanes.
These reactions are highly
exothermic, and release
large amounts of energy.
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Reactions of Alkanes - Combustion
Complete vs Incomplete Combustion
Complete Combustion Reactions
Fuels react with an EXCESS of oxygen and produce carbon dioxide and water:
C3H8(g) + 5 O2(g)
3 CO2(g) + 4 H2O(l)
∆Hºc = -2220 kJ mol-1
Incomplete Combustion Reactions
When oxygen is LIMITING, carbon monoxide and water are formed:
C3H8(g) + 7/2 O2(g)
3 CO(g) + 4 H2O(l)
When oxygen is EXTREMELY limiting, carbon (soot) and water are formed:
C3H8(g) + 2 O2(g)
3 C(s) + 4 H2O(l)
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Reactions of Alkanes - Combustion
Complete vs Incomplete Combustion
Write the equation for the complete combustion of octane:
C8H18(l) + 25/2 O2(g)
8 CO2(g) + 9 H2O(l)
Write the equation for the incomplete combustion of octane when
(a) oxygen is limiting and (b) when oxygen is extremely limiting:
a)
C8H18(l) + 17/2 O2(g)
b)
C8H18(l) + 9/2 O2(g)
8 CO(g) + 9 H2O(l)
8 C(s) + 9 H2O(l)
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Reactions of Alkanes - Combustion
Complete vs Incomplete Combustion
complete combustion
produces a blue
coloured flame
http://www.flickr.com/photos/37388341@N00/with/1495100711/
incomplete
combustion
produces a yellow
coloured flame due to
the diffraction of light
by the solid carbon
(soot) particles
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Reactions of Alkanes - Combustion
Complete vs Incomplete Combustion - Impacts
Complete Combustion
The combustion of fossil fuels produces
carbon dioxide and water.
Both carbon dioxide and water are greenhouse
gases which contribute to global warming.
http://www.topnews.in/usa/earth-can-recover-faster-co2-emissions-29232
Incomplete Combustion
Carbon monoxide is a toxin.
http://www.cdc.gov/co/faqs.htm
CO binds to hemoglobin in red blood cells
and prevents it from carrying oxygen through
the body.
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Reactions of Alkanes - Combustion
Complete vs Incomplete Combustion - Impacts
Incomplete Combustion continued
The solid carbon particles
(soot) released during
incomplete combustion
contribute to a form of air
pollution called particulates.
Particulates can damage
your respiratory system
and contribute to the
formation of smog.
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Reactions of Alkanes - Substitution
Substitution Reactions with Halogens
A substitution reaction is one where an atom or functional group on a
molecule is replaced with another atom or functional group.
+
methane + Br2
ethane + Cl2
bromomethane + HBr
chloroethane + HCl
1,2-dichoroethane + H2
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Reactions
Reactions
of Alkanes
of Alkanes
- Substitution
Free Radical Mechanisms
The halogenation of an alkane (i.e. substitution with a halogen)
does not occur in a single step, but rather as a series of smaller steps.
The series of steps is called a reaction mechanism.
overall equation: CH4 + Cl2
initiation
propagation
termination
step 1: Cl2
uv
uv light
CH3Cl + HCl
Cl• + Cl•
step 2: CH4 + Cl•
CH3• + HCl
step 3: CH3• + Cl2
CH3Cl + Cl•
step 4: Cl• + Cl•
Cl2 + CH4
Cl2
CH3Cl + HCl
This reaction can not occur in the dark.
It requires sunlight (or ultraviolet radiation), as seen in the next slides.
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Reactions of Alkanes - Substitution
Free Radical Mechanisms
overall equation: CH4 + Cl2
Initiation Stage
step 1: Cl2
uv
uv light
CH3Cl + HCl
Breaking the bond in the halogen (Cl2) requires the
energy of ultaviolet light.
Cl• + Cl•
When the chlorine molecule splits into 2 chlorine
atoms, the bond breaks so that one electron
from the shared pair stays with each atom.
Cl Cl
This is called homolytic fission. The
two electrons in the bond that breaks
are evenly split between the two atoms.
Cl +
Cl
Particles with an odd number of
electrons (or unpaired electrons)
are very reactive and are called
free radicals.
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Reactions of Alkanes - Substitution
Free Radical Mechanisms
overall equation: CH4 + Cl2
Propagation Stage
uv light
CH3Cl + HCl
There are several steps in this stage.
Each step uses a free radical, and produces a different
free radical.
step 2: CH4 + Cl•
CH3• + HCl
One of the products of the overall
reaction forms. A new free radical
forms which is used in the next step.
step 3: CH3• + Cl2
CH3Cl + Cl•
The other product of the overall
reaction forms. The chlorine free
radical is reformed, so step 2 can
continue. This sets up a chain reaction,
so the reaction continues.
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Reactions of Alkanes - Substitution
Free Radical Mechanisms
overall equation: CH4 + Cl2
Termination Stage
step 4: Cl• + Cl•
CH3• + Cl•
CH3• + CH3•
uv light
CH3Cl + HCl
Two free radicals combine in this step.
This removes the free radicals and stops the reaction.
There are several possible reactions.
Cl2
CH3Cl
C 2H 6
Larger alkanes (C2H6) may be produced
as a result of this termination step.
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Reactions of Alkanes - Substitution
Free Radical Mechanisms
Your turn!
Show the overall equation and the steps in the free radical substitution
mechanism for the reaction between ethane (C2H6) and bromine (Br2).
overall equation:
initiation:
propagation:
termination:
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Reactions of Alkenes - Addition Reactions
1. Addition Reactions with Hydrogen = Hydrogenation Reactions
alkene + hydrogen gas
alkane
saturated
unsaturated
example: hydrogenation of ethene
C2H4 + H2
+
ethene
H
C2H6
H
ethane
conditions: catalyst = Ni temperature = 150ºC
24
Reactions of Alkenes - Addition Reactions
1. Addition Reactions with Hydrogen = Hydrogenation Reactions
Economic Importance
Margarine is produced by hydrogenating
unsaturated vegetable oils.
Unsaturated oils are liquid at
room temperature.
Most saturated fats are solid
at room temperature.
25
Reactions of Alkenes - Addition Reactions
2. Addition Reactions with Halogens = Halogenation Reactions
alkene + halogen
halogenoalkane
example 1: bromination of propene
C3H6 + Br2
C3H6Br2
+
Br Br
propene
conditions: catalyst = none
1,2-dibromopropane
temperature = room temperature
These reactions occur quickly at room temperature.
26
Reactions of Alkenes - Addition Reactions
2. Addition Reactions with Halogens = Halogenation Reactions
alkene + hydrogen halide
halogenoalkane
example 2: chlorination of ethene using HCl
C2H4 + HCl
+
ethene
C2H5Cl
H Cl
chloroethane
conditions: catalyst = none temperature = room temperature
27
Reactions of Alkenes - Addition Reactions
2. Addition Reactions with Halogens = Halogenation Reactions
Importance - Testing for Saturation
saturated HC
unsaturated HC
The reaction with bromine provides a
simple test for unsaturation.
Bromine is a reddish brown liquid.
http://graingerrevise.blogspot.com/2011/05/natural-fats-and-key-points.html
Alkanes (saturated compounds) do NOT react with Br2, so the
reddish brown colour remains.
Alkenes (unsaturated compounds) DO react with Br2.
The Br-Br bond is broken when bromine atoms are added to the
double bond so the reddish brown colour disappears.
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Reactions of Alkenes - Addition Reactions
3. Addition Reactions with Water = Hydration Reactions
alkene + water
alcohol
example 1: hydration of ethene
C2H4 + H2O
+
C2H5OH
O
H
ethene
conditions: catalyst = H2SO4
H
ethanol
temperature = heat with steam
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Reactions of Alkenes - Addition Reactions
3. Addition Reactions with Water = Hydration Reactions
alkene + water
alcohol
example 2: hydration of but-2-ene
C4H8 + H2O
+
C4H9OH
O
H
but-2-ene
conditions: catalyst = H2SO4
H
butan-2-ol
temperature = heat with steam
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Reactions of Alkenes - Addition Reactions
3. Addition Reactions with Water = Hydrogenation Reactions
Economic Importance
The industrial production of
ethanol occurs on a very
large scale.
http://www.chemicals-technology.com/projects/braskem-ethanol/braskem-ethanol3.html
Ethanol produced this way is important
as a SOLVENT in many industrial processes.
(The ethanol in alcoholic beverages is NOT produced by this type of reaction. A biological process known as FERMENTATION is used instead.)
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Reactions of
Alkenes of
- Addition
Reactions
Alkenes Reactions
4. Addition Polymerisation Reactions
A polymer forms when many small molecules (monomers)
bond together to form a large molecule called a polymer.
We explore this type of reaction in more detail
in Organic Chemistry 8 - Polymers
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