Alkenes
IB Chemistry Topic 10.3
10.3 Alkenes Asmt. Stmts
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10.3.1 Describe, using equations, the
reactions of alkenes with hydrogen and
halogens.
10.3.2 Describe, using equations, the
reactions of symmetrical alkenes with
hydrogen halides and water.
10.3.3 Distinguish between alkanes and
alkenes using bromine water.
10.3.4 Outline the polymerization of alkenes.
10.3.5 Outline the economic importance of
the reactions of alkenes.
10.3.1
Describe, using equations, the
reactions of alkenes with
hydrogen and halogens.
Reactions of Alkenes:
The general mechanism
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Alkenes react with many substances to form
a new substance. Catalysts, acids or other
substances may be required to complete the
reaction:
C2H4 + XY → CH2X CH2Y
Process occurs by breaking the double bond.
Reactions of Alkenes:
with hydrogen

Alkenes react hydrogen gas to create an
alkane, using nickel as a catalyst at 150ºC:
C2H4 + H2 → CH3CH3
Reactions of Alkenes:
with halogens

Alkenes react readily with chlorine or bromine
to create a di-halo-alkane (what is this?):
C2H4 + 2 Cl2 → CH2Cl CH2Cl
H
H
H
H
+
Cl Cl
Cl Cl
H
H
H H
I would draw Cl’s opposite of each
other.
10.3.2
Describe, using equations, the
reactions of symmetrical alkenes
with hydrogen halides and water.
Reactions of Alkenes:
with hydrogen halogens

Alkenes react readily with hydrogen halides
to create a halo-alkane :
C2H4 + HBr → CH3CH2Br
H
H
H
H
+
H Cl
H Cl
H
H
H H
Reactions of Alkenes:
with water
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Alkenes do not react readily with water.
If sulfuric acid is used, then an intermediate
substance will be made, allowing an alcohol
to be created :
C2H4 + H2O → CH3CH2OH
Remember that H2O is the same as HOH,
which can be dissociated into H+ and OHH
H
H
H
H H
acid
+
H
O
H
H
O
H H
H
10.3.3
Distinguish between alkanes and
alkenes using bromine water.
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http://www.youtube.com/watch?v=PE1CDR1S5pk
Distinguish between alkanes and
alkenes using bromine water.
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Bromine water tests for unsaturated
hydrocarbons
Alkanes (cyclohexane) →
reddish/brown/orange…
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No color change because no reaction
Alkenes (cyclohexene) → clear / colorless
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Decolorization because of reaction with
unsaturated hydrocarbon
10.3.4
Outline the polymerization of
alkenes.
Reactions of Alkenes:
Polymerization
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Mono- = one
-mer = part
Poly- = many
So if several monomers are connected (think
Lego’s), you get a … ???
Reactions of Alkenes:
Polymerization
Naming polymers: put “poly-” in front of the
name of the monomer
There are 3 polymerization mechanisms
that you need to be familiar with:
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1.
2.
3.
Polyethene
Polychloroethene
Polypropene
Reactions of Alkenes:
Polymerization: Polyethene
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Reaction: nCH2=CH2 [-CH2-CH2-]n
Monomer: ethene (aka ethylene), CH2=CH2
Reactions of Alkenes:
Polymerization: Polychloroethene
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Each chloroethene contains 1 chlorine
Therefore, when the chloroethene
molecules polymerize, every
other carbon will bond to 1 chlorine
Aka PVC
Reactions of Alkenes:
Polymerization: Polypropene
+
=
10.3.5
Outline the economic importance of
the reactions of alkenes.
Uses of alkene products
Ethene can be used for a variety of purposes:
 Margarine
 Ethanol
 Plastics