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4.2.4 State and explain the relationship between the number of bonds, bond length and bond strength.
The more pairs of Electrons that are
shared between two atoms (bonds) in a molecule will
make the attraction between the atoms
stronger and shorter
Lengthnm
Strength(kj mol-1)
C-O 0.143 356
C-C 0.154 348
C=O 0.121 736
C=C 0.134 657
C C 0.120 908
10.3 Alkenes Asmt. Stmts 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 Reactions of Alkenes:The general mechanism
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.
Addition Reactions
An addition reaction occurs when atoms are added to a molecule
There are different types of addition reactions: Hydrogenation Bromination / chlorination Hydration Polymerization
http://www.youtube.com/watch?v=8G1NXYZKGL0
10.3.1 Reactions of Alkenes: with hydrogen
Alkenes react hydrogen gas to create an alkane, using nickel as a catalyst at 150ºC:
C2H4 + H2 → CH3CH3
10.3.1 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
+HH
Cl
HH
ClCl Cl
10.3.2
Describe, using equations, the reactions of symmetrical alkenes with hydrogen halides and water.
10.3.2 Reactions of Alkenes: with hydrogen halogens
Alkenes react readily with hydrogen halides to create a halo-alkane:
C2H4 + HBr → CH3CH2Br
H
H H
H
+HH
H
HH
ClH Cl
10.3.2 Reactions of Alkenes: with water Alkenes do not react readily with water. If sulfuric acid is used (an intermediate
substance will be made), an alcohol will be created :
C2H4 + H2O → CH3CH2OH
Remember that H2O is the same as HOH,
which can be dissociated into H+ and OH-H
H H
H
+ OHH
a c idO
H
H
H H
H
H
10.3.3 Distinguish between alkanes and alkenes using bromine water.
Bromine water (a red liquid) tests for unsaturated hydrocarbons (alkenes) Alkanes (cyclohexane) → stay yellow/orange
no reaction Alkenes (cyclohexene) → turn clear /
colourless because of reaction with unsaturated hydrocarbon
10.3.3
Distinguish between alkanes and alkenes using bromine water.
• The Fuse School: https://www.youtube.com/watch?v=1r9aYS4Ndac&list=PLW0gavSzhMlReKGMVfUt6YuNQsO0bqSMV&index=84
• http://www.youtube.com/watch?v=NjIuBvod2eM
10.3.4 Reactions of Alkenes: Polymerization
Mono- = one -mer = part Poly- = many So if several monomers are connected (think
Lego’s), you get a … ???
10.3.4 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:
1. Polyethene
2. Polychloroethene
3. Polypropene
10.3.4 Reactions of Alkenes: Polymerization: Polyethene
Monomer: ethene (aka ethylene), CH2=CH2
Conditions: Temp: 200º C Pressure: 2000 atm Initiator: O2
10.3.4 Reactions of Alkenes: Polymerization: Polychloroethene
Each chloroethene contains 1 chlorine Therefore when the chloroethene
molecules polymerize, every other carbon will bond to 1 chlorine
This is PVC
10.3.4 Reactions of Alkenes: Polymerization: Types
Addition:
All of the examples we’ve seen are classified as “Addition Polymerization”
vs.
Condensation
The Fuse School: https://www.youtube.com/watch?v=nz1ucI6gCIg&list=PLW0gavSzhMlReKGMVfUt6YuNQsO0bqSMV&index=3
10.3.4 Reactions of Alkenes: A Hint
Draw the structure of your alkene so that the C=C is in the middle and the other parts bond above
and below the carbons.
For example. A polymer of but-2-ene should look like:
=
+
10.3.4 Reactions of Alkenes: Reaction Type Definitions
Addition:
Bromination:
Dehydration:
Hydration:
Hydrogenation:
10.3.5 Uses of alkene products
Ethene can be used for a variety of purposes: Steam → ethanol Polymerize → polyethene React with chlorine → chloroethene… which
interacts with ethanol → PVC React with benzene → phenylethene
(styrene)… which interacts with ethanol → polystyrene
Outline the economic importance of the reactions of alkenes (10.3.5)
Making margarine Hydrogenation (addition of H) of vegetable oils
(alkenes)