1. Alkenes
IB Chemistry Topic 10

2. 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.

3. Reactions of Alkenes: with hydrogen
Alkenes react with hydrogen gas to create an alkane, using nickel as a catalyst at 150ºC:
C2H4 + H2 → CH3CH3

4. 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

5. Reactions of Alkenes: with hydrogen halogens
Alkenes react readily with hydrogen halides to create a halo-alkane :
C2H4 + HBr → CH3CH2Br

6. Reactions of Alkenes: with water
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 OH-

7. Distinguish between alkanes and alkenes using bromine water.

8. Distinguish between alkanes and alkenes using bromine water.
Bromine water tests for unsaturated hydrocarbons
Alkanes (cyclohexane) → yellow/orange…
No color change because no reaction
Alkenes (cyclohexene) → clear / colourless
Decolourization because of reaction with unsaturated hydrocarbon

9. Reactions of Alkenes: Polymerization
Mono- = one
-mer = part
Poly- = many
So if several monomers are connected (think Lego’s), you get a … ???

10. 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:
Polyethene
Polychloroethene
Polypropene

11. Reactions of Alkenes: Polymerization: Polyethene
Reaction: nCH2=CH2 [-CH2-CH2-]n
Monomer: ethene (aka ethylene), CH2=CH2
Conditions:
Temp: 200º C
Pressure: 2000 atm
Initiator: small amts O2

12. Reactions of Alkenes: Polymerization: Polychloroethene
Each chloroethene contains 1 chlorine
Therefore, when the chloroethene
molecules polymerize, every
other carbon will bond to 1 chlorine
Aka PVC

13. Reactions of Alkenes: Polymerization: Polypropene
+ =

14. 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