1. Access to Science: Chemistry
Ready for learning:
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Drinks and food away
Bags and coats to one side.
Access to Science: Chemistry
Have a go at the activity on your bench
Organic alkenes

2. Learning aims: Recall important terms and prior learning.
Review alkenes and describe their functional group
Describe the structure of alkenes including isomers
Outline the mechanism of electrophilic addition to alkenes with bromine and hydrogen bromide
Describe a test for unsaturation

3. Practical results: Test 1: solubility
Alkanes and alkenes have little polarity so will only be soluble in non-polar solvents.
In polar solvents like water they will not mix – form immiscible layers.
Which was the densest?
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4. Synthesising new compounds
We saw how:
You can make haloalkanes from free radicle substitution reactions with halogens in the presence of UV light: cyclohexane and bromine
Haloalkanes can also be made from alcohols, something we will look at next week.
Practical: reaction: 2
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5. Haloalkanes can then undergo reactions to make other useful products
Nucleophilic substitution reactions:
With strong bases like sodium hydroxide and potassium hydroxide will form alcohols
Used as drinks, fuel or solvent.
With potassium or sodium cyanide they form nitriles
Main use is nitrile rubber, highly chemically resistant.
With ammonia they form amines
used in medicine – basis for analgesic, decongestants and anaesthesia
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6. Haloalkanes can also undergo elimination reactions
Undergo elimination reactions with sodium and potassium hydroxide to give alkenes
Alkenes are used as fuel, to make other useful organic chemicals, particularly polymers.
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7. Synthesis map
Chemists often produce a synthesis map to show how one functional group can undergo many different reactions to produce other useful chemicals.
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8. Alkenes
Alkenes are unsaturated hydrocarbons, which means we can add other atoms to them.
Contain at least one double bond between carbon atoms
General formula CnH2n
Named by replacing the -ane ending by -ene.
The position of the double bond is indicated by numbering the chain
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9. Alkenes H C CH2=CH2 CH3 C CH3CH=CH2 H CH3 C CH3CH=CHCH3 H ethene C2H4
propene C
CH3 H
C3H6
CH3CH=CH2
2-butene or but-2-ene C
CH3 H
C4H8
CH3CH=CHCH3

10. Isomers – we have already met.
Isomers - same molecular formula -atoms are arranged differently to give different compounds.
We have already met structural isomers:
Chain isomerism – hydrocarbon chain is arranged differently.
Positional isomerism – non-hydrocarbons substituents, or multiple bonds e.g. halogens, are in different places on the hydrocarbon chain.
Functional group isomers – same formula isomers have different functional groups aldehydes and ketones.
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11. Positional isomers of butene C4H8
CH2CH3 C
CH3 H
CH2=CHCH2CH3
CH3CH=CHCH3
but-1-ene
but-2-ene

12. Geometric isomers of but-2-ene
CH3 H C
CH3 H
cis-but-2-ene
trans-but-2-ene
This type of isomerism arises because unlike the single bonded alkanes, the double bond cannot rotate.

13. build the geometric isomers of dichloroethene
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14. Reactions of alkenes
Alkenes are much more reactive than alkanes because of the double bond where addition reactions occur.
They are unsaturated which means the double bond can ‘pop’ open and add other atoms to itself.
Addition means that an unsaturated compound becomes saturated
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15. Reaction types:
Nucleophilic Substitution – replacement of one functional group with another or a hydrogen atom
Elimination – loss of a small molecule
We have seen how alkenes can form from elimination of haloalkanes by the hydroxide ion of substance such as KOH.
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16. Alkene reactions:
Alkenes undergo electrophilic addition reactions to form new homologous series.
Addition – addition of a small molecule across the double bond opposite of elimination.
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17. Alkene functional group C=C
The double bond is the functional group of an alkene.
This is the reactive centre because it is a region of high electron density due to the double bond, (two pairs of electrons).
What do you think electrophilic means?
‘electron-loving’
Partially positive or positively charged species attracted to the electron rich double bond
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18. Addition of hydrogen bromide
CH2=CH HBr  CH3CH2Br
1-bromoethane C H +
HBr  Br
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19. Hydrogen bromide
highly polar molecule reacts with alkenes in an electrophilic addition reaction.
Partially positive hydrogen atom attracted to the rich electron dense carbon-carbon double bond.
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20. Electrophilic Addition mechanism
Remember curly arrows show the movement of electrons.
Partially positive hydrogen atom of HBr, comes close to the highly negative alkene double bond.
The electrons are attracted to hydrogen, forming a covalent bond to it.
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21. Electrophilic Addition mechanism
This attaches a carbon to a hydrogen atom. The other carbon of the double bond is left with only 3 negative bonds to other atoms, so is left overall positively charged. (carbocation)
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22. Electrophilic Addition mechanism
Bromine draws the electrons from the H-Br bond onto itself, becoming negatively charged. The negative charge on Br is attracted to the positively charged carbocation, so it forms a bond with carbon by the donation of an electron pair, leaving the molecule overall neutral
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23. Stretch and challenge
The carbocation intermediate can take different forms depending on how many other carbons are attached either side of it
H R R
R C H R C H R C R
Primary Secondary Tertiary
The carbon groups ( R ) attached to the carbocation have an electron donating effect as the carbocation being positively charged draws some of the charge, so stability of the carbocation increases with the number of donating carbon groups attached to the carbocation.
Why does this matter?
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24. Asymmetrical addition to propene
Addition of HBr to our symmetrical alkene.
Underneath is the asymmetrical alkene propene.
Asymmetry arises from different substituents on either side of the carbon-carbon double bond.
Major product forms from the most substituted carbocation
Add hydrogen to the middle carbon atom forms a primary carbocation.
Add hydrogen to the end carbon atom the middle carbon forms secondary carbocation
Secondary more stable - preferred product.
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25. Electrophilic Addition of bromine
CH2=CH Br2  CH2BrCH2Br
1,2-dibromoethane C H +
Br2  Br

26. When an alkene undergoes electrophilic addition with bromine
Br-Br non-polar
However, double bond repels electrons away from nearest bromine atom, leaving it slightly positively charged.
Br – Br polar
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27. Electrons of carbon-carbon double bond attracted to the partial positive charge on bromine and donate their electron pair to it.
The Br-Br bond breaks and the electrons add to the furthest bromine atom.
The C=C bond breaks a carbocation forms, highly reactive positive charge attracts second bromine atom and a dibromoalkane forms.
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28. Let’s practise some..
Draw the mechanism of the addition of HCl and then Cl2 to ethene
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29. Test for unsaturation – bromine water and cyclohexene or alkene
In the presence of a double bond, bromine water will be decolourised from yellow/brown
Why do you think this happens?
What sort of products will you obtain?
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30. Alkenes undergo many reactions
Of notable importance are their reactions to form polymers.
Practical test 4
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31. Poly(ethene) (common name polythene)
3 monomer molecules
Polymerisation
Compounds containing C=C double bonds can form polymers - long hydrocarbon chains - thousands of atoms long. C H
poly(ethene)

32. The polymerisation process can be represented like this
Poly(ethene) (b) n C H
monomer
polymer
(repeating unit)
The polymerisation process can be represented like this

33. Poly(tetrafluoroethene)C F
monomer
polymer
(repeating unit)
Here fluorine substituents result in the polymer PTFE
Used as Teflon, Gortex, non-stick coatings.

34. Learning aims: Recall important terms and prior learning.
Review alkenes and describe their functional group
Describe the structure of alkenes including isomers
Outline the mechanism of electrophilic addition to alkenes with bromine and hydrogen bromide