1. Title: Lesson 4 Alkenes Learning Objectives:
Describe the main addition reactions of the alkenes
Extract an alkene from a citrus fruit

2. Refresh
Write an equation for the reaction between methane and chlorine to form chloromethane.
Explain this reaction in terms of a free- radical mechanism.
Reviewing Your Notes
You should spend 60 seconds reviewing your notes from last lesson before attempting this.

3. Alkenes General structure CnH2n Unsaturated hydrocarbons
The double bond is made of sigma and pi bond. The carbon atoms are sp2 hybridized, forming trigonal planar shape with angles of 120o
Chemically, alkenes are more reactive than alkanes (double bond is site of reactivity)
Pi bond is easily broken, creating two new bonding positions on the carbon atoms.
Alkenes undergo addition reactions, forming saturated products

4. Reactivity of Alkenes
Alkenes are considerably more reactive than alkanes and are a major industrial feedstock
The reactivity is due to the double bond:
The double bond contains 4 electrons
This is a significant amount of charge which:
Makes it attractive to electrophiles
Enables it to polarise approaching molecules
Most reactions of alkenes are addition reactions where two molecules come together to make one new one

5. Alkenes and hydrogen Alkene + hydrogen  alkane Reaction conditions:
Ni catalyst
This is an addition reaction, in which the hydrogen adds across the double bond. This is known as hydrogenation.
Used to convert unsaturated liquid vegetable oils to solid saturated oils (at room temp) by increasing melting point
Used to make margarine

6. Alkenes and halogens Alkene + halogen  dihalogenoalkane
Reaction conditions:
This reaction occurs very readily and needs no special conditions
If the halogen used is an aqueous solution of bromine (bromine water), the orange-brown colour of bromine solution is decolourised.
This is the standard test for alkenes.

7. Alkenes and hydrogen halides
Alkene + hydrogen halide  halogenoalkane
Reaction conditions:
This reaction occurs very readily and needs no special conditions
This is an addition reaction, in which the hydrogen halide adds across the double bond
All hydrogen halides react in this way but reactivity is in the order is HI > HBr > HCl due to decreasing strength of hydrogen halide group down group 17.
HCl reacts most readily

8. Alkenes and water Alkene + water  alcohol Reaction is hydration
Reaction conditions:
Water must be steam
Phosphoric or sulphuric acid catalyst
300o
Pressure 6-7 MPa
This is the process used to make industrial ethanol
Fermentation from sugar would be far too expensive!

9. Extracting an alkene
Alkenes occur widely in nature, one such being the compound limonene which is found in many citrus fruits.
In this experiment you will extract limonene from orange skin and test it to confirm its alkene properties.
Follow the instructions here

10. Polymerisation
Alkenes break their double bonds to undergo addition reactions
Under the right conditions, alkene molecules (monomers) will add to each other creating a polymer
Polymers typically contain thousands of monomers and many useful plastics are polymers of alkenes
In this case, 1-bromo-2-fluoroethene polymerises to form poly-1-bromo-2-fluroethene
Conditions:
Vary from alkene to alkene but often include high pressure, temperature and a catalyst
The carbons in the C=C double bonds form the carbon chain, everything else hangs off this chain

11. Drawing polymers
Polymers are large molecules, so structures are shown using a repeating unit
n denotes the number of repeating units
E.g. Ethene  Poly(ethene)
More examples are poly(chlorethene) or PVC, and PTFE, poly(tetrafluoroethene), often known as Teflon.

12. Drawing polymers Draw three-monomer lengths of the polymers formed by:
Propene
Styrene
Pent-2-ene
C2H5
H3C

13. Summary of Alkenes Undergo addition reactions
Starting materials in the manufacture of many industrially important chemicals
Conc. H3PO4

15. Solutions

16. Homework Research the economic importance of alkenes including:
Manufacture of margarine
Manufacture of ethanol
Polymerisation

17. Key Points Alkenes undergo addition reactions with:
Hydrogen
Hydrogen halides
Halogens
Water (steam)
Alkenes undergo addition polymerisation
Alkenes are very economically important due to the range of products they can make