1. 10.3 Alkenes

2. References

3. Assessment Objectives 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 betweeen alkanes and alkenes using bromine water. 10.3.4 Outline the polymerisation of alkenes. 10.3.5 Outline the economic importance of the reactions of alkenes.

4. Starter. From last lesson: Write the reaction between an alkane and a halogen. Draw the structural formula for the following alkenes.

6. Assessment Objectives 10.3.1 Describe, using equations the reactions of alkenes with hydrogen and halogens. 10.3.3 Distinguish between alkanes and alkenes using bromine water.

7. Alkenes When carbon forms compounds each carbon atom always forms four bonds. This does not, however, mean that each carbon is joined to four other atoms. It is possible to have bonds grouped into pairs. These are called double bonds. Alkenes contain carbon atoms joined by double covalent bonds. Single covalent bond C C Double covalent bond C C

8. Alkenes - homologous series A series of alkanes exist differing only in the number of CH 2 groups. The same is true for alkenes. This leads to a homologous series with the general formula C n H 2n hydrogen carbon n= 1 n=2 n=3 n=4 Alkenes none = = =

9. What will be the formula for alkenes containing the following numbers of carbons? Number of carbonsFormula 11 13 32 21 C 11 H 22 C 13 H 26 C 32 H 64 C 21 H 42 Activity

10. Saturated or Unsaturated? Saturated means “full up”. They have a double bond that could instead become two single bonds. This means that other atoms can be added. It is not “full up”. unsaturatedAlkenes are unsaturated. Every carbon atom has already used all four of it’s bonds to join to four other atoms. No other atoms can be added. saturatedAlkanes are saturated.

11. Alkenes are unsaturated and so extra atoms can be added to alkene molecules. This forms the basis of a test to distinguish between alkanes and alkenes. When bromine water is added to an alkane nothing happens but when bromine is added to an alkene the red colour of the bromine disappears. + Br 2  Gas red colourless Bromine loses this red colour

12. 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

13. Which of these is a true statement about alkenes? A. They turn bromine water from colourless to red B. They contain a double bond C. The smallest alkene has 1 carbon atom D. They have names that end in “ane.”

14. Reactions of Alkenes: with hydrogen Alkenes react hydrogen gas to create an alkane, using nickel as a catalyst at 150ºC: C 2 H 4 + H 2 → CH 3 CH 3

15. Assessment Statement 10.3.2 Describe, using equations, the reactions of symmetrical alkenes with hydrogen halides and water.

16. Reactions of Alkenes: with hydrogen halogens Alkenes react readily with hydrogen halides to create a halo-alkane : C 2 H 4 + HBr → CH 3 CH 2 Br

17. Markovnikoff’s Rule Actually there are two possible carbocations that could be formed. In may cases this would result in two possible products. However only one form is preferred The hydrogen ion will tend to migrate to the side with the greater number of hydrogen atoms. This preference is known as Markovnikoffs Rule. “ Birds of a feather flock together!” 17

18. 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 : C 2 H 4 + H 2 O → CH 3 CH 2 OH Remember that H 2 O is the same as HOH, which can be dissociated into H+ and OH-

19. Assessment Statement 10.3.4 Outline the polymerization of alkenes.

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

21. Reactions of Alkenes: Polymerisation 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

22. Reactions of Alkenes: Polymerization: Polyethene Reaction: nCH 2 =CH 2  [-CH 2 -CH 2 -] n Monomer: ethene (aka ethylene), CH 2 =CH 2 Conditions: Temp: 200º C Pressure: 2000 atm Initiator: O 2

23. Uses The most common plastic Also commonly called polyethylene Plastic bags, bottles Chemically resistant.

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

25. Uses Polyvinylchloride (PVC) Glue Hard plastic Replacing building materials

26. Reactions of Alkenes: Polymerization: Polypropene +=+=

27. Uses Thermoplastic polymer Food containers (does not promote growth of bacteria) Medical equipment and research equipment (e.g. petri dishes) Rope (high impact property) Car bumpers Battery cases

28. 10.3.5 Outline the economic importance of the reactions of alkenes.

29. 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

30. Research Create a poster for the importance of alkenes. Reactions Uses of products Polymerisation. Biological