Position Isomers Alkenes and alkanes

1. Position IsomersAlkenes and alkanes • Use the models to find the isomers of butene and butane • Draw them • Name them • Also identify the types of bonding in each • How does the bonding affect the reactivity? • How can I tell the difference between cyclohexene and cyclohexane?

2. Alkenes and bromine water • IGCSE recall • What is the product made when ethene reacts with bromine • Using bromine water as a test for alkenes • If you shake an alkene with bromine water (or bubble a gaseous alkene through bromine water), the solution becomes colourless. Alkenes decolourise bromine water.

3. The chemistry of the test • This is complicated by the fact that the major product isn't 1,2-dibromoethane. The water also gets involved in the reaction, and most of the product is 2-bromoethanol. • However, there will still be some 1,2-dibromoethane formed, so at this sort of level you can probably get away with quoting the simpler equation:

5. Draw the structure of : • hex-2-ene • pent-2-ene • Why is it not possible to have pent-4-ene? • Give the molecular formula of: • pent-1-ene • 2,2-dimethylbutane

6. Give the molecular and structural formulae of the following: • propane • propene • How many chain isomers are possible for: • C7H16 • C8H18

8. Stereo isomers • Make but-2-ene with your models • Can you make any isomers of but-2-ene? • Why is it possible with butene and not butanol?

9. The new E-Z naming system • Historically, alkenes have been named using cis- and trans- to represent stereochemistry around the double bond as you have just seen • cis- for compounds where the "main substituents" are on the same side of the double bond, and trans- when they are on opposite sides. • This system clearly breaks down, however, in more complex molecules where decisions concerning the "main substituents" are not easily made • The E-Z system provides a set of rules to aid in these decisions. You will see later that these same rules are used in assigning priorities for determining absolute configuration in chiral molecules.

10. Which is the main constituents? Which group get the priority?

11. The rules for assigning E-Z designations are as follows: • rank atoms directly attached to the double bond according to their atomic number • if there is a "tie" at any substituent, look at the second, third, etc., until a difference is found • multiple bonds count as multiples of that same atom • if the highest priority groups are on the same side of the double bond, the molecule is Z; if the highest priority groups are on opposite sides, the molecule is E

12. Name this compound

13. Name these… if you can

14. Organic Chemical Reactions

15. Electrophilic Addition • Ethene reacts with Halogens or sulphuric acid • An electrophile is a positive (relative), species that is attracted towards a negative centre • Which bond on ethene is a very negative centre? • Recall: Addition of bromine to ethene …the test for an alkene • Recall: Addition of steam to make alcohols

16. Reactions of halogens with ethene • What is the order of reactivity with the halogens? • CH2=CH2 + X2 CH2X=CH2X • Write the equation for bromine and name the product • Brδ+-Brδ- This is an induced dipole, now the Brδ+ is attracted to the π (pi) bond • This is the electrophile • A more stable sigma bond forms and a bromine ion is produced

17. There is a positive charge on the carbon without the bromine which then attracts the negative bromine ion • H2C=CH2 + Brδ+-Brδ- • H2C+-CH2Br + Br- BrH2C-CH2Br • Result formation of dibromoethane

18. Hydrogen Halides • The bond strength increases with increasing electronegativity • Predict which will react the fastest HCl/HBr/HI • This time the Hydrogen ion acts as the electrophile as the bond is Hδ+-Brδ- • Hydrogen is the positive or electron deficient, it goes by the same mechanism except there is a permanent dipole not induced which will have what effect on the rate?

19. There is a positive charge on the carbon without the bromine which then attracts the negative bromine ion • H3CHC=CH2 + Hδ+-Brδ- • H3CHC+-CH2H+ Br- H3CBrC-CH2H • Result formation of 2-bromopropane

20. Sulphuric acid catalysed reactions with ethene • H2C=CH2 + δ+H-Oδ-SO3H H2C+-CH2H + HSO4- • H3C-H2Cδ+-O δ-SO3H + H2O CH3CH2OH + H2SO4 • Why does the oxygen of water attack the positive centre?

21. Hydrogenation This is the catalytic addition of hydrogen to alkenes • What catalyst is used and what other conditions are necessary? • What industry uses this reactions? • Answers • Nickel and 150oC • Food industry in the manufacture of margarine

22. Addition of oxidising agents to alkenes • What is an oxidising agent? • Name some • If you add oxygen to an alkene what might you get? • H2C=CH2 + [O] = HO-H2CCH2-OH • This is called a diol as there are two alchol groups attached • This works with potassium manganate in alkali and potassium dichromate in acid

23. Polymerisation • Names some plastics and derive the monomer • Assign a use to these polymers • What is the reaction mechanism for this reaction? • See if you can draw the repeating unit

24. What are these polymers and what are their monomers?

26. These are the two monomers for the formation of Terylene What are these molecules used to make? Hint- look at the joining bonds

27. What is this polymer? Note that each monomer in both terylene and nylon has two functional groups –why would this be?

28. Benefits and drawbacks for plastics In you groups discuss the above • What do the polymers you have looked at remind you of when you look at the long chains? • What have they a lot of stored in their bonds • Why is it a good idea to recycle plastics? • What would you have to consider when recycling them?

29. What an optical isomer? Name the compound Give the molecular formula Make the optical isomers Try to superimpose them on top of one another It is like trying to put your left hand over your right Optical isomers