Bonding in methane, ethane and ethene  and  bonds

1. AS Chemistry Bonding in methane, ethane and ethene  and  bonds

2. Learning Objectives • Candidates should be able to: • describe covalent bonding in terms of orbital overlap, giving  and  bonds. • explain the shape of, and bond angles in, ethane and ethene molecules in terms of  and  bonds.

3. Starter activity

4. Alkenes pent-2-ene CH3CH=CHCH2CH3 hex-3-ene CH3CH2CH=CHCH3 2,3-dimethylpent-2-ene cyclopenta-1,3-diene 3-ethylhept-1-ene CH2=CHCH2CH(CH2CH3)CH2CH2CH3

5. 2p 2 2s 1 1s Hybridisation of orbitals The electronic configuration of a carbon atom is 1s22s22p2

6. 2p 2 2s 1 1s HYBRIDISATION OF ORBITALS If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3 The extra energy released when the bonds form more than compensates for the initial input.

7. Hybridisation of orbitals in alkanes The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent. Because one s and three p orbitals are used, it is called sp3hybridisation. 2s22p2 2s12p3 4 x sp3

8. Hybridisation of orbitals in alkanes In ALKANES, the four sp3orbitals repel each other into a tetrahedral arrangement. sp3orbitals

9. Bonding in methane

10. Bonding in ethane

11. Bonding in ethene Alternatively, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged. 2s22p2 2s12p3 3 x sp2 2p

12. What about ethene? sp2 hybrids

13.  - bonds

14. AS Chemistry Geometric Isomerism

15. Learning Objectives • Candidates should be able to: • describe cis-trans isomerism in alkenes, and explain its origin in terms of restricted rotation due to the presence of π bonds. • deduce the possible isomers for an organic molecule of known molecular formula. • identify cis-trans isomerism in a molecule of given structural formula.

16. Starter activity

17. What is stereoisomerism? In stereoisomerism, the atoms making up the isomers are joined up in the same order, but still manage to have a different arrangement in space ISOMERISM STRUCTURAL ISOMERISM STEREOISOMERISM GEOMETRIC ISOMERISM OPTICAL ISOMERISM

18. Geometric Isomerism?

19. GEOMETRIC ISOMERISM RESTRICTED ROTATION OF C=C BONDS Single covalent bonds can easily rotate. What appears to be a different structure in an alkane is not. Due to the way structures are written out, they are the same. ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION Animation doesn’t work in old versions of Powerpoint

20. Geometric Isomerism?

21. Geometric isomers of but-2-ene

22. Geometric Isomerism? X 

23. GEOMETRIC ISOMERISM How to tell if it exists Two differentatoms/groups attached Two different atoms/groups attached  GEOMETRICAL ISOMERISM  Two similar atoms/groups attached Two similar atoms/groups attached Once you get two similar atoms/groups attached to one end of a C=C, you cannot have geometrical isomerism  Two similar atoms/groups attached Two different atoms/groups attached Two different atoms/groups attached Two different atoms/groups attached  GEOMETRICAL ISOMERISM

24. GEOMETRIC ISOMERISM Isomerism in butene There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism. BUT-1-ENE 2-METHYLPROPENE cis BUT-2-ENE trans BUT-2-ENE *YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING

25. Summary • To get geometric isomers you must have: • restricted rotation (involving a carbon-carbon double bond for A-level purposes); • two different groups on the left-hand end of the bond and two different groups on the right-hand end. It doesn't matter whether the left-hand groups are the same as the right-hand ones or not.

26. The effect of geometric isomerism on physical properties • You will notice that: • the trans isomer has the higher melting point; • the cis isomer has the higher boiling point.

27. Why is the boiling point of the cis isomers higher? The difference between the two is that the cis isomer is a polar molecule whereas the trans isomer is non-polar.

28. Why is the melting point of the cis isomers lower? In order for the intermolecular forces to work well, the molecules must be able to pack together efficiently in the solid. Trans isomers pack better than cis isomers. The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer.

29. AS Chemistry Optical Isomerism

30. Learning Objectives • Candidates should be able to: • explain what is meant by a chiral centre and that such a centre gives rise to optical isomerism. • deduce the possible isomers for an organic molecule of known molecular formula. • identify chiralcentres in a molecule of given structural formula.

31. Starter activity

32. Optical isomerism Chiral centre Chiral molecule When four different atoms or groups are attached to a carbon atom, the molecules can exist in two isomeric forms known as optical isomers. These are non-superimposable mirror images.

33. Optical Isomerism What is a non-superimposable mirror image? Animation doesn’t work in old versions of Powerpoint

34. Optical isomerism Amino acids (the building blocks of proteins) are optically active. They affect plane polarised light differently.

35. Butan-2-ol

36. A B D C E F Optical Isomerism The polarimeter A Light source produces light vibrating in all directions B Polarising filter only allows through light vibrating in one direction C Plane polarised light passes through sample D If substance is optically active it rotates the plane polarised light E Analysing filter is turned so that light reaches a maximum F Direction of rotation is measured coming towards the observer If the light appears to have turned to the rightturned to the left DEXTROROTATORYLAEVOROTATORY

37. Enantiomers – how do they differ? Usually have the same chemical and physical properties – but behave differently in presence of other chiral compounds.

38. Enantiomers – how do they differ?

39. TYPES OF ISOMERISM CHAIN ISOMERISM STRUCTURAL ISOMERISM POSITION ISOMERISM Same molecular formula but different structural formulae FUNCTIONAL GROUP ISOMERISM GEOMETRICAL ISOMERISM Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS STEREOISOMERISM Same molecular formula but atoms occupy different positions in space. OPTICAL ISOMERISM Occurs when molecules have a chiral centre. Get two non-superimposable mirror images.

40. AS Chemistry Electrophilic Addition to Alkenes

41. Learning Objectives • Candidates should be able to: • describe the mechanism of electrophilic addition in alkenes, using bromine/ethene as an example. • describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: addition of hydrogen, steam, hydrogen halides and halogens.

42. Starter activity

43. Electrophilic addition CH2=CH2 + Br2 CH2BrCH2Br

44. Electrophilic addition bromine with ethene CH2=CH2 + Br2 CH2BrCH2Br 1,2-dibromoethane hydrogen bromide with ethene CH2=CH2 + HBr CH3CH2Br bromoethane

45. bromine with ethene H H C C H H H H H C C H + Br - Br Br Br + Br Br H H - C C H H Br Br Electrophilic addition mechanism carbocation 1,2-dibromoethane

46. hydrogen bromide with ethene H H C C H H H H H H C C + + H H - Br Br H H - H C C H Br H Electrophilic addition mechanism carbocation bromoethane

47. Electron flow during electrophilic addition

48. Addition reactions of alkenes

49. Addition to unsymmetrical alkenes Electrophilic addition to propene 2-bromopropane 1-bromopropane

50. Addition to unsymmetrical alkenes In the electrophilic addition to alkenes the major product is formed via the more stable carbocation (carbonium ion) least stable most stable methyl < primary (1°) < secondary (2°) < tertiary (3°)