1. Delocalization and Conjugation in molecules with > one C = C double bonds School of Pharmacy Bandung Institutes of Technology FA2202

2. . This lecture is about the properties of molecules  that have several double bonds and  that depend on the joining up or conjugation of the electrons in these double bonds And see how a large pi framework spread over many atoms and how this dominates the chemistry of such compounds. how this pi framework is responsible for the otherwise unexpected stability of certain cyclic polyunsaturated compounds, including benzene and other aromatic compounds. how this framework gives rise to the many colours in our world. 2

3. As you look around you, you will be aware of many different colours— from the greens and browns outside to the bright blues and reds of the clothes you are wearing All these colours result from the interaction of light with the pigments in these different things— some frequencies of light are absorbed, others scattered. As you 3

4. . 4 A large pi framework gives rise to the many colours in the world

5. . Chlorophyll: highly conjugated compound the ring shown in green is fully conjugated 5

6. Unconjugated bonds 6

7. Conjugated bonds 7

8. sp 3 hybridized orbitals of C (a)Electron configuration of carbon in its most stable state. (b)An electron is “promoted” from the 2s orbital to the vacant 2p orbital. (c)The 2s orbital and the three 2p orbitals are combined to give a set of four equal-energy sp3- hybridized orbitals, each of which contains one electron. 8

9. Four sp 3 hybridized orbitals of C 9

10. sp 2 hybridized orbitals of C 10 (a)Electron configuration of carbon in its most stable state. (b)An electron is “promoted” from the 2s orbital to the vacant 2p orbital. (c)The 2s orbital and two of the three 2p orbitals are combined to give a set of three equal-energy sp2-hybridized orbitals. One of the 2p orbitals remains unchanged.

11. ethylene the simplest unsaturated compound The structure of ethene (ethylene) is planar (all atoms are in the same plane) The carbon atoms are roughly trigonal the C= C bond distance is shorter than that of a C–C sigma bond. 11

12. combination of two sp 2 orbitals of C atoms in formation of acethylene 12

13. No rotation is possible about a double bond The π bond has electron density both above and below the σ bond as the parallel p orbitals overlap locking the bond rigid. - π bond must be broken before rotation can occur. 13

14. Trans-hexatriene 14

15. Cis- and trans- characteristic 15.

16. sp hybridized orbitals of C 16 (a) Electron configuration of carbon in its most stable state. (b) An electron is “promoted” from the 2s orbital to the vacant 2p orbital. (c) The 2s orbital and one of the three 2p orbitals are combined to give a set of two equal-energy sp-hybridized orbitals. Two of the 2p orbitals remain unchanged.

17. π & π * ORBITAL 17

18. CIS – TRANS CONVERSION 18

19. σ & π bond 19

20. Conjugated double bond system 20

21. Conjugated double bond ‘conjugated’ : ‘joined together, especially in pairs’ ‘acting or operating as if joined’. the properties of a conjugated system are often different from those of the component parts. We are using the word conjugation  to describe bonds and delocalization  to describe electrons. 21

22. Addition and substitution reactions 22

23. Distinct characteristic of aromatic double bonds 23 Benzene does react with Br 2 in the presence of iron(III) bromide as a catalyst, but even then addition isn’t observed. Benzene and its derivatives reflect a special stability Substitution occurs instead!

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25. Different length of single and double bonds 25

26. Delocalized double bonds od benzene 26

27. Drawing reaction mechanism for benzene structure it does not matter which way round you draw the benzene ring – both drawings give the same result but one uses more arrows to get there 27

28. Rotation about single bond 28

29. Overlapping all p orbitals in a planar molecule 29

30. Energy for rotation 30

31. Conjugation 31

32. Nonplanarity of Unconjugated double bonds 32

33. 33 Nonplanar molecule of allene

34. 34

35. Bonding & nonbonding MO 35

36. Allyl cation 36

37. Allyl anion 37

38. Delocalization of pi bond in Allyl anion 38

39. Resonance in carboxylate ion 39

40. Resonance at Nitro group 40

41. Resonance at amide system 41

42. Special amide group:Peptide unit 42

43. 43

44. Butadiene systems 44

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48. Approximate wavelengths for different colours 48

49. Structure of colored molecule 49

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52. Nonaromatic structure of double bond system 52

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55. Margarine manufacture 55

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58. Drug having conjugated bonds 58

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