3. What is the structure of benzene? (C 6 H 6 ) first ideas:

4. or, it could be this...

5. actual structure (refer to models)

6. orbitals and electron densities:

9. 8-membered version (cyclo-octa-tetra-ene): not in same plane – less delocalization, single/double bonds

10. Resonance contributors

11. four  electrons in three p orbitals

12. Rules for resonance: 1: Only electrons move – never atoms 2: Only  electrons and lone pair electrons can move – never  electrons 3: Number of electrons never changes. Net charge is conserved

13. carbocations two  electrons in three p orbitals

19. Relative stability (importance) of resonance contributors A is more stable – no sep of charge. A is closer to reality

20. equal stability

21. neg on carbon, pos on oxygen

23. rules for stability: contributor is less stable if it has: 1: atom(s) with incomplete octet 2: neg charge NOT on most electronegative atom 3: charge separation

24. which is more stable?

29. resonance delocalization gives stability! which is more stable carbocation?

30. conjugated dienes more stable:

31. heats of hydrogenation: Why? conjugated p electrons are delocalized (refer to model) also – sp2-sp2 overlap is closer than sp2-sp3 – meaning closer  overlap too

33. cumulated double bonds - allene

34. stability of allylic / benzylic carbocations

37. rate of reaction with HBr?

38. MO theory

39. 1,3-butadiene LUMO HOMO

40. 1,3,5-hexatriene HOMO-LUMO gap getting smaller!

41. Benzene don’t worry about pictures of higher energy MO’s (p. 311 fig)

42. Resonance delocalization and acid/base strength

48. what about ortho? meta?

49. explain regioselectivity:

50. 1,2 vs 1,4 addition

51. Thermodynamic vs kinetic control

52. kinetic control: low temps

53. thermodynamic control: higher temps

54. more sub alkene – more stable

58. kinetic control: why is 1,2 product formed faster? consider the reaction:

59. proximity effect:

60. kinetic control, but no thermodynamic control:

61. Diels-Alder

62. increases reactivity of dienophile

63. MO picture:

67. prediciting regiochem:

69. prefered:

70. conformation of diene:

73. in general, endo favored

74. stereochem of Diels-ALder