1. History of Benzene

2. History of Benzene
Isolated from burnt whale oil by Michael Faraday
1834 Eilhardt Mitscherlich finds benzene has formula of C6H6

3. History of Benzene
Isolated from burnt whale oil by Michael Faraday
1834 Eilhardt Mitscherlich finds benzene has formula of C6H6
1861 Josef Loschmidt proposes structure for benzoic acid and aniline

4. History of Benzene
Isolated from burnt whale oil by Michael Faraday
1834 Eilhardt Mitscherlich finds benzene has formula of C6H6
1861 Josef Loschmidt proposes structure for benzoic acid and aniline

5. History of Benzene
1865 Friedrich August Kekulé steps into the picture

6. History of Benzene
1865 Kekulé has a dream

7. History of Benzene 1865 Kekule has a dream
1865 Kekule invokes sausage diagrams
Bulletin de la Société Chimique de France, 3, 98 (1865)

8. History of Benzene 1865 Kekule has a dream
1865 Kekule invokes sausage diagrams
Bulletin de la Société Chimique de France, 3, 98 (1865)
1866 Kekule introduces ring model for benzene +

9. History of Benzene 1865 Kekule has a dream
1865 Kekule invokes sausage diagrams
Bulletin de la Société Chimique de France, 3, 98 (1865)
1866 Kekule introduces ring model for benzene +

10. Alternate Benzene Structures

11. Figure 15.2: A comparison of the heats of hydrogenation for cyclohexene, 1,3-cyclohexadiene, and benzene. Benzene is 150 kJ/mol (36 kcal/mol) more stable than might be expected for “cyclohexatriene.”

12. Hückel’s Rules for aromaticity1. 2 3. 4.

13. Benzene electron distribution map

14. Figure 15.12: An orbital picture and electrostatic potential map of naphthalene, showing that the ten π electrons are fully delocalized throughout both rings.

15. Hückel’s Rules for aromaticity
Aromaticity vs. antiaromaticity

18. Why 4n + 2 for aromatic and 4n for antiaromatic?
Hückel’s Rules for aromaticity
Aromaticity vs. antiaromaticity
Why 4n + 2 for aromatic and 4n for antiaromatic?
Frost’s cycle

19. Benzene Molecular Orbitals
Figure 15.3: The six benzene π molecular orbitals. The bonding orbitals ψ2 and ψ3 have the same energy and are said to be degenerate, as are the antibonding orbitals ψ4* and ψ5*. The orbitals ψ3 and ψ4* have no electron density on two carbons because of a node passing through these atoms.

20. Cyclopentadienyl ions

21. Figure 15.11: Energy levels of the five cyclopentadienyl molecular orbitals. Only the six-π-electron cyclopentadienyl anion has a filled-shell configuration leading to aromaticity.

22. Figure 15.5: An orbital view of the aromatic cyclopentadienyl anion, showing the cyclic conjugation and six π electrons in five p orbitals. The electrostatic potential map further indicates that the ion is symmetrical and that all five carbons are electron-rich (red).

23. 1. methoxycyclobutadiene
Aromatic
Antiaromatic
Neither

24. 2. tropyllium
Aromatic
Antiaromatic
Neither

25. Figure 15.7: Reaction of cycloheptatriene with bromine yields cycloheptatrienylium bromide, an ionic substance containing the cycloheptatrienyl cation. The electrostatic potential map shows that all seven carbon atoms are equally charged and electron-poor (blue).

26. 3. pyrrole
Aromatic
Antiaromatic
Neither

27. Figure 15.9: Pyrrole and imidazole are five-membered, nitrogen-containing heterocycles but have six π electron arrangements, much like that of the cyclopentadienyl anion. Both have a lone pair of electrons on nitrogen in a p orbital perpendicular to the ring.

28. 4. imidazole
Aromatic
Antiaromatic
Neither

29. Figure 15.9: Pyrrole and imidazole are five-membered, nitrogen-containing heterocycles but have six π electron arrangements, much like that of the cyclopentadienyl anion. Both have a lone pair of electrons on nitrogen in a p orbital perpendicular to the ring.

30. Figure 15. 14: The origin of aromatic ring-current
Figure 15.14: The origin of aromatic ring-current. Aromatic protons are deshielded by the induced magnetic field caused by delocalized π electrons circulating in the molecular orbitals of the aromatic ring.

31. 5. 14-annulene
Aromatic
Antiaromatic
Neither

32. x x

33. 6. 12-Annulene
Aromatic
Antiaromatic
Neither

34. More than an electronic effect.
Not the full show, just an introduction.
Yes, there is a lot going on here.

35. Inside:
d – 0.5 ppm
Outside:
d 6.9 – 7.3 ppm

38. CHE 311 Starts Here
Section 9.6

39. Reactivity of Benzene Electrophilic Aromatic Substitution

40. Reactivity of Benzene Electrophilic Aromatic Substitution
The Mechanism

41. Reactivity of Benzene Electrophilic Aromatic Substitution
The electrophiles
1. Cl+
2. Br+
3. I+
4. “F+”
5. NO2+
6. SO3H+
7. R+ (carbocation)
8. R-C O+

42. Reactivity of Benzene Substituent Group Effects on
Electrophilic Aromatic Substitution
Activators and deactivators

43. Reactivity of Benzene Substituent Group Effects on
Electrophilic Aromatic Substitution
Figure 9.16
Classification of substituent effects in electrophilic aromatic substitution. All activating groups are ortho- and para-directing, and all deactivating groups other than halogen are meta-directing. The halogens are unique in being deactivating but ortho- and para-directing.

44. Reactivity of Benzene Substituent Group Effects on
Electrophilic Aromatic Substitution
Directing Group Effects

46. Activation energy vs. substituent for EAS
Y, directing effect

47. p. 340

48. p. 345

49. p. 338

50. Aromatic Biomolecules
Amino acids
Nucleic acids

51. Figure 15.8: Pyridine and pyrimidine are nitrogen-containing aromatic heterocycles with π electron arrangements much like that of benzene. Both have a lone pair of electrons on nitrogen in an sp2 orbital in the plane of the ring.
Fig. 15-8, p. 528

52. Figure 15.9: Pyrrole and imidazole are five-membered, nitrogen-containing heterocycles but have six π electron arrangements, much like that of the cyclopentadienyl anion. Both have a lone pair of electrons on nitrogen in a p orbital perpendicular to the ring.
Fig. 15-9, p. 529

53. p. 533

54. p. 529

55. p. 533

56. End