1. Dr. Wolf's CHM 201 & 202 11-1 Chapter 11 Arenes and Aromaticity

2. Dr. Wolf's CHM 201 & 202 11-2 BenzeneToluene Naphthalene Examples of Aromatic Hydrocarbons HHH HH H CH 3 H H HH H HH H HH HHH

3. Dr. Wolf's CHM 201 & 202 11-3 11.1 Benzene

4. Dr. Wolf's CHM 201 & 202 11-4 Some history 1834 Eilhardt Mitscherlich isolates a new hydrocarbon and determines its empirical formula to be C n H n. Compound comes to be called benzene. 1845 August W. von Hofmann isolates benzene from coal tar. 1866 August Kekulé proposes structure of benzene.

5. Dr. Wolf's CHM 201 & 202 11-5 11.2 Kekulé and the Structure of Benzene

6. Dr. Wolf's CHM 201 & 202 11-6 Kekulé proposed a cyclic structure for C 6 H 6 with alternating single and double bonds. Kekulé Formulation of Benzene HHH HH H

7. Dr. Wolf's CHM 201 & 202 11-7 Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. Kekulé Formulation of Benzene HHH HH H HHH HH H

8. Dr. Wolf's CHM 201 & 202 11-8 However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. Kekulé Formulation of Benzene HXX HH H HXX HH H

9. Dr. Wolf's CHM 201 & 202 11-9 Structural studies of benzene do not support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds are the same length. Structure of Benzene Benzene has the shape of a regular hexagon.

10. Dr. Wolf's CHM 201 & 202 11-10 140 pm All C—C bond distances = 140 pm

11. Dr. Wolf's CHM 201 & 202 11-11 140 pm 146 pm 134 pm All C—C bond distances = 140 pm 140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene.

12. Dr. Wolf's CHM 201 & 202 11-12 11.3 A Resonance Picture of Bonding in Benzene

13. Dr. Wolf's CHM 201 & 202 11-13 Instead of Kekulé's suggestion of a rapid equilibrium between two structures: HHH HH H HHH HH H Kekulé Formulation of Benzene

14. Dr. Wolf's CHM 201 & 202 11-14 express the structure of benzene as a resonance hybrid of the two Lewis structures. Electrons are not localized in alternating single and double bonds, but are delocalized over all six ring carbons. Resonance Formulation of Benzene HHH HH H HHH HH H

15. Dr. Wolf's CHM 201 & 202 11-15 Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures) Resonance Formulation of Benzene

16. Dr. Wolf's CHM 201 & 202 11-16 11.4 The Stability of Benzene benzene is the best and most familiar example of a substance that possesses "special stability" or "aromaticity" aromaticity is a level of stability that is substantially greater for a molecule than would be expected on the basis of any of the Lewis structures written for it

17. Dr. Wolf's CHM 201 & 202 11-17 heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene"  H°= – 208 kJ Thermochemical Measures of Stability + 3H 2 Pt

18. Dr. Wolf's CHM 201 & 202 11-18 120 kJ/mol 231 kJ/mol 208 kJ/mol 360 kJ/mol 3 x cyclohexene Figure 11.2 (p 404)

19. Dr. Wolf's CHM 201 & 202 11-19 120 kJ/mol 360 kJ/mol 3 x cyclohexene Figure 11.2 (p 404) "expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene

20. Dr. Wolf's CHM 201 & 202 11-20 208 kJ/mol 360 kJ/mol 3 x cyclohexene Figure 11.2 (p 404) observed heat of hydrogenation is 152 kJ/mol less than "expected" benzene is 152 kJ/mol more stable than expected 152 kJ/mol is the resonance energy of benzene

21. Dr. Wolf's CHM 201 & 202 11-21 hydrogenation of 1,3- cyclohexadiene (2H 2 ) gives off more heat than hydrogenation of benzene (3H 2 )! 231 kJ/mol 208 kJ/mol Figure 11.2 (p 404)

22. Dr. Wolf's CHM 201 & 202 11-22 heat of hydrogenation = 208 kJ/mol heat of hydrogenation = 337 kJ/mol 3H 2 Pt Pt Cyclic conjugation versus noncyclic conjugation

23. Dr. Wolf's CHM 201 & 202 11-23 compared to localized 1,3,5-cyclohexatriene 152 kJ/mol compared to 1,3,5-hexatriene 129 kJ/mol exact value of resonance energy of benzene depends on what it is compared to, but regardless of model, benzene is more stable than expected by a substantial amount Resonance Energy of Benzene

24. Dr. Wolf's CHM 201 & 202 11-24 11.5 An Orbital Hybridization View of Bonding in Benzene

25. Dr. Wolf's CHM 201 & 202 11-25 Orbital Hybridization Model of Bonding in Benzene Figure 11.3 Planar ring of 6 sp 2 hybridized carbons

26. Dr. Wolf's CHM 201 & 202 11-26 Orbital Hybridization Model of Bonding in Benzene Figure 11.3 Each carbon contributes a p orbital Six p orbitals overlap to give cyclic  system; six  electrons delocalized throughout  system

27. Dr. Wolf's CHM 201 & 202 11-27 Orbital Hybridization Model of Bonding in Benzene Figure 11.3 High electron density above and below plane of ring

28. Dr. Wolf's CHM 201 & 202 11-28 11.6 The  Molecular Orbitals of Benzene

29. Dr. Wolf's CHM 201 & 202 11-29 Energy Bonding orbitals Antibonding orbitals Benzene MOs 6 p AOs combine to give 6  MOs 3 MOs are bonding; 3 are antibonding

30. Dr. Wolf's CHM 201 & 202 11-30 Energy Bonding orbitals Antibonding orbitals Benzene MOs All bonding MOs are filled No electrons in antibonding orbitals

31. Dr. Wolf's CHM 201 & 202 11-31 The Three Bonding  MOs of Benzene

32. Dr. Wolf's CHM 201 & 202 11-32 11.7 Substituted Derivatives of Benzene and Their Nomenclature

33. Dr. Wolf's CHM 201 & 202 11-33 1) Benzene is considered as the parent and comes last in the name. General Points

34. Dr. Wolf's CHM 201 & 202 11-34 ExamplesExamples Bromobenzene tert-Butylbenzene Nitrobenzene NO 2 C(CH 3 ) 3 Br

35. Dr. Wolf's CHM 201 & 202 11-35 1) Benzene is considered as the parent and comes last in the name. 2) List substituents in alphabetical order 3) Number ring in direction that gives lowest locant at first point of difference General Points

36. Dr. Wolf's CHM 201 & 202 11-36 2-bromo-1-chloro-4-fluorobenzene ExampleExample Br Cl F

37. Dr. Wolf's CHM 201 & 202 11-37 Ortho, Meta, and Para alternative locants for disubstituted derivatives of benzene 1,2 = ortho (abbreviated o-) 1,3 = meta (abbreviated m-) 1,4 = para (abbreviated p-)

38. Dr. Wolf's CHM 201 & 202 11-38 ExamplesExamples o-ethylnitrobenzene NO 2 CH 2 CH 3 ClCl m-dichlorobenzene (1-ethyl-2-nitrobenzene)(1,3-dichlorobenzene)

39. Dr. Wolf's CHM 201 & 202 11-39 Certain monosubstituted derivatives of benzene have unique names Benzene Derivatives

40. Dr. Wolf's CHM 201 & 202 11-40 Benzaldehyde Benzene Derivatives CHO

41. Dr. Wolf's CHM 201 & 202 11-41 Benzoic acid Benzene Derivatives COHO

42. Dr. Wolf's CHM 201 & 202 11-42 Styrene Benzene Derivatives CH 2 CH

43. Dr. Wolf's CHM 201 & 202 11-43 Toluene Benzene Derivatives CH 3

44. Dr. Wolf's CHM 201 & 202 11-44 Acetophenone Benzene Derivatives CCH 3 O

45. Dr. Wolf's CHM 201 & 202 11-45 Phenol Benzene Derivatives OH

46. Dr. Wolf's CHM 201 & 202 11-46 Anisole Benzene Derivatives OCH 3

47. Dr. Wolf's CHM 201 & 202 11-47 Aniline Benzene Derivatives NH 2

48. Dr. Wolf's CHM 201 & 202 11-48 Benzene derivative names can be used as parent OCH 3 NO 2 OCH 3 Anisole p-Nitroanisole or 4-Nitroanisole

49. Dr. Wolf's CHM 201 & 202 11-49 Easily confused names phenylphenolbenzyl OH CH 2 —

50. Dr. Wolf's CHM 201 & 202 11-50 11.8 Polycyclic Aromatic Hydrocarbons

51. Dr. Wolf's CHM 201 & 202 11-51 resonance energy = 255 kJ/mol most stable Lewis structure; both rings correspond to Kekulé benzene NaphthaleneNaphthalene

52. Dr. Wolf's CHM 201 & 202 11-52 Anthracene Phenanthrene resonance energy: 347 kJ/mol 381 kJ/mol Anthracene and Phenanthrene

53. Dr. Wolf's CHM 201 & 202 11-53 11.9 Physical Properties of Arenes

54. Dr. Wolf's CHM 201 & 202 11-54 Resemble other hydrocarbons nonpolar insoluble in water less dense than water Physical Properties

55. Dr. Wolf's CHM 201 & 202 11-55 11.10 Reactions of Arenes: A Preview 1. Some reactions involve the ring. 2. In other reactions the ring is a substituent.

56. Dr. Wolf's CHM 201 & 202 11-56 a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitution (Chapter 12) c) Nucleophilic aromatic substitution (Chapter 23) 1. Reactions involving the ring 2. The ring as a substituent (Sections 11.12-11.17)

57. Dr. Wolf's CHM 201 & 202 11-57 catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) HH HH HH HHH HH H HH Reduction of Benzene Rings HH H HHHH HHH H H