Chapter 11 Arenes and Aromaticity

1. Chapter 11Arenes and Aromaticity

2. CH3 H H H H H H H H H H H H H H H H H H H Examples of Aromatic Hydrocarbons Benzene Toluene Naphthalene

3. 11.1Benzene

4. Some history 1825 Michael Faraday isolates a new hydrocarbon from illuminating gas. 1834 Eilhardt Mitscherlich isolates same substance and determines its empirical formula to be CnHn. Compound comes to be called benzene. 1845 August W. von Hofmann isolates benzene from coal tar. 1866 August Kekulé proposes structure of benzene.

5. 11.2Kekulé and theStructure of Benzene

6. H H H H H H Kekulé Formulation of Benzene Kekulé proposed a cyclic structure for C6H6with alternating single and double bonds.

7. H H H H H H H H H H H H Kekulé Formulation of Benzene Later, Kekulé revised his proposal by suggestinga rapid equilibrium between two equivalentstructures.

8. X X X X H H H H H H H H Kekulé Formulation of Benzene However, this proposal suggested isomers of thekind shown were possible. Yet, none were everfound.

9. Structure of Benzene Structural studies of benzene do not support theKekulé formulation. Instead of alternating singleand double bonds, all of the C—C bonds are thesame length. Benzene has the shape of a regular hexagon.

10. 146 pm 134 pm All C—C bond distances = 140 pm 140 pm 140 pm 140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene. 140 pm 140 pm 140 pm 140 pm

11. 11.3A Resonance Picture of Bonding in Benzene

12. H H H H H H H H H H H H Kekulé Formulation of Benzene Instead of Kekulé's suggestion of a rapidequilibrium between two structures:

13. H H H H H H H H H H H H Resonance Formulation of Benzene express the structure of benzene as a resonancehybrid of the two Lewis structures. Electrons arenot localized in alternating single and double bonds,but are delocalized over all six ring carbons.

14. Resonance Formulation of Benzene Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures)

15. 11.4The 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

16. Thermochemical Measures of Stability heat of hydrogenation: compare experimentalvalue with "expected" value for hypothetical"cyclohexatriene" Pt + 3H2 H°= – 208 kJ

17. 3 x cyclohexene 360 kJ/mol 231 kJ/mol 208 kJ/mol 120 kJ/mol Figure 11.2 (p 452)

18. 3 x cyclohexene Figure 11.2 (p 452) "expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene 360 kJ/mol 120 kJ/mol

19. 3 x cyclohexene Figure 11.2 (p 452) observed heat of hydrogenation is 152 kJ/mol less than "expected" benzene is 152 kJ/mol more stable thanexpected 152 kJ/mol is the resonance energy of benzene 360 kJ/mol 208 kJ/mol

20. Figure 11.2 (p 452) hydrogenation of 1,3-cyclohexadiene (2H2) gives off more heat than hydrogenation of benzene (3H2)! 231 kJ/mol 208 kJ/mol

21. Cyclic conjugation versus noncyclic conjugation 3H2 Pt heat of hydrogenation = 208 kJ/mol 3H2 Pt heat of hydrogenation = 337 kJ/mol

22. Resonance Energy of Benzene 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

23. 11.5An Orbital Hybridization Viewof Bonding in Benzene

24. Orbital Hybridization Model of Bonding in Benzene Planar ring of 6 sp2 hybridized carbons Figure 11.3

25. Orbital Hybridization Model of Bonding in Benzene Each carbon contributes a p orbital Six p orbitals overlap to give cyclic  system;six  electrons delocalized throughout  system Figure 11.3

26. Orbital Hybridization Model of Bonding in Benzene High electron density above and below plane of ring Figure 11.3

27. 11.6The  Molecular Orbitalsof Benzene

28. Energy Benzene MOs Antibondingorbitals 6 p AOs combine to give 6  MOs 3 MOs are bonding; 3 are antibonding Bondingorbitals

29. Energy Benzene MOs Antibondingorbitals All bonding MOs are filled No electrons in antibonding orbitals Bondingorbitals

30. Benzene MOs Figure 11.4 p 454

31. 11.7Substituted Derivatives of Benzene and Their Nomenclature

32. Br NO2 C(CH3)3 Bromobenzene tert-Butylbenzene Nitrobenzene General Points 1) Benzene is considered as the parent andcomes last in the name.

33. General Points 1) Benzene is considered as the parent andcomes last in the name. 2) List substituents in alphabetical order. 3) Number ring in direction that gives lowest locant at first point of difference.

34. Example Cl Br F 2-bromo-1-chloro-4-fluorobenzene

35. 1,3 = meta(abbreviated m-) 1,4 = para(abbreviated p-) Ortho, Meta, and Para Alternative locants for disubstitutedderivatives of benzene 1,2 = ortho(abbreviated o-)

36. Cl Cl o-ethylnitrobenzene m-dichlorobenzene (1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene) Examples NO2 CH2CH3

37. Table 11.1 Certain monosubstituted derivatives of benzene have unique names.

38. O CH Table 11.1 Benzaldehyde

39. O COH Table 11.1 Benzoic acid

40. CH CH2 Table 11.1 Styrene

41. O CCH3 Table 11.1 Acetophenone

42. OH Table 11.1 Phenol

43. OCH3 Table 11.1 Anisole

44. NH2 Table 11.1 Aniline

45. OCH3 OCH3 NO2 Names in Table 11.1 Can be Used as Parent Anisole p-Nitroanisoleor4-Nitroanisole

46. OH CH2— phenyl phenol benzyl a group a compound a group Easily Confused Names

47. 11.8Polycyclic Aromatic Hydrocarbons

48. Naphthalene Resonance energy = 255 kJ/mol Most stable Lewis structure;both rings correspond to Kekulé benzene.

49. resonance energy: 347 kJ/mol 381 kJ/mol Anthracene and Phenanthrene Phenanthrene Anthracene

50. 11.9Physical Properties of Arenes