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Chapter 11 Arenes and Aromaticity

Chapter 11 Arenes and Aromaticity. Dr. Wolf's CHM 201 & 202. 11-1. CH 3. 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. Dr. Wolf's CHM 201 & 202. 11-2. 11.1 Benzene. Dr. Wolf's CHM 201 & 202.

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Chapter 11 Arenes and Aromaticity

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

  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 Dr. Wolf's CHM 201 & 202 11-2

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

  4. Some history 1834 Eilhardt Mitscherlich isolates a new hydrocarbon 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. Dr. Wolf's CHM 201 & 202 11-4

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

  6. H H H H H H Kekulé Formulation of Benzene Kekulé proposed a cyclic structure for C6H6with alternating single and double bonds. Dr. Wolf's CHM 201 & 202 11-6

  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. Dr. Wolf's CHM 201 & 202 11-7

  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. Dr. Wolf's CHM 201 & 202 11-8

  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. Dr. Wolf's CHM 201 & 202 11-9

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

  11. All C—C bond distances = 140 pm 146 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 134 pm 140 pm 140 pm Dr. Wolf's CHM 201 & 202 11-11

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

  13. 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: Dr. Wolf's CHM 201 & 202 11-13

  14. 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. Dr. Wolf's CHM 201 & 202 11-14

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

  16. 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 Dr. Wolf's CHM 201 & 202 11-16

  17. Thermochemical Measures of Stability heat of hydrogenation: compare experimentalvalue with "expected" value for hypothetical"cyclohexatriene" Pt + 3H2 DH°= – 208 kJ Dr. Wolf's CHM 201 & 202 11-17

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

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

  20. 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 thanexpected 152 kJ/mol is the resonance energy of benzene 360 kJ/mol 208 kJ/mol Dr. Wolf's CHM 201 & 202 11-20

  21. Figure 11.2 (p 404) hydrogenation of 1,3-cyclohexadiene (2H2) gives off more heat than hydrogenation of benzene (3H2)! 231 kJ/mol 208 kJ/mol Dr. Wolf's CHM 201 & 202 11-21

  22. Cyclic conjugation versus noncyclic conjugation 3H2 Pt heat of hydrogenation = 208 kJ/mol 3H2 Pt heat of hydrogenation = 337 kJ/mol Dr. Wolf's CHM 201 & 202 11-22

  23. 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 Dr. Wolf's CHM 201 & 202 11-23

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

  25. Orbital Hybridization Model of Bonding in Benzene Planar ring of 6 sp2 hybridized carbons Figure 11.3 Dr. Wolf's CHM 201 & 202 11-25

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

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

  28. 11.6The p Molecular Orbitalsof Benzene Dr. Wolf's CHM 201 & 202 11-28

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

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

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

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

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

  34. Examples Br NO2 C(CH3)3 Bromobenzene tert-Butylbenzene Nitrobenzene Dr. Wolf's CHM 201 & 202 11-34

  35. 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 Dr. Wolf's CHM 201 & 202 11-35

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

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

  38. Cl Cl Examples NO2 CH2CH3 o-ethylnitrobenzene m-dichlorobenzene (1-ethyl-2-nitrobenzene) (1,3-dichlorobenzene) Dr. Wolf's CHM 201 & 202 11-38

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

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

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

  42. CH CH2 Benzene Derivatives Styrene Dr. Wolf's CHM 201 & 202 11-42

  43. CH3 Benzene Derivatives Toluene Dr. Wolf's CHM 201 & 202 11-43

  44. O CCH3 Benzene Derivatives Acetophenone Dr. Wolf's CHM 201 & 202 11-44

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

  46. OCH3 Benzene Derivatives Anisole Dr. Wolf's CHM 201 & 202 11-46

  47. NH2 Benzene Derivatives Aniline Dr. Wolf's CHM 201 & 202 11-47

  48. OCH3 OCH3 NO2 Benzene derivative names can be used as parent Anisole p-Nitroanisoleor4-Nitroanisole Dr. Wolf's CHM 201 & 202 11-48

  49. Easily confused names CH2— OH phenyl phenol benzyl Dr. Wolf's CHM 201 & 202 11-49

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

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