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. Another Example of an Aromatic Hydrocarbon Common source: gum benzoin tree Methyl paraben Used as a preservative in food and cosmetics. American queen bee pheromone

4. 11.1Benzene

5. 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. 1865 August Kekulé proposes cyclic structure of benzene. 1929 Kathleen Lonsdale confirms cyclic structure of benzene by X-ray crystallography.

6. 11.2Kekulé and theStructure of Benzene

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

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

10. Structure of Benzene Structural studies of benzene demonstrate that it does not have alternating single and double bonds. All of the C—C bonds are the same length. Benzene has the shape of a regular hexagon.

11. 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

12. 11.3A Resonance Picture of Bonding in Benzene

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:

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.

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

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.

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

18. 3 x cyclohexene 360 kJ/mol 231 kJ/mol 208 kJ/mol 120 kJ/mol

19. 3 x cyclohexene "Expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene. 360 kJ/mol 120 kJ/mol

20. 3 x cyclohexene 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

21. Hydrogenation of 1,3-cyclohexadiene (2H2) gives off more heat than hydrogenation of benzene (3H2)! 231 kJ/mol 208 kJ/mol

22. Cyclic Conjugation versus Noncyclic Conjugation 3H2 Pt Heat of hydrogenation = 208 kJ/mol 3H2 Pt Heat of hydrogenation = 337 kJ/mol

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.

24. 11.5An Orbital Hybridization Viewof Bonding in Benzene

25. Orbital Hybridization Model of Bonding in Benzene Planar ring of 6 sp2 hybridized carbons

26. 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.

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

28. 11.6The  Molecular Orbitalsof Benzene

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

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

31. Benzene MOs

32. 11.7Substituted Derivatives of Benzene and Their Nomenclature

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

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

35. Example Cl Br F 2-Bromo-1-chloro-4-fluorobenzene

36. 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-)

37. Cl Cl o-Ethylnitrobenzene m-Dichlorobenzene (1-Ethyl-2-nitrobenzene) (1,3-Dichlorobenzene) Examples NO2 CH2CH3

38. Certain Monosubstituted Derivatives of Benzene Have Unique Names

39. O CH Benzaldehyde

40. O COH Benzoic acid

41. CH CH2 Styrene

42. O CCH3 Acetophenone

43. OH Phenol

44. OCH3 Anisole

45. NH2 Aniline

46. OCH3 OCH3 NO2 Names can be used as parent Anisole p-Nitroanisoleor4-Nitroanisole

47. Names can be used as parent Benzoic acid o-Acetoxybenzoic acidor 2-Acetoxybenzoic acid (Acetylsalicylic acid = Aspirin)

48. OH CH2— Phenyl Phenol Benzyl A group A compound A group Easily Confused Names

49. 11.8Polycyclic Aromatic Hydrocarbons

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