CH:11 Arenes and Aromaticity Examples of Aromatic Hydrocarbons - PowerPoint PPT Presentation

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CH:11 Arenes and Aromaticity Examples of Aromatic Hydrocarbons

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  1. CH3 H H H H H H H H H H H H H H H H H H H CH:11 Arenes and AromaticityExamples of Aromatic Hydrocarbons Benzene Toluene Naphthalene

  2. H H H H H H The Structure of BenzeneKekulé Formulation of Benzene Kekulé proposed a cyclic structure for C6H6with alternating single and double bonds.

  3. 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.

  4. 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.

  5. 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.

  6. Resonance Structures of Benzene • Benzene is actually a resonance hybrid between the two Kekulé structures. • The C—C bond lengths in benzene are shorter than typical single-bond lengths, yet longer than typical double-bond lengths (bond order 1.5). • Benzene's resonance can be represented by drawing a circle inside the six-membered ring as a combined representation.

  7. 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.

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

  9. 3 x cyclohexene 360 kJ/mol 231 kJ/mol 208 kJ/mol 120 kJ/mol Figure 11.2 (p 433)

  10. Resonance Energy • Benzene does not have the predicted heat of hydrogenation of –360 kJ/mol. • The observed heat of hydrogenation is –208 kJ/mol, a difference of 152 kJ. • This difference between the predicted and the observed value is called the resonance energy.

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

  12. 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.

  13. Structure of Benzene • Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring that overlaps around the ring. • The six pi electrons are delocalized over the six carbons.

  14. Br NO2 C(CH3)3 Bromobenzene tert-Butylbenzene Nitrobenzene Substituted Derivatives of Benzene and Their Nomenclature 1) Benzene is considered as the parent andcomes last in the name.

  15. 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.

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

  17. 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-)

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

  19. O CH Table 11.1 Certain monosubstituted derivatives of benzene have unique names. Aniline Benzaldehyde Benzoic Acid

  20. CH CH2 Table 11.1 Styrene Acetophenone Anisole Phenol

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

  22. Polycyclic Aromatic HydrocarbonsNaphthalene Resonance energy = 255 kJ/mol Most stable Lewis structure;both rings correspond to Kekulé benzene.

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

  24. Physical Properties of Arenes Arenes (aromatic hydrocarbons) resembleother hydrocarbons. They are: nonpolar insoluble in water less dense than water

  25. Reactions of Arenes 1. Reactions involving the ring a) Reduction Catalytic hydrogenation (Section 11.3) Birch reduction (Section 11.10) b) Electrophilic aromatic substitution (Chapter 12) c) Nucleophilic aromatic substitution (Chapter 12) • The ring as a substituent (Sections 11.11-11.16)

  26. H Birch reduction (Section 11.10) H H H H H H H H H H H H H H H H H H H H H H H H H Reduction of Benzene Rings catalytic hydrogenation (Section 11.3)

  27. H H H H H H H H H H H H H H Birch Reduction of Benzene Product is non-conjugated diene. Reaction stops here. There is no further reduction. Reaction is not hydrogenation. H2 is not involved in any way. Na, NH3 CH3OH (80%)

  28. H H H H H H • + Na+ • Na •• H H H H – H H Mechanism of the Birch Reduction Step 1: Electron transfer from sodium +

  29. H H H • H H H H H – •• • • OCH3 OCH3 • • •• •• Mechanism of the Birch Reduction Step 2: Proton transfer from methanol H H H • •• H H – H

  30. H H – H H H H • •• + + • Na Na+ H H H H H H H H Mechanism of the Birch Reduction Step 3: Electron transfer from sodium

  31. •• • OCH3 – •• • • OCH3 • •• H H H H H H H H H Mechanism of the Birch Reduction Step 4: Proton transfer from methanol H – H H •• H H H H

  32. H H H H H H H H C(CH3)3 H C(CH3)3 H H H Birch Reduction of an Alkylbenzene Na, NH3 If an alkyl group is present on the ring, it ends up asa substituent on the double bond. CH3OH (86%)

  33. C C C C • • Free-Radical Halogenationof AlkylbenzenesThe Benzene Ring as a Substituent Benzylic carbon is analogous to allylic carbon. allylic radical benzylic radical

  34. Recall: Bond-dissociation energy for C—H bond is equal to H° for: The more stable the free radical R•, the weaker the bond, and the smaller the bond-dissociation energy. + R—H R• •H and is about 400 kJ/mol for alkanes.

  35. H H H C • H2C CH C H H H H • H C C H H Bond-dissociation Energies of Propene and Toluene 368 kJ/mol H2C CH Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals. -H• 356 kJ/mol -H•

  36. C Resonance in Benzyl Radical H H • Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it. H H H H H

  37. H H C H H • H H H Resonance in Benzyl Radical Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

  38. H H C H H • H H H Resonance in Benzyl Radical Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

  39. C Resonance in Benzyl Radical H H Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it. H H • H H H

  40. Spin Density in Benzyl Radical(Figure 11.9) Unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it.

  41. Cl2 CH3 CH2Cl lightorheat Toluene Benzyl chloride Free-radical Chlorination of Toluene industrial process highly regioselective for benzylic position

  42. CHCl2 CCl3 Free-radical Chlorination of Toluene Similarly, dichlorination and trichlorination areselective for the benzylic carbon. Furtherchlorination gives: (Dichloromethyl) benzene (Dichloromethyl) benzene

  43. CH2Br CH3 CCl4, 80°C + HBr light NO2 NO2 p-Nitrobenzyl bromide (71%) Benzylic Bromination is used in the laboratory to introduce a halogen at the benzylic position + Br2 p-Nitrotoluene

  44. Br CH2CH3 CHCH3 O O CCl4 + NBr + NH benzoyl peroxide, heat O O (87%) N-Bromosuccinimide (NBS) is a convenient reagent for benzylic bromination

  45. CH3 Na2Cr2O7 O H2SO4 CH2R COH H2O heat CHR2 Oxidation of AlkylbenzenesSite of Oxidation is Benzylic Carbon or or

  46. O COH CH3 Na2Cr2O7 H2SO4 H2O heat NO2 Example NO2 p-Nitrotoluene p-Nitrobenzoicacid (82-86%)

  47. O COH CH(CH3)2 Na2Cr2O7 H2SO4 H2O heat CH3 COH O Example (45%)

  48. CH3 CH3 Cl Cl CH3 C C CH3 CH3 SN1 Reactions of Benzylic Halides Relative solvolysis rates in aqueous acetone: Tertiary benzylic carbocation is formedmore rapidly than tertiary carbocation;therefore, more stable. 620 1

  49. CH3 CH3 + + CH3 CH3 Benzylic SN1 Reactions Relative rates of formation: CH3 C C more stable less stable

  50. C C C C + + Compare Benzylic carbon is analogous to allylic carbon. allylic carbocation benzylic carbocation