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a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitu PowerPoint Presentation
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a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitu

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a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitu - PowerPoint PPT Presentation

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a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitu
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  1. 1. Reactions involving the ring a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitution (Chapter 12) c) Nucleophilic aromatic substitution (Chapter 23) 2. The ring as a substituent (Sections 11.12-11.17)

  2. 11.12Free-Radical Halogenationof Alkylbenzenes

  3. C C C C • • The Benzene Ring as a Substituent benzylic carbon is analogous to allylic carbon allylic radical benzylic radical

  4. Recall: Bond-dissociation energy for C—H bond is equal to DH° 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.

  5. 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•

  6. 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

  7. 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

  8. 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

  9. 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

  10. CH3 CH2Cl Free-radical chlorination of toluene industrial process highly regioselective for benzylic position Cl2 lightorheat Toluene Benzyl chloride

  11. CHCl2 CCl3 Free-radical chlorination of toluene Similarly, dichlorination and trichlorination areselective for the benzylic carbon. Furtherchlorination gives: (Dichloromethyl)benzene (Trichloromethyl)benzene

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

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

  14. 11.13Oxidation of Alkylbenzenes

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

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

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

  18. 11.14Nucleophilic Substitutionin Benzylic Halides

  19. CH2Cl O2N O NaOCCH3 O CH2OCCH3 O2N Primary Benzylic Halides Mechanism is SN2 acetic acid (78-82%)

  20. CH3 CH3 Cl Cl CH3 C C CH3 CH3 What about SN1? Relative solvolysis rates in aqueous acetone tertiary benzylic carbocation is formedmore rapidly than tertiary carbocation;therefore, more stable 600 1

  21. CH3 CH3 + + CH3 CH3 What about SN1? Relative rates of formation: CH3 C C more stable less stable

  22. C C C C + + Compare. benzylic carbon is analogous to allylic carbon allylic carbocation benzylic carbocation

  23. C Resonance in Benzyl Cation 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

  24. C Resonance in Benzyl Cation 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

  25. C Resonance in Benzyl Cation 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

  26. C Resonance in Benzyl Cation 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

  27. CH3 Cl C CH3 CH3 OCH2CH3 C CH3 Solvolysis CH3CH2OH (87%)