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11.11 The Birch Reduction

11.11 The Birch Reduction. Dr. Wolf's CHM 201 & 202. 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. H 2 is not involved in any way. .

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11.11 The Birch Reduction

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  1. 11.11The Birch Reduction Dr. Wolf's CHM 201 & 202

  2. 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%) Dr. Wolf's CHM 201 & 202

  3. H H H H H H • + Na+ • Na •• H H H H – H H Mechanism of the Birch Reduction (Figure 11.8) Step 1: Electron transfer from sodium + Dr. Wolf's CHM 201 & 202

  4. H • OCH3 • •• Mechanism of the Birch Reduction (Figure 11.8) Step 2: Proton transfer from methanol H H H • •• H H – H Dr. Wolf's CHM 201 & 202

  5. H •• • • OCH3 OCH3 • • •• •• Mechanism of the Birch Reduction (Figure 11.8) Step 2: Proton transfer from methanol H H H H H H • • •• H H H H – H H H – Dr. Wolf's CHM 201 & 202

  6. H H H • + • Na H H H H Mechanism of the Birch Reduction (Figure 11.8) Step 3: Electron transfer from sodium Dr. Wolf's CHM 201 & 202

  7. H H H • + • Na H H H H Mechanism of the Birch Reduction (Figure 11.8) Step 3: Electron transfer from sodium H – H H •• + Na+ H H H H Dr. Wolf's CHM 201 & 202

  8. •• • OCH3 • H Mechanism of the Birch Reduction (Figure 11.8) Step 4: Proton transfer from methanol H – H H •• H H H H Dr. Wolf's CHM 201 & 202

  9. •• • OCH3 – •• • • OCH3 • •• H H H H H H H H H Mechanism of the Birch Reduction (Figure 11.8) Step 4: Proton transfer from methanol H – H H •• H H H H Dr. Wolf's CHM 201 & 202

  10. 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%) Dr. Wolf's CHM 201 & 202

  11. 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) Dr. Wolf's CHM 201 & 202

  12. 11.12Free-Radical Halogenationof Alkylbenzenes Dr. Wolf's CHM 201 & 202

  13. C C C C • • The Benzene Ring as a Substituent benzylic carbon is analogous to allylic carbon allylic radical benzylic radical Dr. Wolf's CHM 201 & 202

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

  15. 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• Dr. Wolf's CHM 201 & 202

  16. 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 Dr. Wolf's CHM 201 & 202

  17. 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 Dr. Wolf's CHM 201 & 202

  18. 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 Dr. Wolf's CHM 201 & 202

  19. 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 Dr. Wolf's CHM 201 & 202

  20. CH3 CH2Cl Free-radical chlorination of toluene industrial process highly regioselective for benzylic position Cl2 lightorheat Toluene Benzyl chloride Dr. Wolf's CHM 201 & 202

  21. CHCl2 CCl3 Free-radical chlorination of toluene Similarly, dichlorination and trichlorination areselective for the benzylic carbon. Furtherchlorination gives: (Dichloromethyl)benzene (Trichloromethyl)benzene Dr. Wolf's CHM 201 & 202

  22. 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%) Dr. Wolf's CHM 201 & 202

  23. CH2CH3 CHCH3 O O NBr NH O O N-Bromosuccinimide (NBS) Br is a convenient reagent for benzylic bromination CCl4 + + benzoyl peroxide, heat (87%) Dr. Wolf's CHM 201 & 202

  24. 11.13Oxidation of Alkylbenzenes Dr. Wolf's CHM 201 & 202

  25. CH3 Na2Cr2O7 O H2SO4 CH2R COH H2O heat CHR2 Site of Oxidation is Benzylic Carbon or or Dr. Wolf's CHM 201 & 202

  26. O COH CH3 Na2Cr2O7 H2SO4 H2O heat NO2 Example NO2 p-Nitrotoluene p-Nitrobenzoicacid (82-86%) Dr. Wolf's CHM 201 & 202

  27. O COH CH(CH3)2 Na2Cr2O7 H2SO4 H2O heat CH3 COH O Example (45%) Dr. Wolf's CHM 201 & 202

  28. 11.14 SN1 Reactions of Benzylic Halides Dr. Wolf's CHM 201 & 202

  29. 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 Dr. Wolf's CHM 201 & 202

  30. CH3 CH3 + + CH3 CH3 What about SN1? Relative rates of formation: CH3 C C more stable less stable Dr. Wolf's CHM 201 & 202

  31. C C C C + + Compare. benzylic carbon is analogous to allylic carbon allylic carbocation benzylic carbocation Dr. Wolf's CHM 201 & 202

  32. 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 Dr. Wolf's CHM 201 & 202

  33. 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 Dr. Wolf's CHM 201 & 202

  34. 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 Dr. Wolf's CHM 201 & 202

  35. 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 Dr. Wolf's CHM 201 & 202

  36. CH3 Cl C CH3 CH3 OCH2CH3 C CH3 Solvolysis CH3CH2OH (87%) Dr. Wolf's CHM 201 & 202

  37. 11.15SN2 Reactions of Benzylic Halides Dr. Wolf's CHM 201 & 202

  38. CH2Cl O2N O NaOCCH3 O CH2OCCH3 O2N Primary Benzylic Halides Mechanism is SN2 acetic acid (78-82%) Dr. Wolf's CHM 201 & 202

  39. 11.16Preparation of Alkenylbenzenes • dehydrogenation • dehydration • dehydrohalogenation Dr. Wolf's CHM 201 & 202

  40. CH2CH3 CH CH2 Dehydrogenation • industrial preparation of styrene 630°C ZnO + H2 Dr. Wolf's CHM 201 & 202

  41. Cl Cl CH CHCH3 CH2 OH Acid-Catalyzed Dehydration of Benzylic Alcohols KHSO4 heat + H2O (80-82%) Dr. Wolf's CHM 201 & 202

  42. Cl Cl CH CHCH3 CH2 OH Cl CHCH3 + Acid-Catalyzed Dehydration of Benzylic Alcohols KHSO4 heat (80-82%) Dr. Wolf's CHM 201 & 202

  43. CH2CHCH3 H3C Br CH H3C CHCH3 Dehydrohalogenation NaOCH2CH3 ethanol, 50°C (99%) Dr. Wolf's CHM 201 & 202

  44. 11.17Addition Reactions of Alkenylbenzenes • hydrogenation • halogenation • addition of hydrogen halides Dr. Wolf's CHM 201 & 202

  45. CH3 CH3 C CHCH2CH3 CHCH3 H2 Pt Br Br Hydrogenation (92%) Dr. Wolf's CHM 201 & 202

  46. CH CH2 Halogenation Br2 CH CH2 Br Br (82%) Dr. Wolf's CHM 201 & 202

  47. Cl HCl Addition of Hydrogen Halides (75-84%) Dr. Wolf's CHM 201 & 202

  48. Cl HCl Addition of Hydrogen Halides + via benzylic carbocation Dr. Wolf's CHM 201 & 202

  49. CH CH2CH2Br CH2 Free-Radical Addition of HBr HBr peroxides Dr. Wolf's CHM 201 & 202

  50. CH CH2CH2Br CH2 Free-Radical Addition of HBr HBr peroxides CH CH2Br • via benzylic radical Dr. Wolf's CHM 201 & 202

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