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

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%)