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

11.12 Free-Radical Halogenation of Alkylbenzenes

The Benzene Ring as a Substituent • C • C allylic radical benzylic radical benzylic carbon is analogous to allylic carbon 2

Bond-dissociation energy for C—H bond is equal to DH° for: Recall: Bond-dissociation energy for C—H bond is equal to DH° for: R—H R• + •H and is about 400 kJ/mol for alkanes. The more stable the free radical R•, the weaker the bond, and the smaller the bond-dissociation energy. 13

Bond-dissociation energies of propene and toluene H H2C CH C H C • 368 kJ/mol H2C CH -H• H C H C • 356 kJ/mol -H• Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals. 14

Resonance in Benzyl Radical H H • C H H H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

Resonance in Benzyl Radical H • unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

Resonance in Benzyl Radical H • unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

Resonance in Benzyl Radical H H C H H • H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

Free-radical chlorination of toluene industrial process highly regioselective for benzylic position CH3 Cl2 CH2Cl light or heat Toluene Benzyl chloride 16

Free-radical chlorination of toluene Similarly, dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives: CHCl2 CCl3 (Dichloromethyl)benzene (Trichloromethyl)benzene 16

Benzylic Bromination is used in the laboratory to introduce a halogen at the benzylic position CH3 NO2 CH2Br CCl4, 80°C light + Br2 + HBr NO2 p-Nitrotoluene p-Nitrobenzyl bromide (71%) 22

N-Bromosuccinimide (NBS) is a convenient reagent for benzylic bromination Br NBr O CH2CH3 CHCH3 NH O CCl4 + + benzoyl peroxide, heat (87%) 23

11.13 Oxidation of Alkylbenzenes 24

Site of Oxidation is Benzylic Carbon CH3 or Na2Cr2O7 H2SO4 COH O CH2R H2O heat or CHR2 25

p-Nitrobenzoic acid (82-86%) Example COH O CH3 NO2 Na2Cr2O7 H2SO4 H2O heat NO2 p-Nitrotoluene p-Nitrobenzoic acid (82-86%) 26

Example COH O CH(CH3)2 CH3 Na2Cr2O7 H2SO4 H2O heat (45%) 26

11.14 Nucleophilic Substitution in Benzylic Halides 24

Primary Benzylic Halides CH2Cl O2N NaOCCH3 O Mechanism is SN2 acetic acid CH2OCCH3 O2N O (78-82%) 29

Relative solvolysis rates in aqueous acetone What about SN1? Relative solvolysis rates in aqueous acetone C CH3 Cl C CH3 Cl 600 1 tertiary benzylic carbocation is formed more rapidly than tertiary carbocation; therefore, more stable 30

Relative rates of formation: What about SN1? Relative rates of formation: CH3 + CH3 + C CH3 C more stable less stable 30

benzylic carbon is analogous to allylic carbon Compare. + C + C allylic carbocation benzylic carbocation benzylic carbon is analogous to allylic carbon 2

Resonance in Benzyl Cation H + H C H H H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

Resonance in Benzyl Cation H H C H H + H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

Resonance in Benzyl Cation H H C H H + H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

Resonance in Benzyl Cation H H C H H + H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it 18

Solvolysis C CH3 Cl CH3CH2OH C CH3 OCH2CH3 (87%) 37