11-1 Dr. Wolf's CHM 201 & The Birch Reduction

11-2 Dr. Wolf's CHM 201 & 202 (80%)HH HH HH HHH HH H HH Na, NH 3 CH 3 OH 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.

11-3 Dr. Wolf's CHM 201 & 202HH HH HH Step 1: Electron transfer from sodium + Na + Na + Mechanism of the Birch Reduction (Figure 11.8) H H HHH H –

11-4 Dr. Wolf's CHM 201 & 202 Step 2: Proton transfer from methanol Mechanism of the Birch Reduction (Figure 11.8) H H H H H H – OCH 3 H

11-5 Dr. Wolf's CHM 201 & 202 Step 2: Proton transfer from methanol Mechanism of the Birch Reduction (Figure 11.8) H H H H H H – OCH 3 H H H H H H H H –

11-6 Dr. Wolf's CHM 201 & 202 Step 3: Electron transfer from sodium Mechanism of the Birch Reduction (Figure 11.8) H H HHH H H + Na

11-7 Dr. Wolf's CHM 201 & 202 Step 3: Electron transfer from sodium Mechanism of the Birch Reduction (Figure 11.8) H H HHH H H + Na H H H H H H H + Na + –

11-8 Dr. Wolf's CHM 201 & 202 Step 4: Proton transfer from methanol Mechanism of the Birch Reduction (Figure 11.8) H H H H H H H – OCH 3 H

11-9 Dr. Wolf's CHM 201 & 202 Step 4: Proton transfer from methanol Mechanism of the Birch Reduction (Figure 11.8) H H H H H H H – OCH 3 H HHH HH H HH –

11-10 Dr. Wolf's CHM 201 & 202 (86%)HH H C(CH 3 ) 3 HH HHH H H HH Na, NH 3 CH 3 OH Birch Reduction of an Alkylbenzene If an alkyl group is present on the ring, it ends up as a substituent on the double bond.

11-11 Dr. Wolf's CHM 201 & 202 a) Reduction Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) b) Electrophilic aromatic substitution (Chapter 12) c) Nucleophilic aromatic substitution (Chapter 23) 1. Reactions involving the ring 2. The ring as a substituent (Sections )

11-12 Dr. Wolf's CHM 201 & Free-Radical Halogenation of Alkylbenzenes

11-13 Dr. Wolf's CHM 201 & 202 allylic radical The Benzene Ring as a Substituent C C C C benzylic radical benzylic carbon is analogous to allylic carbon

11-14 Dr. Wolf's CHM 201 & 202 The more stable the free radical R, the weaker the bond, and the smaller the bond-dissociation energy. Recall:Recall: R—HRH + Bond-dissociation energy for C—H bond is equal to  H° for: and is about 400 kJ/mol for alkanes.

11-15 Dr. Wolf's CHM 201 & 202 Bond-dissociation energies of propene and toluene 368 kJ/mol 356 kJ/mol H H2CH2CH2CH2C CH C HH H C HH H2CH2CH2CH2C CH -H -H H CH H CH Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals.

11-16 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Radical C H H HH H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

11-17 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Radical CHH HH H HH unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

11-18 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Radical CHH HH H HH unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

11-19 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Radical C H H HH H H H unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

11-20 Dr. Wolf's CHM 201 & 202 industrial process highly regioselective for benzylic position CH 3 Free-radical chlorination of toluene Cl 2 light or heat CH 2 Cl Toluene Benzyl chloride

11-21 Dr. Wolf's CHM 201 & 202 Similarly, dichlorination and trichlorination are selective for the benzylic carbon. Further chlorination gives: Free-radical chlorination of toluene CCl 3 (Dichloromethyl)benzene CHCl 2 (Trichloromethyl)benzene

11-22 Dr. Wolf's CHM 201 & 202 is used in the laboratory to introduce a halogen at the benzylic position Benzylic Bromination CH 3 NO 2 + Br 2 CCl 4, 80°C light + HBr NO 2 CH 2 Br p-Nitrotoluene p-Nitrobenzyl bromide (71%)

11-23 Dr. Wolf's CHM 201 & 202 is a convenient reagent for benzylic bromination N-Bromosuccinimide (NBS) CCl 4 benzoylperoxide,heat CH 2 CH 3 + NBr OO CHCH 3 NHOO + Br (87%)

11-24 Dr. Wolf's CHM 201 & Oxidation of Alkylbenzenes

11-25 Dr. Wolf's CHM 201 & 202 Site of Oxidation is Benzylic Carbon CH 3 CH 2 R CHR 2 or or COHO Na 2 Cr 2 O 7 H 2 SO 4 H2OH2OH2OH2Oheat

11-26 Dr. Wolf's CHM 201 & 202 ExampleExample Na 2 Cr 2 O 7 H 2 SO 4 H2OH2OH2OH2Oheat COHO CH 3 NO 2 p-Nitrotoluene NO 2 p-Nitrobenzoic acid (82-86%)

11-27 Dr. Wolf's CHM 201 & 202 ExampleExample Na 2 Cr 2 O 7 H 2 SO 4 H2OH2OH2OH2Oheat CH(CH 3 ) 2 CH 3 (45%) COHOCOH O

11-28 Dr. Wolf's CHM 201 & S N 1 Reactions of Benzylic Halides

11-29 Dr. Wolf's CHM 201 & 202 tertiary benzylic carbocation is formed more rapidly than tertiary carbocation; therefore, more stable What about S N 1? C CH 3 Cl 6001 C Cl Relative solvolysis rates in aqueous acetone

11-30 Dr. Wolf's CHM 201 & 202 What about S N 1? C more stable less stable C CH 3 Relative rates of formation: CH 3 + +

11-31 Dr. Wolf's CHM 201 & 202 allylic carbocation Compare.Compare. + C C C + C benzylic carbocation benzylic carbon is analogous to allylic carbon

11-32 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Cation C H H HH H H H + unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

11-33 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Cation C H H HH H H H + unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

11-34 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Cation C H H HH H H H + unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

11-35 Dr. Wolf's CHM 201 & 202 Resonance in Benzyl Cation C H H HH H H H + unpaired electron is delocalized between benzylic carbon and the ring carbons that are ortho and para to it

11-36 Dr. Wolf's CHM 201 & 202 SolvolysisSolvolysis C CH 3 Cl CH 3 CH 2 OH C CH 3 OCH 2 CH 3 (87%)

11-37 Dr. Wolf's CHM 201 & S N 2 Reactions of Benzylic Halides

11-38 Dr. Wolf's CHM 201 & 202 Primary Benzylic Halides acetic acid CH 2 Cl O2NO2NO2NO2N NaOCCH 3 O CH 2 OCCH 3 O2NO2NO2NO2NO Mechanism is S N 2 (78-82%)

11-39 Dr. Wolf's CHM 201 & Preparation of Alkenylbenzenes dehydrogenation dehydration dehydrohalogenation

11-40 Dr. Wolf's CHM 201 & 202 industrial preparation of styrene Dehydrogenation CH 2 CH 3 630°C ZnO CH 2 CH + H 2

11-41 Dr. Wolf's CHM 201 & 202 Acid-Catalyzed Dehydration of Benzylic Alcohols KHSO 4 heat (80-82%) CH 2 CH CHCH 3 OHClCl+ H2OH2OH2OH2O

11-42 Dr. Wolf's CHM 201 & 202 Acid-Catalyzed Dehydration of Benzylic Alcohols KHSO 4 heat (80-82%) CH 2 CH CHCH 3 OHClCl Cl+

11-43 Dr. Wolf's CHM 201 & 202 Dehydrohalogenation NaOCH 2 CH 3 ethanol, 50°C (99%) H3CH3CH3CH3C CH 2 CHCH 3 Br CH H3CH3CH3CH3C CHCH 3

11-44 Dr. Wolf's CHM 201 & 202 hydrogenation halogenation addition of hydrogen halides Addition Reactions of Alkenylbenzenes

11-45 Dr. Wolf's CHM 201 & 202 Hydrogenation H2H2H2H2Pt (92%) Br C CH 3 CHCH 3 Br CHCH 2 CH 3 CH 3

11-46 Dr. Wolf's CHM 201 & 202 Halogenation CH 2 CH Br 2 CH 2 CH BrBr (82%)

11-47 Dr. Wolf's CHM 201 & 202 Addition of Hydrogen Halides HCl(75-84%)Cl

11-48 Dr. Wolf's CHM 201 & 202 Addition of Hydrogen Halides HCl via benzylic carbocation Cl+

11-49 Dr. Wolf's CHM 201 & 202 Free-Radical Addition of HBr CH 2 CH CH 2 CH 2 Br HBr peroxides

11-50 Dr. Wolf's CHM 201 & 202 Free-Radical Addition of HBr CH 2 CH CH 2 CH 2 Br HBr peroxides via benzylic radical CH 2 Br CH