11-1 dr. wolf's chm 201 & 202 11.11 the birch reduction
TRANSCRIPT
11-1Dr. Wolf's CHM 201 & 202
11.11The Birch Reduction
11-2Dr. Wolf's CHM 201 & 202
(80%)(80%)
HH
HH
HH HH
HHHH
HH HH
HH
HH HH
HH
HH HH
Na, NHNa, NH33
CHCH33OHOH
Birch Reduction of BenzeneBirch Reduction of BenzeneBirch Reduction of BenzeneBirch Reduction of Benzene
Product is non-conjugated diene.Product is non-conjugated diene.
Reaction stops here. There is no further reduction.Reaction stops here. There is no further reduction.
Reaction is not hydrogenation. HReaction is not hydrogenation. H22 is not involved in any way. is not involved in any way.
11-3Dr. Wolf's CHM 201 & 202
HH
HH
HH HH
HHHH
Step 1: Electron transfer from sodiumStep 1: Electron transfer from sodium
++ NaNa•• ++ NaNa++
Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)
HH
HH
HH
HH
HH
HH••
••••––
11-4Dr. Wolf's CHM 201 & 202
Step 2: Proton transfer from methanolStep 2: Proton transfer from methanol
Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)
HH
HH
HH
HH
HH
HH
––
••
••••
OCHOCH33
HH
••••••••
11-5Dr. Wolf's CHM 201 & 202
Step 2: Proton transfer from methanolStep 2: Proton transfer from methanol
Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)
HH
HH
HH
HH
HH
HH
––
••
••••
OCHOCH33
HH
••••••••
HH
HH
HH
HH
HH
HH
HH
••
OCHOCH33••••••••••••––
11-6Dr. Wolf's CHM 201 & 202
Step 3: Electron transfer from sodiumStep 3: Electron transfer from sodium
Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8) HH
HH
HH
HH
HH
HH
HH
••++ NaNa••
11-7Dr. Wolf's CHM 201 & 202
Step 3: Electron transfer from sodiumStep 3: Electron transfer from sodium
Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8) HH
HH
HH
HH
HH
HH
HH
••++ NaNa••
HH
HH
HH
HH
HH
HH
HH
••••++ NaNa++
––
11-8Dr. Wolf's CHM 201 & 202
Step 4: Proton transfer from methanolStep 4: Proton transfer from methanol
Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8) HH
HH
HH
HH
HH
HH
HH
••••––
•• OCHOCH33
HH
••••••
11-9Dr. Wolf's CHM 201 & 202
Step 4: Proton transfer from methanolStep 4: Proton transfer from methanol
Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8)Mechanism of the Birch Reduction (Figure 11.8) HH
HH
HH
HH
HH
HH
HH
••••––
•• OCHOCH33
HH
••••••
HH HH
HH
HH HH
HH
HH HH
––•• OCHOCH33••••
••••••
11-10Dr. Wolf's CHM 201 & 202
(86%)(86%)
HH
HH
HH C(CHC(CH33))33
HHHH
HH HH
HH
HH C(CHC(CH33))33
HH
HH HH
Na, NHNa, NH33
CHCH33OOHH
Birch Reduction of an AlkylbenzeneBirch Reduction of an AlkylbenzeneBirch Reduction of an AlkylbenzeneBirch Reduction of an Alkylbenzene
If an alkyl group is present on the ring, it ends up asIf an alkyl group is present on the ring, it ends up asa substituent on the double bond.a substituent on the double bond.
11-11Dr. Wolf's CHM 201 & 202
a) Reductiona) Reduction
Catalytic hydrogenation (Section 11.4)Catalytic hydrogenation (Section 11.4) Birch reduction (Section 11.11) Birch reduction (Section 11.11)
b) Electrophilic aromatic substitutionb) Electrophilic aromatic substitution(Chapter 12)(Chapter 12)
c) Nucleophilic aromatic substitutionc) Nucleophilic aromatic substitution(Chapter 23)(Chapter 23)
1. Reactions involving the ring1. Reactions involving the ring
2. The ring as a substituent (Sections 11.12-11.17)2. The ring as a substituent (Sections 11.12-11.17)
11-12Dr. Wolf's CHM 201 & 202
11.1211.12
Free-Radical HalogenationFree-Radical Halogenation
of Alkylbenzenesof Alkylbenzenes
11-13Dr. Wolf's CHM 201 & 202
allylic radicalallylic radical
The Benzene Ring as a SubstituentThe Benzene Ring as a SubstituentThe Benzene Ring as a SubstituentThe Benzene Ring as a Substituent
••CCCC
CC ••CC
benzylic radicalbenzylic radical
benzylic carbon is analogous to allylic carbonbenzylic carbon is analogous to allylic carbon
11-14Dr. Wolf's CHM 201 & 202
The more stable the free radical R•, the weaker The more stable the free radical R•, the weaker the bond, and the smaller the bond-dissociation the bond, and the smaller the bond-dissociation energy.energy.
Recall:Recall:Recall:Recall:
R—HR—H R•R• ••HH++
Bond-dissociation energy for C—H bond Bond-dissociation energy for C—H bond is equal to is equal to HH° for:° for:
and is about 400 kJ/mol for alkanes.and is about 400 kJ/mol for alkanes.
11-15Dr. Wolf's CHM 201 & 202
Bond-dissociation energies of propene and tolueneBond-dissociation energies of propene and tolueneBond-dissociation energies of propene and tolueneBond-dissociation energies of propene and toluene
368 kJ/mol368 kJ/mol
356 kJ/mol356 kJ/mol
HH
HH22CC CHCH CC HH
HH HH
CC HH
HH
HH22CC CHCH-H•-H•
-H•-H•
HH
CC
HH
••
HH
CC
HH
••
Low BDEs indicate allyl and benzyl radical are Low BDEs indicate allyl and benzyl radical are more stable than simple alkyl radicals.more stable than simple alkyl radicals.
11-16Dr. Wolf's CHM 201 & 202
Resonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl Radical CCHH
HH
HHHH
HH
HH
HH••
unpaired electron is delocalized between unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho and para to itortho and para to it
11-17Dr. Wolf's CHM 201 & 202
Resonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl Radical
CCHH
HH
HHHH
HH
HH
HH
••
unpaired electron is delocalized between unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho and para to itortho and para to it
11-18Dr. Wolf's CHM 201 & 202
Resonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl Radical
CCHH
HH
HHHH
HH
HH
HH
••
unpaired electron is delocalized between unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho and para to itortho and para to it
11-19Dr. Wolf's CHM 201 & 202
Resonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl RadicalResonance in Benzyl Radical
CCHH
HH
HHHH
HH
HH
HH
••
unpaired electron is delocalized between unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho and para to itortho and para to it
11-20Dr. Wolf's CHM 201 & 202
industrial processindustrial process
highly regioselective for benzylic positionhighly regioselective for benzylic position CHCH33
Free-radical chlorination of tolueneFree-radical chlorination of tolueneFree-radical chlorination of tolueneFree-radical chlorination of toluene
ClCl22
lightlightoror
heatheat
CHCH22ClCl
TolueneToluene Benzyl chlorideBenzyl chloride
11-21Dr. Wolf's CHM 201 & 202
Similarly, dichlorination and trichlorination areSimilarly, dichlorination and trichlorination areselective for the benzylic carbon. Furtherselective for the benzylic carbon. Furtherchlorination gives:chlorination gives:
Free-radical chlorination of tolueneFree-radical chlorination of tolueneFree-radical chlorination of tolueneFree-radical chlorination of toluene CClCCl33
(Dichloromethyl)benzene(Dichloromethyl)benzene
CHClCHCl22
(Trichloromethyl)benzene(Trichloromethyl)benzene
11-22Dr. Wolf's CHM 201 & 202
is used in the laboratory to introduce a is used in the laboratory to introduce a halogen at the benzylic positionhalogen at the benzylic position
Benzylic BrominationBenzylic BrominationBenzylic BrominationBenzylic Bromination CHCH33
NONO22
+ Br+ Br22
CClCCl44, 80°C, 80°C
lightlight+ HBr+ HBr
NONO22
CHCH22BrBr
pp-Nitrotoluene-Nitrotoluene pp-Nitrobenzyl-Nitrobenzylbromide (71%)bromide (71%)
11-23Dr. Wolf's CHM 201 & 202
N-Bromosuccinimide (NBS)N-Bromosuccinimide (NBS)N-Bromosuccinimide (NBS)N-Bromosuccinimide (NBS)
CClCCl44
benzoylbenzoylperoxide,peroxide,
heatheat
CHCH22CHCH33 ++
NNBrBr
OO
OO
CHCHCHCH33 NHNH
OO
OO
++
BrBr
(87%)(87%)
11-24Dr. Wolf's CHM 201 & 202
11.1311.13
Oxidation of AlkylbenzenesOxidation of Alkylbenzenes
11-25Dr. Wolf's CHM 201 & 202
Site of Oxidation is Benzylic CarbonSite of Oxidation is Benzylic CarbonSite of Oxidation is Benzylic CarbonSite of Oxidation is Benzylic Carbon CHCH33 CHCH22RR CHRCHR22
oror
oror
COHCOH
OONaNa22CrCr22OO77
HH22SOSO44
HH22OO
heatheat
11-26Dr. Wolf's CHM 201 & 202
ExampleExampleExampleExample
NaNa22CrCr22OO77
HH22SOSO44
HH22OO
heatheat
COHCOH
OO CHCH33
NONO22
pp-Nitrotoluene-Nitrotoluene
NONO22
pp-Nitrobenzoic-Nitrobenzoicacid (82-86%)acid (82-86%)
11-27Dr. Wolf's CHM 201 & 202
ExampleExampleExampleExample
NaNa22CrCr22OO77
HH22SOSO44
HH22OO
heatheat
CH(CHCH(CH33))22
CHCH33
(45%)(45%)
COHCOH
OO COHCOH
OO
11-28Dr. Wolf's CHM 201 & 202
11.14 11.14
S SNN1 Reactions of1 Reactions of
Benzylic Halides Benzylic Halides
11-29Dr. Wolf's CHM 201 & 202
tertiary benzylic carbocation is formedtertiary benzylic carbocation is formedmore rapidly than tertiary carbocation;more rapidly than tertiary carbocation;therefore, more stabletherefore, more stable
What about SWhat about SNN1?1?What about SWhat about SNN1?1? CC
CHCH33
CHCH33
ClCl
600600 11
CC
CHCH33
CHCH33
ClClCHCH33
Relative solvolysis rates in aqueous acetoneRelative solvolysis rates in aqueous acetone
11-30Dr. Wolf's CHM 201 & 202
What about SWhat about SNN1?1?What about SWhat about SNN1?1? CC
more stablemore stable less stableless stable
CCCHCH33
Relative rates of formation:Relative rates of formation:
CHCH33
CHCH33
++
CHCH33
CHCH33
++
11-31Dr. Wolf's CHM 201 & 202
allylic carbocationallylic carbocation
Compare.Compare.Compare.Compare.
++CCCC
CC ++CC
benzylic carbocationbenzylic carbocation
benzylic carbon is analogous to allylic carbonbenzylic carbon is analogous to allylic carbon
11-32Dr. Wolf's CHM 201 & 202
Resonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl Cation CCHH
HH
HHHH
HH
HH
HH++
unpaired electron is delocalized between unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho and para to itortho and para to it
11-33Dr. Wolf's CHM 201 & 202
Resonance in Benzyl Cation Resonance in Benzyl Cation Resonance in Benzyl Cation Resonance in Benzyl Cation
CCHH
HH
HHHH
HH
HH
HH
++
unpaired electron is delocalized between unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho and para to itortho and para to it
11-34Dr. Wolf's CHM 201 & 202
Resonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl Cation
CCHH
HH
HHHH
HH
HH
HH
++
unpaired electron is delocalized between unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho and para to itortho and para to it
11-35Dr. Wolf's CHM 201 & 202
Resonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl CationResonance in Benzyl Cation
CCHH
HH
HHHH
HH
HH
HH
++
unpaired electron is delocalized between unpaired electron is delocalized between benzylic carbon and the ring carbons that are benzylic carbon and the ring carbons that are ortho and para to itortho and para to it
11-36Dr. Wolf's CHM 201 & 202
SolvolysisSolvolysisSolvolysisSolvolysis CC
CHCH33
CHCH33
ClCl
CHCH33CHCH22OHOH
CC
CHCH33
CHCH33
OCHOCH22CHCH33
(87%)(87%)
11-37Dr. Wolf's CHM 201 & 202
11.1511.15
SSNN2 Reactions of2 Reactions of
Benzylic Halides Benzylic Halides
11-38Dr. Wolf's CHM 201 & 202
Primary Benzylic HalidesPrimary Benzylic HalidesPrimary Benzylic HalidesPrimary Benzylic Halides
acetic acidacetic acid
CHCH22ClCl
OO22NN
NaOCCHNaOCCH33
OO
CHCH22OCCHOCCH33
OO22NN
OO
Mechanism is SMechanism is SNN22
(78-82%)(78-82%)
11-39Dr. Wolf's CHM 201 & 202
11.16Preparation of
Alkenylbenzenes
•dehydrogenation
•dehydration
•dehydrohalogenation
11-40Dr. Wolf's CHM 201 & 202
•industrial preparation of styrene
DehydrogenationDehydrogenation CHCH22CHCH33
630°C630°C
ZnOZnO
CHCH22CHCH
+ H+ H22
11-41Dr. Wolf's CHM 201 & 202
Acid-Catalyzed Dehydration of Benzylic Alcohols
Acid-Catalyzed Dehydration of Benzylic Alcohols
KHSOKHSO44
heatheat
(80-82%)(80-82%)
CHCH22CHCHCHCHCHCH33
OHOH
ClCl ClCl ++ HH22OO
11-42Dr. Wolf's CHM 201 & 202
Acid-Catalyzed Dehydration of Benzylic Alcohols
Acid-Catalyzed Dehydration of Benzylic Alcohols
KHSOKHSO44
heatheat(80-82%)(80-82%)
CHCH22CHCHCHCHCHCH33
OHOH
ClCl ClCl CHCHCHCH33
ClCl ++
11-43Dr. Wolf's CHM 201 & 202
DehydrohalogenationDehydrohalogenation
NaOCHNaOCH22CHCH33 ethanol, 50°Cethanol, 50°C
(99%)(99%)
HH33CC CHCH22CHCHCHCH33
BrBr
CHCHHH33CC
CHCHCHCH33
11-44Dr. Wolf's CHM 201 & 202
•hydrogenation
•halogenation
•addition of hydrogen halides
11.17Addition Reactions of
Alkenylbenzenes
11-45Dr. Wolf's CHM 201 & 202
HydrogenationHydrogenation
HH22
PtPt
(92%)(92%)
BrBr
CC
CHCH33
CHCHCHCH33
BrBr
CHCHCHCH22CHCH33
CHCH33
11-46Dr. Wolf's CHM 201 & 202
HalogenationHalogenation CHCH22CHCH
BrBr22
CHCH22CHCH
BrBrBrBr
(82%)(82%)
11-47Dr. Wolf's CHM 201 & 202
Addition of Hydrogen HalidesAddition of Hydrogen Halides
HClHCl
(75-84%)(75-84%)
ClCl
11-48Dr. Wolf's CHM 201 & 202
Addition of Hydrogen HalidesAddition of Hydrogen Halides
HClHCl
via benzylic carbocationvia benzylic carbocation
ClCl ++
11-49Dr. Wolf's CHM 201 & 202
Free-Radical Addition of HBrFree-Radical Addition of HBr CHCH22CHCH
CHCH22CHCH22BrBr
HBrHBr
peroxidesperoxides
11-50Dr. Wolf's CHM 201 & 202
Free-Radical Addition of HBrFree-Radical Addition of HBr CHCH22CHCH
CHCH22CHCH22BrBr
HBrHBr
peroxidesperoxides
via benzylic radicalvia benzylic radical
CHCH22BrBrCHCH
••