11.11 The Birch Reduction

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11.11The Birch Reduction
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Birch Reduction of Benzene
Na, NH3
CH3OH
(80)

• Product is non-conjugated diene.
• Reaction stops here. There is no further
  reduction.
• Reaction is not hydrogenation. H2 is not involved
  in any way.

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Mechanism of the Birch Reduction (Figure 11.8)

• Step 1 Electron transfer from sodium

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Mechanism of the Birch Reduction (Figure 11.8)

• Step 2 Proton transfer from methanol

H
H
H


H
H

H
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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

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Mechanism of the Birch Reduction (Figure 11.8)

• Step 3 Electron transfer from sodium

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Mechanism of the Birch Reduction (Figure 11.8)

• Step 3 Electron transfer from sodium

H

H
H


Na
H
H
H
H
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Mechanism of the Birch Reduction (Figure 11.8)

• Step 4 Proton transfer from methanol

H

H
H

H
H
H
H
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Mechanism of the Birch Reduction (Figure 11.8)

• Step 4 Proton transfer from methanol

H

H
H

H
H
H
H
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Birch Reduction of an Alkylbenzene
Na, NH3
CH3OH
(86)

• If an alkyl group is present on the ring, it ends
  up asa substituent on the double bond.

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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)
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11.12Free-Radical Halogenationof Alkylbenzenes
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The Benzene Ring as a Substituent
allylic radical
benzylic radical

• benzylic carbon is analogous to allylic carbon

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Recall
Bond-dissociation energy for CH bond is equal
to DH for

RH
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.

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Bond-dissociation energies of propene and toluene
368 kJ/mol
H2C
CH
-H
356 kJ/mol
-H

• Low BDEs indicate allyl and benzyl radical are
  more stable than simple alkyl radicals.

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Resonance in Benzyl Radical
H
H

H
H
H
H
H

• unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it

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Resonance in Benzyl Radical

• unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it

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Resonance in Benzyl Radical

• unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it

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Resonance in Benzyl Radical
H
H
H
H

H
H
H

• unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it

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Free-radical chlorination of toluene

• industrial process
• highly regioselective for benzylic position

Cl2
lightorheat
Toluene
Benzyl chloride
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Free-radical chlorination of toluene

• Similarly, dichlorination and trichlorination
  areselective for the benzylic carbon.
  Furtherchlorination gives

(Dichloromethyl)benzene
(Trichloromethyl)benzene
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Benzylic Bromination

• is used in the laboratory to introduce a halogen
  at the benzylic position

Br2
HBr
NO2
p-Nitrotoluene
p-Nitrobenzyl bromide (71)
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N-Bromosuccinimide (NBS)

• is a convenient reagent for benzylic bromination

Br
CCl4


benzoyl peroxide, heat
(87)
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11.13Oxidation of Alkylbenzenes
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Site of Oxidation is Benzylic Carbon
or
or
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Example
NO2
p-Nitrotoluene
p-Nitrobenzoicacid (82-86)
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Example
(45)
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11.14 SN1 Reactions of Benzylic Halides
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What about SN1?
Relative solvolysis rates in aqueous acetone
600
1

• tertiary benzylic carbocation is formedmore
  rapidly than tertiary carbocationtherefore,
  more stable

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What about SN1?
Relative rates of formation
CH3
C
C
more stable
less stable
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Compare.
allylic carbocation
benzylic carbocation

• benzylic carbon is analogous to allylic carbon

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Resonance in Benzyl Cation
H
H

H
H
H
H
H

• unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it

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Resonance in Benzyl Cation
H
H
H
H

H
H
H

• unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it

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Resonance in Benzyl Cation
H
H
H
H

H
H
H

• unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it

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Resonance in Benzyl Cation
H
H
H
H

H
H
H

• unpaired electron is delocalized between
  benzylic carbon and the ring carbons that are
  ortho and para to it

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Solvolysis
CH3CH2OH
(87)
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11.15SN2 Reactions of Benzylic Halides
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Primary Benzylic Halides

• Mechanism is SN2

acetic acid
(78-82)
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11.16Preparation of Alkenylbenzenes

• dehydrogenation
• dehydration
• dehydrohalogenation

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Dehydrogenation

• industrial preparation of styrene

630C
ZnO
H2
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Acid-Catalyzed Dehydration of Benzylic Alcohols
KHSO4
heat

H2O
(80-82)
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Acid-Catalyzed Dehydration of Benzylic Alcohols
KHSO4
heat
(80-82)
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Dehydrohalogenation
NaOCH2CH3
ethanol, 50C
(99)
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11.17Addition Reactions of Alkenylbenzenes

• hydrogenation
• halogenation
• addition of hydrogen halides

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Hydrogenation
(92)
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Halogenation
Br2
CH
CH2
Br
Br
(82)
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Addition of Hydrogen Halides
(75-84)
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Addition of Hydrogen Halides

via benzylic carbocation
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Free-Radical Addition of HBr
HBr
peroxides
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Free-Radical Addition of HBr
HBr
peroxides
CH
CH2Br

via benzylic radical
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