a) Reduction

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
The Benzene Ring as a Substituent
allylic radical
benzylic radical

• benzylic carbon is analogous to allylic carbon

4
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.

5
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.

6
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

7
Resonance in Benzyl Radical

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

8
Resonance in Benzyl Radical

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

9
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

10
Free-radical chlorination of toluene

• industrial process
• highly regioselective for benzylic position

Cl2
lightorheat
Toluene
Benzyl chloride
11
Free-radical chlorination of toluene

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

(Dichloromethyl)benzene
(Trichloromethyl)benzene
12
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
N-Bromosuccinimide (NBS)

• is a convenient reagent for benzylic bromination

Br
CCl4


benzoyl peroxide, heat
(87)
14
11.13Oxidation of Alkylbenzenes
15
Site of Oxidation is Benzylic Carbon
or
or
16
Example
NO2
p-Nitrotoluene
p-Nitrobenzoicacid (82-86)
17
Example
(45)
18
11.14Nucleophilic Substitutionin Benzylic
Halides
19
Primary Benzylic Halides

• Mechanism is SN2

acetic acid
(78-82)
20
What about SN1?
Relative solvolysis rates in aqueous acetone
600
1

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

21
What about SN1?
Relative rates of formation
CH3
C
C
more stable
less stable
22
Compare.
allylic carbocation
benzylic carbocation

• benzylic carbon is analogous to allylic carbon

23
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

24
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

25
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

26
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

27
Solvolysis
CH3CH2OH
(87)