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Question
Chemical Behavior of Aromatic Compounds
• Which compound has the lowest pKa?
•
Acidity
• A)
B)
• C)
D)
The Benzene Ring as a Substituent
Free-Radical Halogenation
of Alkylbenzenes
C
C
C•
C•
allylic radical
benzylic radical
Benzylic carbon is analogous to allylic carbon.
Resonance in Benzyl Radical
H
•
C
Resonance in Benzyl Radical
H
H
H
H
H
H
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.
Unpaired electron is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Resonance in Benzyl Radical
H
C
H
H
•
Resonance in Benzyl Radical
H
H
H
H
H
H
H
C
•
H
H
H
H
Unpaired electron is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Unpaired electron is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Density in Benzyl Radical
N-Bromosuccinimide (NBS)
is a convenient reagent for benzylic bromination
Br
CH2CH3
O
NBr +
O
Unpaired electron is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
CCl4
benzoyl
peroxide,
heat
CHCH3
O
NH +
O
Question
• Which reagent would you use for the
reaction below?
Oxidation of Alkylbenzenes
•
•
•
•
A)
B)
C)
D)
Br2, CCl4
Br2, FeCl3
HBr, peroxides
NBS, peroxides, heat
(87%)
Example
Site of Oxidation is Benzylic Carbon
O
CH3
CH3
or
CH2R
or
Na2 Cr2 O7
H2 SO4
COH
Na2 Cr2 O7
H2 SO4
O
H2 O
heat
COH
H2 O
heat
NO2
NO2
p-Nitrotoluene
CHR2
p-Nitrobenzoic
acid (82-86%)
Example
Question
O
CH(CH3)2
COH
Na2 Cr2 O7
H2 SO4
H2 O
heat
CH3
• Select the best reagent(s) to accomplish the
transformation shown.
•
COH
O
(45%)
•
•
•
•
A)
B)
C)
D)
PCC, CH2 Cl2
K 2 Cr2 O7, H2 SO4, heat
LiAlH4 then H2 O
NBS then CH3ONa
Benzylic SN 1 Reactions
Relative solvolysis rates in aqueous acetone:
CH3
SN 1 Reactions of
Benzylic Halides
C
CH3
620
CH3
Cl
CH3
C
Cl
CH3
1
Tertiary benzylic carbocation is formed
more rapidly than tertiary carbocation;
therefore, more stable.
Benzylic SN 1 Reactions
Compare
Relative rates of formation:
CH3
CH3
C+
CH3
CH3
C+
C
C
C+
C+
CH3
more stable
less stable
allylic carbocation
benzylic carbocation
Benzylic carbon is analogous to allylic carbon.
Resonance in Benzyl Cation
H
+
C
Resonance in Benzyl Cation
H
H
H
H
H
H
H
H
H
C
H
+
H
H
H
Positive charge is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Positive charge is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Resonance in Benzyl Cation
Resonance in Benzyl Cation
H
C
H
H
+
H
H
H
H
H
H
H
C
+
H
H
H
H
Positive charge is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Positive charge is delocalized between
benzylic carbon and the ring carbons that are
ortho and para to it.
Solvolysis
Question
CH3
C
Cl
• Which compound would undergo
solvolysis at a faster rate?
CH3
CH3CH2 OH
•
A)
B)
•
C)
D)
CH3
C
OCH2CH3
CH3
(87%)
Primary Benzylic Halides
O2 N
CH2Cl
Mechanism is SN2
SN 2 Reactions of
Benzylic Halides
O
Like allylic halide,
substitution is faster
than a normal
primary halide.
NaO
NaOCCH3
acetic acid
O
O2 N
CH2OCCH3
(78-82%)
Question
• Which product will be siolated when 4chlorobenzyl chloride is treated with
K2CO3 in H2O?
• A)
•
• C)
B)
Preparation of Alkenylbenzenes
•dehydrogenation
•dehydration
D)
•dehydrohalogenation
Acid-Catalyzed Dehydration of
Benzylic Alcohols
Dehydrogenation
•Industrial preparation of styrene
Cl
Cl
•Almost 12 billion lbs. produced annually in U.S.
KHSO4
CHCH3
CH2CH3
630°C
ZnO
CH
CH
heat
OH
CH2
CH2
(80-82%)
Cl
+ H2
CHCH3
+
Dehydrohalogenation
H3 C
CH2CHCH3
Question
• Which product(s) will be isolated from
the reaction shown here?
Br
NaO
NaOCH2 CH3
ethanol, 50°C
• A)
• B)
H3 C
CH
CHCH3
(99%)
• C) Equal amounts of A and B.
• D) Neither A nor B.
Hydrogenation
CH3
Addition Reactions of Alkenylbenzenes
C
CHCH3
CHCH2 CH3
H2
•hydrogenation
•halogenation
•addition of hydrogen halides
CH3
Pt
Br
Br
(92%)
Addition of Hydrogen Halides
Halogenation
Cl
CH
HCl
Br2
CH2
CH
CH2
Br
Br
(75-84%)
+
(82%)
via benzylic carbocation
Free-Radical Addition of HBr
CH
CH2
HBr
CH2CH2 Br
peroxides
CH
•
Polymerization of Styrene
CH2Br
via benzylic radical
Polymerization of Styrene
H2 C
CHC
CHC6H5
H2 C
CH2
CH
C6 H5
CH2
Mechanism
••
RO ••
•
CH
C6 H5
polystyrene
CH2
CH
C6 H5
CHC
CHC6H5
Mechanism
••
RO ••
H2 C
CHC
CHC6H5
•
H2 C
H2 C
H2 C
Mechanism
CHC
CHC6H5
H2 C
CHC
CHC6H5
CHC
CHC6H5
CHC
CHC6H5
H2 C
CHC
CHC6H5
CHC
CHC6H5
•
H2 C
CHC
CHC6H5
•
H2 C
Mechanism
••
RO ••
H2 C
Mechanism
H2 C
CHC
CHC6H5
•
CHC
CHC6H5
••
RO ••
H2 C
H2 C
H2 C
CHC
CHC6H5
•
CHC
CHC6H5
••
RO ••
H2 C
Mechanism
••
RO ••
CHC
CHC6H5
CHC
CHC6H5