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Chapter 23: Aryl Halides
23.1: Bonding in Aryl Halides. Halide bonded directly to an
aromatic ring. C-X bond of aryl halides are shorter and
stronger than the C-X bond of alkyl halides.
23.2: Sources of Aryl Halides. (see Ch. 23.4)
23.3: Physical Properties of Aryl Halides. (please read)
23.4: Reactions of Aryl Halides: A Review and Preview.
a. Electrophilic aromatic substitution of arenes (Ch. 12.5)
Cl2, FeCl3
Cl
Br2, FeBr3
Br
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b. From arylamines via the aryl diazonium ion (Ch. 22.17)
F
HBF4
- N2
Cl
CuCl
NH2
NaNO2,
H2SO4
N
N
- N2
Br
CuBr
- N2
I
KI
- N2
23.5: Nucleophilic Subititution in Nitro-Substituted Aryl
Halides. Nucleophiles such as hydroxide, alkoxides, thiolate,
ammonia, and 1° and 2° amines can displace an aryl halide
when a p- or o-nitro group is present
OCH3
Cl
+
CH3O
+
Na
NaCl
229
NO2
NO2
Reactivity:
Nature of the leaving group:
OCH3
X
50° C
+
CH3O
+
Na
H3COH
NO2
NO2
NaX
X=F
Cl
Br
I
krel= 312
1.0
0.8
0.4
Rate = k [aryl halide][nucleophile]
230
Addition-elimination mechanism (Mech. 23.1, p. 971): The
nucleophile adds to the carbon bearing the halide giving leading
to an intermediate aryl anion (Meisenheimer complex), which is
stabilzed by the nitro group. The halide is the eliminated giving
the nucleophilic aromatic substitution product.
OCH3
F
+
NO2
CH3O
+
Na
NaF
NO2
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23.7: Related Nucleophilic Aromatic Substitution. Halides of
heteroaromatic compounds are reactive toward nucleophilic
aromatic substitution
+
N
Cl
+
Nu:
Cl
N
Nu
This is an important biochemical reaction.
CO2H
O
N
PO
O
N
O-PO3
NH
N
N
aspartic acid,
GTP
2-
O3P-O
O
H
2-
N
N
N
CO2H
N
N
PO
O
N
N
N
+ GDP
adenylosuccinate
synthase
HO
OH
HO
OH
Inosine monophosphate
(IMP)
OH
NH2
N
adenylosuccinate
lyase
HO
2-
O3P-O
O
HO
N
N
N
OH
Adenosine monophosphate
(AMP)
+
HO2C
CO2H
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23.8: The Elimination-Addition Mechanism of Nucleophilic
Aromatic Substitution: Benzyne.
Br
H3C
NH2
NaNH2, NH3
H3C
H3C
+
H3C
23.9: Diels-Alder Reactions of Benzyne. (please read)
23.10: m-Benzyne and p-Benzyne. (please read)
NH2
233