aryl halides - govt.college for girls sector 11...
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Aryl Halides
By:- Dr.Harpreet Kaur
PGGCG11 CHD
DEPARTMENT OF CHEMISTRY
Aryl Halides Ar-X
Organic compounds with a halogen atom attached to an
aromatic carbon are very different from those compounds
where the halogen is attached to an aliphatic compound.
While the aliphatic compounds readily undergo
nucleophilic substitution and elimination reactions, the
aromatic compounds resist nucleophilic substitution, only
reacting under severe conditions or when strongly
electron withdrawing groups are present ortho/para to the
halogen.
Aryl halides, syntheses:
1. From diazonium salts
Ar-N2+ + CuCl Ar-Cl
Ar-N2+ + CuBr Ar-Br
Ar-N2+ + KI Ar-I
Ar-N2+ + HBF4 Ar-F
2. Halogenation
Ar-H + X2, Lewis acid Ar-X + HX
X2 = Cl2, Br2
Lewis acid = FeCl3, AlCl3, BF3, Fe…
reactions of alkyl halides Ar-X
1. SN2 NR
2. E2 NR
3. organo metallic compounds similar
4. reduction similar
C C X
X
aryl halide
vinyl halide
Ag+
-OH
-OR
NH3
-CN
ArH
AlCl3
NO REACTION
Bond Lengths (Å)
C—Cl C—Br
CH3—X 1.77 1.91
C2H5—X 1.77 1.91 sp3
(CH3)3C—X 1.80 1.92
CH2=CH—X 1.69 1.86
C6H5—X 1.69 1.86
sp2
In aryl halides, the carbon to which the halogen is attached
is sp2 hybrizided. The bond is stronger and shorter than the
carbon-halogen bond in aliphatic compounds where the
carbon is sp3 hybridized. Hence it is more difficult to break
this bond and aryl halides resist the typical nucleophilic
substitution reactions of alkyl halides.
The same is true of vinyl halides where the carbon is also
sp2 hybridized and not prone to nucleophilic substitution.
In a manner analogous to the phenols & alcohols, we have
the same functional group in the two families, aryl halides
and alkyl halides, but very different chemistries.
Aryl halides, reactions:
1. Formation of Grignard reagent
2. EAS
3. Nucleophilic aromatic substitution (bimolecular displacement)
(Ar must contain strongly electron withdrawing groups ortho
and/or para to X)
4. Nucleophilic aromatic substitution (elimination-addition)
(Ring not activated to bimolecular displacement)
1) Grignard reagent
Br
Cl
Mg
Mg
anhyd. Et2O
THF
MgBr
MgCl
2) EAS The –X group is electron-withdrawing and
deactivating in EAS, but is an ortho/para director.
Br
HNO3, H2SO4
H2SO4,SO3
Br2,Fe
CH3CH2-Br, AlCl3
+
+
+
+
Br Br
Br
Br Br
Br
NO2
SO3H
Br
CH2CH3
Br
NO2
Br SO3H
Br CH2CH3
3) Nucleophilic aromatic substitution (bimolecular
displacement)
Ar must contain strongly electron withdrawing groups
ortho and/or para to the X.
Cl
NO2
NO2
+ NH3
NH2
NO2
NO2
Br
NO2
NO2
+ NaOCH3
OCH3
NO2
NO2
O2N O2N
Cl
NO2
OH
NO2
Cl
NO2
NO2
OH
NO2
NO2
O2N O2N
Cl
+ NaOH NR
350oC, 4500 psi H+
OH
15% NaOH, 160oC H+
warm water
Cl
NO2
NH2
NO2
Cl
NO2
NO2
NH3
NO2
NO2
O2N O2N
Cl NH2
NH3, 170oC
NH3, room temp.
NH3, Cu2O, 200oC, 900 psi
NO2 NO2
bimolecular displacement (nucleophilic aromatic substitution)
mechanism:
1) + :ZXX
ZRDS
X
Z2) Z + :X
X
Z
Z
X
Z
X
Z
X
Z
X
Z
X
G
G
If G is an electron withdrawing group in the ortho andpara positions, it will stabilize the intermediate anion.
evidence for the bimolecular displacement mechanism:
no element effect : Ar-I Ar-Br Ar-Cl Ar-F
(the C—X bond is not broken in the RDS)
4) Elimination-Addition, nucleophilic aromatic substitution.
When the ring is not activated to the bimolecular
displacement and the nucleophile is an extremely good
one.
Br
+ NaNH2, NH3
NH2
F
+
Li
LiH2O
Elimination-Addition mechanism (nucleophilic aromatic
substitution)
1)
X
H
+ :NH2
X
+ NH3
2)
X
+ :X
benzyne
3) + :NH2
NH2
NH2
4) + NH3
NH2
H
+ :NH2
elimination
addition
:
:
:
:
While the concept of ―benzyne‖ may appear to be strange,
there is much evidence that this mechanism is correct.
Cl
*
* = 14C
NaNH2
NH3
NH2
* *+
NH2
47% 53%
OCH3
Br
H3C NaNH2
NH3
NR
Cl
D+ :NH2
Cl
H
+
benzyne intermediate has been trapped in a Diels-Alder
condensation:
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