Aryl halides (Ar-x)

Report
Aryl halides
Dr. Talat R. Al-Ramadhany
Aryl halides (Ar-x)
Aryl halides are organic compounds
containing halogen atom attached to an aromatic
ring. They have the general formula ArX, where (Ar) is phenyl or substituted phenyl and (x) is F, Cl, Br,
I.
X
I
Cl
Chlorobenzene
Br
NO2
m-Bromonitrobenzene
OH
p-Iodophenol
COOH
Cl
o-Chlorobenzoic acid
Dr. Talat R.Al-Ramadhany
An aryl halide is not just any halogen
compound containing an aromatic ring. Benzyl
chloride, for example, is not an aryl halide; its
halogen is not attached to the aromatic ring, in
structure and properties it is simply a substituted
alkyl halide.
CH2 Cl
Benzyl chloride
C
C
X
A vinyl halide
Dr. Talat R.Al-Ramadhany
Physical properties:
1. Aryl halides are insoluble in water but soluble in
organic solvents.
2. Para-isomer has a melting point about (70-100ºC)
degree higher than ortho- or meta- isomers.
3. Para- isomer is more symmetrical which fits better
into a crystalline lattice and the higher m.p.
4. Because of the strong intracrystalline forces,
p- Isomer is less soluble in a given solvent than
ortho-isomer.
Dr. Talat R.Al-Ramadhany
Preparation of aryl halide compounds:
1. Direct halogenations of aromatic compounds:
ArH + X2
Lewis acid
ArX + HX
X2 = Cl2, Br2
Lewis acid = FeCl3, AlCl3, BFe3, ....etc
Example:
NO2
NO2
Cl2 , AlCl3
Cl
Dr. Talat R.Al-Ramadhany
2. From aryl diazonium salts
A rH
H N O
H 2S O
3
A rN O
2
re d u c tio n
A rN H
H O N O
2
0
4
BF 4CuCl
ArN 2+
Diaznium
salt
CuBr
KI

A rN
2
+
D ia z n iu m
s a lt
ArF
ArCl
+ N2
ArBr
ArI
Dr. Talat R.Al-Ramadhany
Sandmeyer reaction
Replacement of the diazonium group by Cl, or Br is
carried out by mixing the solution of the freshly
prepared diazonium salt with cuprous chloride or
cuprous bromide at room temperature.
ArN2+X-
CuX
ArX + N2
Dr. Talat R.Al-Ramadhany
3. From arylthallium compounds
Aryl iodides can be prepared by simple treatment of
arylthallium with iodine. The thallation route has the advantages of
speed, high yield, and orientation control.
[ArH + Tl(OOCCF3)3 ]
Arylthallium
triflouroacetate
CH3
For iodides
only
CH3
CH3
Tl(OOCCF3)3
ArI
ArTl(OOCCF3)2 + I2
I2
Tl(OOCCF3)2
I
Dr. Talat R.Al-Ramadhany
The preparation of aryl halides from diazonium salts is more
important than direct halogenation for several reasons:
First of all, fluorides and iodides, which can seldom be prepared by
direct halogenation, can be obtained from the diazonium salts.
Second, where direct halogenation yields a mixture of ortho- and
para- isomers; the ortho isomer, at least, is difficult to obtain pure.
On the other hand, the ortho and para isomers of corresponding nitro
compounds, from which the diazonium salts ultimately come, can
often be separated by fractional distillation.
Dr. Talat R.Al-Ramadhany
Low reactivity of aryl and vinyl halides:
Br
alcoholic AgNO3
Dry /
H2C
CHBr
alcoholic AgNO3
Dry /
N. R.
N. R.
Dr. Talat R.Al-Ramadhany
Ag+
X
OH
OR
Aryl halide
NH3
No Reaction
or
CN
C C X
Vinyl halide
ArH,
AlCl3
Dr. Talat R.Al-Ramadhany
The low reactivity attributed to two factors:
A. Delocalization of electrons by resonance.
B. Differences in σ bond energies due to differences
in hybridization of carbon.
Cl
Cl
Cl
Cl
H
H
H
I
II
III
IV
Dr. Talat R.Al-Ramadhany
Reactions of aryl halides:
1. Formation of Grignard reagents
Br
MgBr
Mg
anhyd.Et2O
Cl
MgCl
Mg
THF
Dr. Talat R.Al-Ramadhany
G may not be:
Br
MgBr
Mg
C O
OH
COOR
NH2
C N
SO3H
ether
G
COOH
G
NO2
and many others....
G may be:
R
OR
Ar
Cl
Dr. Talat R.Al-Ramadhany
2. Electrophilic Aromatic Substitution (EAS)
Br
NO2
HNO3, H2SO4
+ Br
NO2
Br
SO3H
H2SO4 , SO3
+ Br
Br
SO3H
Br
Br
Br2, Fe
+
Br
Br
Br
CH3CH2Br
AlCl3
CH2CH3
+
Br
CH2CH3
Dr. Talat R.Al-Ramadhany
3. Nucleophilic aromatic substitution:
Ar:Z + :X-
Ar:X + :Z
(Ar-) must contain strongly electron withdrawing groups
ortho and/or para to the X.
N.R
Cl
+ NaOH
ONa
350oC
4500 psi
OH
H+
Dr. Talat R.Al-Ramadhany
Cl
ONa
H+
15% NaOH
160oC
NO2
OH
NO2
Cl
NO2
OH
NO2
NO2
boiling aq.Na2CO3
130oC
NO2
NO2
Dr. Talat R.Al-Ramadhany
Cl
OH
ONa
NO2
NO2
NO2
H+
+ NaOH
NO2
NO2
NO2
2,4-Dinitrophenol
2,4-Dinitrochlorobenzene
Cl
O2N
OH
O2N
NO2
NO2
warm water
NO2
NO2
Dr. Talat R.Al-Ramadhany
4. Nucleophilic aromatic substitution.
Ar:Z + :X-
Ar:X + :Z
When the ring is not activated toward bimolecular
displacement
Br
NH2
+ NaNH2
Bromobenzene
NH3
Aniline
Dr. Talat R.Al-Ramadhany
F
Li
H2O
+
Fluorobenzene
Li
Phenyllithium
Biphenyl
OCH3
Br
3-Bromo-4-methoxybiphenyl
NaNH2 , NH3
OCH3
NH2
2-Amino-4-methoxybiphenyl
Dr. Talat R.Al-Ramadhany
Analysis of aryl halides
► Insoluble in cold sulfuric acid.
► Inertness toward bromine in carbon tetrachloride
(CCl4) and toward permanganate solution (KMnO4).
► Formation of orange to red colors when treated
with chloroform and aluminum chloride.
► dissolution in cold fuming sulfuric acid, but at a
slower rate than that of benzene.
Dr. Talat R.Al-Ramadhany

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