aryl halides
DESCRIPTION
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. m - Bromonitrobenzene. - PowerPoint PPT PresentationTRANSCRIPT
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
Dr. Talat R.Al-Ramadhany
Cl Br
NO2
I
OH
COOHCl
Chlorobenzene m-Bromonitrobenzene p-Iodophenol
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 C C X
Benzyl chloride 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 ArX + HX
X2 = Cl2, Br2
Lewis acid = FeCl3, AlCl3, BFe3, ....etc
Lewis acid
NO2 NO2
Cl2 , AlCl3
Cl
Example:
Dr. Talat R.Al-Ramadhany
2. From aryl diazonium salts
ArH HNO3H2SO4
ArNO2reduction ArNH2
HONO0
ArN2+
Diazniumsalt
BF4-
ArF
KI ArI
ArCl
ArBr
CuCl
CuBr+ N2ArN2
+
Diazniumsalt
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 compoundsAryl 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 ] ArTl(OOCCF3)2 + I2 ArIArylthallium
triflouroacetate
CH3
Tl(OOCCF3)3 I2
CH3
I
CH3
Tl(OOCCF3)2
For iodides only
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 /N. R.
alcoholic AgNO3
Dry /N. R.H2C CHBr
Dr. Talat R.Al-Ramadhany
X
Aryl halide
or
C C X
Vinyl halide
Ag+
OH
OR
NH3
CN
ArH,
AlCl3
No Reaction
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
H
Cl
H
Cl
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
Br
G
MgBr
G
Mg
ether
G may not be:
COOH C O
OH COOR
NH2 C N
SO3H NO2
and many others....
G may be:
R OR
Ar Cl
Dr. Talat R.Al-Ramadhany
Br
HNO3, H2SO4
Br
NO2Br NO2+
H2SO4 , SO3
Br
SO3HBr SO3H+
Br
BrBrBr+
Br2, Fe
Br
CH2CH3 Br CH2CH3+CH3CH2Br
AlCl3
2 .Electrophilic Aromatic Substitution (EAS)
Dr. Talat R.Al-Ramadhany
3. Nucleophilic aromatic substitution:
Ar:X + :Z Ar:Z + :X-
(Ar-) must contain strongly electron withdrawing groups ortho and/or para to the X.
H+
Cl
+ NaOH
N.R
350oC4500 psi
ONa OH
Dr. Talat R.Al-Ramadhany
Cl
NO2
15% NaOH160oC
ONa
NO2
H+
OH
NO2
Cl
NO2
NO2
boiling aq.Na2CO3130oC
OH
NO2
NO2
Dr. Talat R.Al-Ramadhany
Cl
NO2
NO2
O2Nwarm water
OH
NO2
NO2
O2N
Cl
NO2
NO2
+ NaOH
ONa
NO2
NO2
OH
NO2
NO2
H+
2,4-Dinitrophenol2,4-Dinitrochlorobenzene
Dr. Talat R.Al-Ramadhany
4. Nucleophilic aromatic substitution.
When the ring is not activated toward bimolecular displacement
Ar:X + :Z Ar:Z + :X-
Br
+ NaNH2
NH2
NH3
Bromobenzene Aniline
Dr. Talat R.Al-Ramadhany
F
+
Li
PhenyllithiumFluorobenzene
Li
H2O
Biphenyl
OCH3
Br3-Bromo-4-methoxybiphenyl
NaNH2 , NH3OCH3
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.