aryl halides dr. talat r. al-ramadhany. aryl halides (ar-x) aryl halides are organic compounds...

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

COOH

Cl

Chlorobenzene m-Bromonitrobenzenep-Iodophenol

o-Chlorobenzoic acid


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


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.


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:


2. From aryl diazonium salts

ArH HNO3

H2SO4

ArNO2reduction ArNH2

HONO

0ArN2

+

Diazniumsalt

BF4-

ArF

KIArI

ArCl

ArBr

CuCl

CuBr+ N2

ArN2+

Diazniumsalt


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


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 ] ArTl(OOCCF3)2 + I2 ArIArylthallium

triflouroacetate

CH3

Tl(OOCCF3)3 I2

CH3

I

CH3

Tl(OOCCF3)2

For iodides only


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.


Low reactivity of aryl and vinyl halides:

Br

alcoholic AgNO3

Dry /N. R.

alcoholic AgNO3

Dry /N. R.H2C CHBr


X

Aryl halide

or

C C X

Vinyl halide

Ag+

OH

OR

NH3

CN

ArH,

AlCl3

No Reaction


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


Reactions of aryl halides:

1. Formation of Grignard reagents

Br MgBr

Mg

anhyd.Et2O

Cl MgCl

Mg

THF


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


Br

HNO3, H2SO4

Br

NO2

Br NO2+

H2SO4 , SO3

Br

SO3H

Br SO3H+

Br

Br

BrBr+Br2, Fe

Br

CH2CH3Br CH2CH3

+CH3CH2Br

AlCl3

2 .Electrophilic Aromatic Substitution (EAS)


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


Cl

NO2

15% NaOH160oC

ONa

NO2

H+

OH

NO2

Cl

NO2

NO2

boiling aq.Na2CO3

130oC

OH

NO2

NO2


Cl

NO2

NO2

O2N

warm water

OH

NO2

NO2

O2N

Cl

NO2

NO2

+ NaOH

ONa

NO2

NO2

OH

NO2

NO2

H+

2,4-Dinitrophenol2,4-Dinitrochlorobenzene


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


F

+

Li

PhenyllithiumFluorobenzene

Li

H2O

Biphenyl

OCH3

Br

3-Bromo-4-methoxybiphenyl

NaNH2 , NH3OCH3

NH2

2-Amino-4-methoxybiphenyl


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.

aryl halides dr. talat r. al-ramadhany. aryl halides (ar-x) aryl halides are organic compounds...

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