aryl halides - govt.college for girls sector 11...

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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