22.17

Synthetic Transformations

of Aryl Diazonium Salts

Transformations of Aryl Diazonium Salts

Ar N N +

Ar H

Ar OH

Ar I

Ar F

Ar Br Ar Cl

Ar CN

Preparation of Phenols

Ar N N +

Ar OH

H2O, heat

Example

2. H2O, heat

(CH3)2CH NH2

1. NaNO2, H2SO4

H2O, 0-5°C

(CH3)2CH OH

(73%)

Transformations of Aryl Diazonium Salts

Ar N N +

Ar H

Ar OH

Ar I

Ar F

Ar Br Ar Cl

Ar CN

Preparation of Aryl Iodides

Ar N N +

Ar I

reaction of an aryl diazonium salt with

potassium iodide

KI

Example

2. KI, room temp.

1. NaNO2, HCl

H2O, 0-5°C

(72-83%)

NH2

Br

I

Br

Transformations of Aryl Diazonium Salts

Ar N N +

Ar H

Ar OH

Ar I

Ar F

Ar Br Ar Cl

Ar CN

Preparation of Aryl Fluorides

Ar N N +

Ar F

heat the tetrafluoroborate salt of a diazonium ion;

process is called the Schiemann reaction

Example

(68%)

NH2

CCH2CH3

O

2. HBF4

1. NaNO2, HCl,

H2O, 0-5°C

3. heat

F

CCH2CH3

O

Transformations of Aryl Diazonium Salts

Ar N N +

Ar H

Ar OH

Ar I

Ar F

Ar Br Ar Cl

Ar CN

Preparation of Aryl Chlorides and Bromides

Ar N N +

Ar Br Ar Cl

aryl chlorides and aryl bromides are prepared by

heating a diazonium salt with copper(I) chloride or

bromide

substitutions of diazonium salts that use copper(I)

halides are called Sandmeyer reactions

Example

(68-71%)

NH2

NO2

2. CuCl, heat

1. NaNO2, HCl,

H2O, 0-5°C

Cl

NO2

Example

(89-95%)

2. CuBr, heat

1. NaNO2, HBr,

H2O, 0-10°C

NH2

Cl

Br

Cl

Transformations of Aryl Diazonium Salts

Ar N N +

Ar H

Ar OH

Ar I

Ar F

Ar Br Ar Cl

Ar CN

Preparation of Aryl Nitriles

Ar N N +

Ar CN

aryl nitriles are prepared by heating a diazonium

salt with copper(I) cyanide

this is another type of Sandmeyer reaction

Example

(64-70%)

2. CuCN, heat

1. NaNO2, HCl,

H2O, 0°C

NH2

CH3

CN

CH3

Transformations of Aryl Diazonium Salts

Ar N N +

Ar H

Ar OH

Ar I

Ar F

Ar Br Ar Cl

Ar CN

Transformations of Aryl Diazonium Salts

Ar N N +

Ar H

hypophosphorous acid (H3PO2) reduces diazonium

salts; ethanol does the same thing

this is called reductive deamination

Example

(70-75%)

NaNO2, H2SO4,

H3PO2

NH2

CH3 CH3

Value of Diazonium Salts

1) allows introduction of substituents such as

OH, F, I, and CN on the ring

2) allows preparation of otherwise difficultly

accessible substitution patterns

Example

Br

Br Br

NH2

Br

Br

Br

(74-77%)

NaNO2, H2SO4,

H2O, CH3CH2OH

NH2

Br2

H2O

(100%)

22.18

Azo Coupling

Azo Coupling

Diazonium salts are weak electrophiles.

React with strongly activated aromatic

compounds by electrophilic aromatic

substitution.

Ar N N +

Ar' H + Ar N N Ar'

an azo compound

Ar' must bear a strongly electron-releasing group

such as OH, OR, or NR2.

Example

OH

+ C6H5N N +

OH

N NC6H5

Cl–