Diazonium Salts, Azo Dyes c5n2c

1.

Preparation of Diazonium salt

2. Coupling Reaction with Phenols

Student handout

Dotsha J. Raheem College of Science/ Department of Chemistry University of Salahadin May 2010

1

Diazonium salt formation (diazotization reaction)

NaNO2 / HCl

NH2

N

N Cl

o

0-5 C aniline or any other primary aromatic amine

Diazonium salt

Mechanism of the Reaction 1. Formation of the nitrosonium ion NaNO2 + HCl

H O N O

+

NaCl +

HNO2

H H O N O

H

H H O

+

N O

N O

Nitrosonium ion 2. Nitrosation of the amine

H NH2 +

N O

N N O H

H2O

N N O + H3O H N-nitrosoamine

3. Tautomerism

Tautomerism N N O

N N OH

H N-nitrosoamine

diazenol

2

4. Formation of the diazonium salt H3O

H N N OH

N N

N N

N N OH

-H2O N N

N N

The Diazonium Salt:

N N

N N Cl

Diazotization reaction of different amines Diazonium salts obtained from aliphatic amines are less stable than aromatic diazonium salts, and they undergo decomposition to the corresponding carbonium ion with the liberation of N2 gas. NaNO2

RCH2-NH2 primary aliphatic amine

HCl

RCH2-N N Cl unstable diazonium salt

NaNO2

NH2

N N Cl

HCl

primary aromatic amine

RCH2 + N2 carbonium ion

diazonium salt

Secondary amines both aliphatic and aromatic, form N-nitrosoamines upon reaction with HNO2 (R)

Ar NH R secondary amines

NaNO2 HCl

(R)

Ar N N O R N-nitrosoamine

3

N

R R

NaNO2 HCl

tertiary aromatic amine R 3N tertiary aliphatic amine

O N

N

R R

p-nitrosoamine NaNO2

no reaction

HCl

Reactions of diazonium compounds: 1. Coupling 2. Displacement

X

N N Coupling N N X

X = -OH, -NH2, -OR

CuX

X

+ N2

I

+ N2

HBF4

F

+ N2

CuCN

CN

+ N2

OH

+ N2

H

+ N2

X= Cl or Br Displacement

X

KI

H2O heat H3PO4

4

Diazonium salts A class of organic compounds with the general formula ArN +≡NX-, where Ar is an aryl group and X is ananion; made by the action of nitrous acid on aromatic amines, and used in dyeing and as useful intermediates in organic synthesis. Q/ Aromatic diazonium salts are more stable than the aliphatic ones, why? A/ That is because in aromatic diazonium salts the azo group is stabilized by becoming a part of a conjugated system, an effect which is absent in the aliphatic ones.

Importance of Diazotization Reaction Aryldiazonium salts are very important class of compounds because they can be transformed into a large number of other functional groups. 1. Used in the preparation of organic compounds that can’t be prepared

otherwise [e.g. 3,5-dibromotoluene from toluene]. 2. Used in the preparation of dyes and indicators.

Azo Dyes

OH HNO2 NH2

N NX 10% NaOH

HO N N

azo dye

The coupling reaction between diazonium salts and a phenol (and some other activated arenes) will result in colored compounds called ”azo dyes”. The color of these compounds is due to the high levels of conjugation which extends through the –N=N- group to the aryl system. The basic structure (Ar-N=N-Ar) may be substituted with different functional groups (e.g. –NO2, -SO3Na, -NH2, -CH3, -OH, -OCH3) that affect the color of the dye due to their conjugation with the azo system. 5

Mechanism of the reaction (coupling with phenols)

O H

Na OH

O

O Na

H2O +

O O N N

N N H

+

Cl

2-naphthol Cl

O

-H

HO

+H

N N

N N

red azo dye 1-phenylazo-2-naphthol

Examples on azo dyes: Some common dyes synthesized by diazo coupling reaction:

H3C H3C

N

N

O2N

N

SO3Na

Methyl orange red in acid - yellow in base

N N Alizarin yellow R

OH

COOH

6

NH2

H2N

N NaO3S

N N

N

SO3Na

Congo red

NaO3S

N

O2N

N

OH

Orange 1 4-[(p-benezenesulfonyl)azo]-1-naphthol

HO

N

N

Para red

Try to fill this table with different azo dye combinations Aroma tic amines Coupling agents

7

Procedure: Preparation of the diazonium salt 1. Place 1 g of the aniline, 5 ml of water and 2.5 ml of conc. HCl in a flask and shake. Keep the flask in ice bath. 2. Dissolve 1g NaNO2 in 5 ml of water and put it in ice too. Add this

solution dropwise to the solution of aniline. Keeping the temperature between 0-5 oC.

Coupling reaction 1. Dissolve 1 g 2-naphthol in 10 ml 10% NaOH keep this solution also in ice. 2. Now add the diazonium solution slowly to the cold solution of 2maphthol: a brilliant red dye is produced. 3. Filter on a buchner funnel.

8