1.Preparation of Diazonium salt2.Coupling Reaction with Phenols

Student handout

Dotsha J. RaheemCollege of Science/ Department of Chemistry

University of SalahadinMay 2010

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Diazonium salt formation (diazotization reaction)

Mechanism of the Reaction

1. Formation of the nitrosonium ion

2. Nitrosation of the amine

3. Tautomerism

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4. Formation of the diazonium salt

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.

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Reactions of diazonium compounds:

1. Coupling2. Displacement

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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

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.

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Mechanism of the reaction (coupling with phenols)

Examples on azo dyes:

Some common dyes synthesized by diazo coupling reaction:

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Try to fill this table with different azo dye combinations

Aromatic amines

Coupling agents

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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 2-maphthol: a

brilliant red dye is produced.3. Filter on a buchner funnel.

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