Azo dyes

Preparation & properties

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This is to certify that Purushottam Gupta,Of class 12 AOf Delhi Public School, Noida Has prepared the following project onPreparation & properties of azo dyesFor the practicals of class 12 conduced by Central Board Of Secondary EducationFor the session 2009-2010With complete sincerity under my supervision Sign: (Mrs. Sandesh Arora) Senior chemistry

I would like to take this opportunity To thank my chemistry teacher Mrs. Sandesh Arora & Lab asst Mr. Joshi Whose able guidance & encouragement Has enabled me to finish this project.

Introduction

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Dyes play an indispensable role in human history

since ancient time. Dyeing

processes are often considered as an important

characteristic of a particular civilisation or

culture. Dyes are used in almost every

commercial product such as food, clothing,

pigments and paints, etc.

There are many different classes of dyes in

which azo dyes are certainly one of the

most important classes. About half of the dyes

used in industry are azo dyes. Azo

dyes have the basic structure, Ar−N=N−Ar’,

where Ar and Ar’ are two aromatic

groups.

What are they?

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The unit containing the nitrogen-nitrogen double

bond is called an azo group. The nature of the

aromatic substituents on both sides of the azo

group controls the colours of the azo

compounds as well as the water-solubility of

the dyes and how well they bind to a particular

fabric.

Aromatic azo compounds are used as acid-base

indicators, biological stains, and commercial

colorants for clothing, plastics, cosmetics, and

food beverages. Many azo-dyes, such as

methyl red, methyl orange, and Congo red, can

be used as acid-base indicators due to their

ability to function as weak acids or bases.

Color changes are caused by changes in extent of

delocalization of electrons: more

delocalization shifts the absorption max to

longer wavelengths and makes the light

absorbed redder, while less delocalization

shifts the absorption max to shorter

wavelengths. Color changes can also be due to

geometrical isomerism of the azo group. UV

radiation can cause a trans azo group to

become cis

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Advantages

Azo dyes form 60-70% of all synthetic dyes used

as commercial colorants. Azo dyes have several

advantages over other commercial dyes

including their

1.Wide color range,

2.Good color fastness and

3.Ability to absorb light.

4.They can also be synthesized cheaply

because the starting materials are

5.Readily available,

6.Inexpensive compounds; most of the

chemistry is completed at or below room

temperature;

7.The environmental impact is low due to the

use of water as a solvent in all of the

reactions.

Cost advantages tend to compensate for the

lower resistance to bleaching and lower

brilliance of azo dyes compared to

anthraquinones, the second most used dye class.

The color differences are caused by different

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substituents on the aromatic rings which lead to

differences in the extent of conjugation of the π

system in the azo dye. In general, the less

extensive the conjugated π system of a molecule,

the shorter the wavelength of visible light it will

absorb.

Colorless (shortest π system) → yellow →

orange → red → green →blue (longest π

system)

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The synthesis of an azo dye requires two organic compounds - a diazonium salt and a coupling component. The general synthesis of azo dyes is shown above.

Mechanism

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Combinatorial chemistry is heavily used in drug

discovery research. Combinatorial methods

allow for the simultaneous synthesis of many

potentially valuable compounds. These are

screened for desirable biological activity. In a

“parallel synthesis”, a variety of reactants

(several nucleophiles which are assigned to the

rows) is allowed to react with a variety of

substrates (several electrophiles which are

assigned to the columns). In this way, an array of

chemicals can be synthesized.

Attachment

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