Azo dyes

Preparation & properties

1

2

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.

3

Introduction

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. Azodyes have the basic structure, Ar−N=N−Ar’, where Ar and Ar’ are two aromaticgroups.

4

What are they?

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

5

geometrical isomerism of the azo group. UV radiation can cause a trans azo group to become cis

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

6

anthraquinones, the second most used dye class.

The color differences are caused by different 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)

.

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.

7

Mechanism

8

9

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

10

Dye molecules bind to the fiber molecules in cloth in one of the following ways:via electrostatic attraction, by van der Waals forces, through hydrogen bonding or evenby covalent bonds. In general, the greater the number of polar groups that are present inthe fiber molecule, the easier the fabric is to dye.

11

Experiment

Apparatus300mL beaker, balance, measuring glass, ice bath,Chemicals= paranitro aniline 2g,sodium nitrite, HCl 8mL, napthol 0.2g,NaOH

Procedure1.solutions were prepared as follows:

A 2g paranitro aniline, 20ml water, 8ml HClB 2g sodium nitrite, 20ml waterC napthol 0.2g dissolved in 10%NaOH and diluted with 10ml water

All three solutions are cooled in an ice bath for 30 min. solution B was added slowly above it with stirring, then filtered & dried.

2.Para Red was obtained

Observation-the filtrate after drying was deep red in colour.It gave needle shaped crystals on drying.

12

Precautions1.The experiment should be performed in a

well-ventilatedlaboratory or fume hood. Handle all chemicals with greatcare. Avoid direct contact of chemicals with skin.

2.Sodium hydroxide solution and concentrated hydrochloric

acid are highly corrosive. Avoid contact with skin and puton appropriate protective gloves whenever available.Concentrated hydrochloric acid gives the choking gaseous hydrogen chloride, whichis corrosive to the respiratory organs.

13

BIBLIOGRAHY

Laboratory Manual in Chemistry for class 12www.Wikipedia.com

Text book of chemistry class12 ncert.

14

15