Long Way Education

By Eman Abdallah Kamel

Eman is a writer and an engineer.

Here you will learn about azo dyes, their classification, chemistry, physical and chemical properties, and the steps of cotton fabric dyeing with azo dyes.

Azo Dyes

Azo dyes are synthetic aromatic organic compounds. It is estimated that azo dyes account for roughly half of the types of dyes on the market. Due to the diversity of their chemical structures, azo dyes are classified into approximately 25 classes. Azo dyes provide a diverse range of colours with excellent fastness and fixing power. In cotton fabrics, vibrant colours such as crimson, deep red, navy blue, orange, and black are produced using azo dyes. There are many types of azo dyes and various classification systems. Types of azo dyes used in the textile industry include disperse dyes, reactive dyes, direct dyes, metal-based dyes, and acid dyes. They are used for numerous purposes, including colouring natural and synthetic materials such as,

• Inks,
• Cosmetics,
• Food,
• Leather,
• And paint solutions.

Characterises

Azo dyes are characterised by

1. Their ease of manufacture,
2. Wide structural diversity,
3. High molar absorption coefficient,
4. Low cost,
5. Moderate-to-high stability towards light and moisture.

Classification

According to the number of azo links present in a single dye molecule, azo dyes are classified as

• Monoazo,
• Disazo,
• Trisazo,
• Polyazo,
• Azoic.

Chemistry

Azo compounds have the chemical formula R-N=N-R’, where R or R’ can be aryl or alkyl compounds and -N=N- is the azo group.

Most azo dyes are made by reacting diazotised aromatic primary amines with one or more electron-rich nucleophiles, such as hydroxyl or amine groups. Additional techniques for preparing azo dyes include

1. Reducing aromatic nitro derivatives in an alkaline medium.
2. Oxidising primary amines using lead tetraacetate or potassium permanganate.
3. Condensing hydrazines and quinones.
4. Condensing primary amines with nitroso derivatives.

Properties

1. Physical Properties

Azo dyes are solid substances, mostly salts, where the colouring component is the anion, although some azo dyes are cationic. The anionic nature of most dyes arises from the presence of one to three sulfonic acid groups, which ionise completely at the pH of the dyeing medium: RSO3H → RSO3− + H+

Did You Know?

Most proteins are cationic; therefore, the dyeing of wool and leather is achieved through an ion-exchange reaction. Anionic dyes adhere to these materials by electrostatic forces. Quaternary ammonium centres are commonly found in cationic azo dyes.

2. Chemical properties

• Isomerism: It is the process by which one molecule transforms into another molecule with the same atoms but in a different arrangement. In some molecules and under certain conditions, isomerism occurs spontaneously.
• Tautomerism: Azo/hydrazone tautomerism results from the transfer of a proton within the molecule from one element to another. This synergistic tautomerism is of paramount importance for azo dyes, as it contributes to the development of specific properties such as optical stability, colour strength, and others.
• Reactivity: In general, the effectiveness of a dye depends on the structure of its active component. Dichlorotriazines are the most effective, followed by dichloropyrimidines or tetrafluoropyrimidines and dichloroquinoxaline, then monofluoropyrazines. Intermediately effective dyes include phenylsulfonates, a type of monochloropyrazine/phenylsulfone bifunctional dye. Next are oxymethylchloropyrazines, derived from the aminochloropyrazine structure and less effective. Finally, trichloropyrimidines are the least effective.

Gewald Reaction

This reaction has applications in numerous fields, such as peptide analogues, dyes, electronics, conjugated carbohydrates, agrochemicals, pharmaceuticals/biomedical products, sugarcane processing materials, and others. According to Ram W. Sabnis, the condensation of benzothiophene-3(2H)-one-1,1-dioxide with ethyl cyanoacetate, followed by the addition of a diazo group using sulphuric nitrosyl acid and its subsequent linkage with N,N-dialkyl-substituted aryl amines (acid ligation), yields an azo dye.

Dyeing Cotton Fabrics with Azo Dyes

The process of dyeing cotton fabrics with azo dyes involves three main steps:

1. Naphtholation,
2. Diazotisation,
3. Coupling.

1. Naphtholation: Naphthalates are insoluble in water and require treatment with an alkali to become soluble. The fabric is then impregnated with this saline solution.

2. Diazotisation: At a temperature of 0-5°C, a base having an amino group (-NH₂) combines with sodium nitrite (NaNO₂) to generate a diazonium chloride solution of that base in the presence of HCl.

3. Coupling: The saturated material requires treatment in a diazonium solution bath to facilitate conjugation, which produces colour within the fabric. Maintaining the correct pH is crucial.

Sources

• Azo dyes: Synthesis, Classification and Utilisation in the Textile Industry. Egyptian Journal of Chemistry.
• Classifications, properties, recent synthesis, and applications of azo dyes.pdf
• Azo Dye

© Eman Abdallah Kamel, 2026

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