Azo Compound: Types, Preparation, Uses, and Harmful effects

Azo groups are chemical compounds with the general formula R-N=N-R', where R and R' can be either aryl (aromatic) or alkyl (aliphatic) functional groups. The N=N group is called an azo group, although the parent compound, HN=NH, is called diamide. The stable azo compounds contain two aryl groups. Azo compounds with alkyl functional groups are particularly unstable and should be handled with care, to prevent them from shattering. In this article, we will learn more about the azo group, its uses in dye preparation, and its harmful effects on humans.

Key Terms: Azo group, aryl, alkyl, dye, isomer

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What is an Azo Group?

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Azo compounds are compounds bearing the functional group diazenyl R−N=N−R′, in which R and R′ can be either alkyl or aryl. Any chemical or organic compound in which the azo group (—N=N—) is part of the molecular structure. The atomic groups get attached to the nitrogen atoms may be of any other organic compound, but for trading purposes azo compounds are important, those who make up more than half the mercantile dyes, have the benzene group.

Most fragrant azo compounds are prepared by the reaction of a diazonium salt with an organic substance that contains easily replaced hydrogen atoms. The combination of azobenzene from nitrobenzene by treatment with certain oxygen-removing reagents is an example of another method useful for symmetrical azo compounds.

In Azo compounds get the attached groups are aliphatic organic groups that are usually made by dehydrogenation of the corresponding hydrazo compounds (containing the group —HN—NH—) made from hydrazine, N₂H₄. 

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Types of Azo Compound

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Aryl Azo Compound

Aryl azo compounds are normally stable, crystalline species. Azobenzene is the prototypical aromatic azo compound. It exists mainly as the trans isomer, but upon photolysis, converts to the isomer. Aromatic azo compounds can be synthesized by azo coupling, which entails an electrophilic substitution reaction where an aryl diazonium cation is attacked by another aryl ring, especially those substituted with electron-donating groups.

\(ArN^{+}_{2} + Ar'H \rightarrow ArN = NAr' + H^{+}\)

Hence diazonium salts are usually unstable at room temperature, the azo coupling reactions are typically conducted near 0 °C. The oxidation of hydrazines (R−NH−NH−R′) also gives azo compounds. Azo dyes are also prepared by the condensation of nitroaromatics with anilines followed by reduction of the resulting azoxy intermediate:

Chemical reaction:

\(ArNO_{2} + Ar'NH_{2} \rightarrow ArN(O) = NAr' + H_{2}O\)

\(ArN(O) = NAr' + C_{6}H_{12}O_{6} \rightarrow ArN= NAr' + C_{6}H_{12}O_{6} + H_{2}O\)

Alkyl Azo Compound

Aliphatic azo compounds (R and/or R' = aliphatic) are less encountered than the aryl azo compounds. 

For example: diethyl diazene, EtN=NEt. At elevated temperatures or upon irradiation, the carbon-nitrogen (C-N) bonds. In certain alkyl azo compounds cleaving with the loss of nitrogen gas to generate radicals is azobisisobutyronitrile (AIBN), which is mainly used as an initiator in free-radical polymerizations and other radical-induced reactions. It got this initiation by decomposition, eliminating a molecule of nitrogen gas to form two 2-cyanoprop-2-yl radicals:

2-cyanoprop-2-yl radicals

AIBN is produced by converting acetone cyanohydrin to the hydrazine derivative followed by oxidation.

\(2 (CH_{3})_{2}C(CN)OH + N_{2}H_{4} \rightarrow [(CH_{3})_{2}C(CN)]_{2}N_{2}H_{2} +H_{2}O\)

\([(CH_{3})_{2}C(CN)]_{2}N_{2}H_{2} +Cl_{2} \rightarrow [(CH_{3})_{2}C(CN)]_{2}N_{2} +HCl\)

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Preparation of Azo Dye

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A compound that contains two aromatic fragments connected by an N=N double bond. You may have experienced these in chemistry experiments as they are typical pH indicators. Methyl yellow, methyl orange, methyl red, congo red, and alizarin yellow are some examples. Azo dyes are prepared in a two-step reaction, the first the synthesis of an aromatic diazonium ion from an aniline derivative. The second step is the coupling of the diazonium salt with an aromatic compound (shown below). The colors of azo dyes contain different shades of yellow, red, orange, brown, and blue. The color of the dye is dependent on the azo bond and the associated chromophores and auxochromes. Azo dyes are mainly used in textile, print, and other industries. They make up to 70% of the world’s commercial dyes.

Preparation of Azo Dye

Azo dyes are signified by chemical groups having the potential to form covalent bonds with textile substrates. The common method of production is the diazotization of a primary amine aromatic and the coupling with a diazonium salt.

Preparation of Azo Dye

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Uses of Azo Dye

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Azo dyes have many uses and that is the reason that they consist on an average- 70% of the world’s commercial dyes. It is assumed that approximately 2000 azo dyes are presently present in the market. They are usually used in textile industries, the printing market, the biomedical industry, cosmetics, leather, paint, and other industries.

There are two types of azo dyes:

Azo direct dyes: it has been found to be carcinogenic, mutagenic, and have negative reproductive effects.

Azo reactive dyes: it has been linked to increasingly active allergy risk.

Both of these types of diets are non-biodegradable, harmful to the environment as they accumulate in the oceans and affect marine life. 

These dyes are especially effective for the health of children and infants and their exposure causes various problems for them.

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Harmful effects of Azo Compounds

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The harmful effects of azo compounds are not very well known. Researchers showed that these compounds can be carcinogenic, reproductive toxins, and have cellular and neurological toxicants, irritants, allergens, environmental hazards, and developmental toxicants.

The most majorly used dyes belong to the class of azo compounds, some of which are known for their toxic and genotoxic properties. They are used in great quantities in textile industries and are of environmental concern because of their discharge into water. 

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Things to Remember

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• Azo groups are chemical compounds with the general formula R-N=N-R, where R and R' can be either aryl (aromatic) or alkyl (aliphatic) functional groups.
• Azo dyes are also used in tattoos, inks for printers, insecticides, pesticides, paints, varnishes, lacquers, and personal care products.
• Reduction of nitroso compounds by lithium aluminum hydride.
• Another method of preparing azo dyes is by first reducing nitroaromatic derivatives in an alkaline medium.
• The N=N group is called an azo group
• The stable azo compounds contain two aryl groups.
• Types of azo compound
  1. Aryl azo compound
  2. Alkyl azo compound

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