How Are Azo Compounds Prepared and Used in Chemistry?
Azo compound refers to those compounds that have within them the azo group i.e the molecular structure —N=N—. They have the functional group diazenyl in them. The term azo is derived from the French word azote meaning nitrogen, which itself is derived from a Greek word.
Azo compounds are derivatives of diazine, in which both hydrogens are substituted with hydrocarbyl groups. Azo compounds are used to make a number of azo dyes and pigments which are used in a variety of industries.
These commercial dyes have the benzene group or its derivatives attached instead of the usual nitrogen. The attachment of the benzene groups gives the azo compound an aroma and is referred to as the aromatic azo compound.
Some samples of popular azo dyes are- methyl orange azo dye- widely used in textiles, dimethylamino azobenzene- which is the name of the red azo dye, benzene azo beta naphthol, etc.
Preparation of Azo Dye
Azo dyes are prepared using a simple process of diazotization and coupling. Their importance has been growing massively. Azo dyes today make up for more than half of the commercial dyes and dominate the printing press market.
Generally, azo dyes are prepared by diazotization of a primary aromatic amine, then followed by coupling with one or more than one electron-rich nucleophile. Examples of electron-rich nucleophiles include- amino and hydroxy.
Another method of preparing azo dyes is by first reducing nitroaromatic derivatives in an alkaline medium.
It can also be prepared by the oxidation of primary amines by potassium permanganate.
Reduction of nitroso compounds by lithium aluminium hydride.
Condensation of primary amines with nitroso derivatives.
Conclusively, azo dyes have a chemical structure in which there is a dominant auxochrome group, followed by the chromophoric group and then the solubilizing groups.
The color of the dye is dependent on the azo bond and the associated chromophores and auxochromes. Azo dyes are widely used in textile, print, and other industries. They make up for 70% of the world’s commercial dyes.
Azo dyes are characterized by chemical groups having the ability to form covalent bonds with textile substrates. The most common method of production is the diazotization of a primary amine aromatic and the coupling with a diazonium salt.
Use of Azo Dyes
Azo dyes have a variety of uses and this is the reason that they comprise on an average- 70% of the world’s commercial dyes. It is believed that around 2000 azo dyes are currently in the market. They are widely used in textile industries, printing market, biomedical industry, cosmetic, leather, paint, and other industries.
They Have the Characteristic Features of Being:
Coloring functions
Antibacterial
Antiviral
Antifungal
Cytotoxic
Biomedical
Within the biomedical field, the name of red azo dye is used as drug carriers, either by acting as a cargo that carries therapeutic agents or with the use of a prodrug approach.
They are also used in cellular staining to better view cellular compounds and metabolic processes. Researchers are also working on finding more possible uses of azo dyes in other biomedical fields, especially cancer diagnosis and chemotherapy, where its role is still in the early stages.
Cosmetic
Azo dyes are present in cosmetic and skincare products such as hair dyes, face masks, make-up, conditioners, skincare lotions, nail paints, lipsticks, bath products such as scrubs, soaps, etc. and also in personal hygiene products such as sanitary napkins.
Manufacturing Industry
Azo dyes are a popular choice in the manufacturing industry because of the easy availability, cheap price point as well as easy usage. Azo dyes are widely used in the textile industry because they easily dye most fabrics like- silk, wool, viscose, and even synthetic ones.
Azo dyes are also used in tattoos, inks for printers, insecticides, pesticides, paints, varnishes, lacquers, and personal care products.
Harmful Effects of Azo Compounds
Unfortunately, the harmful effects of azo compounds are not as well known as their usage is. Research studies have shown that these compounds can be carcinogenic, reproductive toxins, have cellular and neurological toxicants, irritants, allergens, environmental hazards, and developmental toxicants.
There are two types of azo dyes namely azo direct dyes and azo reactive dyes. The former has been found to be carcinogenic, mutagenic, and to have negative reproductive effects. The latter has been linked to increasingly active allergy risk.
Both of these types of diet are non-biodegradable, harmful to the environment as they accumulate in the oceans and affect the marine food chain.
The Swedish chemicals agency said that these dyes can cause allergy on the skin, irritation to the eyes, and can also be toxic if swallowed or inhaled for a long period of time.
Another aspect of these dyes is that they can be absorbed by the skin and accumulate in the body, leading to a variety of problems.
These dyes are especially detrimental to the health of children and infants and their exposure causes various problems for them. Some samples of harmful azo dyes are:
Congo Red- it has been classified as a substance of very high concern. It can cause cancer, damage to the unborn child, and even be toxic for reproduction.
Aniline Yellow or Para Amino Azo Benzene– can cause cancer, be harmful to aquatic life, cause allergy reactions, and have long-term effects if swallowed.
Pigment Red 4- It is a color that is restricted for use in Canada due to its potential detrimental health concerns.
C.I Acid Red 114- It is again on the red list in Canada and can be carcinogenic.
FAQs on Azo Compounds: Meaning, Synthesis, and Applications
1. What is an azo compound?
An azo compound is a type of organic chemical that contains the functional group R-N=N-R', which is known as the azo group. In this general structure, the 'R' and 'R'' can be either aryl (aromatic) or alkyl groups. These compounds are most famous for their ability to produce bright and intense colours.
2. What is the general structure and formula for azo compounds?
The general formula for an azo compound is R-N=N-R'. The core of this structure is the diazenyl (-N=N-) bridge that links two organic groups. Aromatic azo compounds, like azobenzene (C₆H₅-N=N-C₆H₅), where both R and R' are aryl groups, are typically more stable and widely studied than their alkyl counterparts.
3. Can you give some common examples of azo compounds?
Certainly. Some of the most well-known examples of azo compounds that students encounter are:
- Methyl Orange: A common acid-base indicator used in laboratory titrations.
- Azobenzene: The simplest aromatic azo compound, often used as a reference.
- Para Red: One of the first azo dyes ever produced.
- Congo Red: Used as a pH indicator and a biological stain in microscopy.
4. Where are azo compounds most commonly used in everyday life?
The most widespread application of azo compounds is their use as dyes and pigments. Because of their vibrant colours and cost-effective production, they are used extensively in:
- The textile industry for dyeing fabrics like cotton, silk, and wool.
- The manufacturing of paints, lacquers, and printing inks.
- Colouring leather, paper, and some plastics.
- Analytical chemistry, where compounds like Methyl Orange serve as pH indicators.
5. Why are most azo compounds so brightly coloured?
The intense colour of azo compounds is due to their molecular structure. The azo group (-N=N-) connects two aromatic rings, creating an extended conjugated system of alternating single and double bonds. This large system is very effective at absorbing light in the visible part of the spectrum. The colour our eyes perceive is the complementary colour to the light that is absorbed by the molecule.
6. How do azo compounds differ from diazonium salts?
While they are chemically related, they serve different roles. A diazonium salt (with the formula Ar-N₂⁺X⁻) is a highly reactive and often unstable intermediate compound. In contrast, an azo compound (Ar-N=N-Ar') is a stable final product. The key difference is that a diazonium salt reacts with another aromatic compound in a 'coupling reaction' to form the stable, coloured azo compound.
7. What is the fundamental chemical reaction used to prepare an azo dye?
The preparation of an azo dye is a classic organic synthesis known as azo coupling. This process typically involves two main steps:
1. Diazotization: A primary aromatic amine, such as aniline, is treated with nitrous acid (HNO₂) at very low temperatures (0–5 °C) to create a diazonium salt.
2. Coupling Reaction: The freshly made diazonium salt is then immediately reacted with an electron-rich compound, like phenol or another amine, to form the stable and brightly coloured azo compound (the dye).
8. Are all azo compounds safe for use?
Not all of them. While many azo dyes are stable and safe for use in textiles and paints, certain types can break down under specific conditions to release compounds called aromatic amines. Some of these amines are known to be harmful. For this reason, governments and health organisations worldwide strictly regulate which azo dyes can be used in products that have direct contact with consumers, such as clothing, toys, and food products.
