Azo dyes are the most diverse group of synthetic dyes. Azo dyes are prepared in two steps. The diazonium salt is produced in the first step, which then reacts with highly reactive aromatic compounds such as phenol is the second step. Different pathways are followed to obtain the desired color properties of dyes.
“The highly reactive phenols and tertiary aromatic amines react with weak electrophiles such as diazonium ions to form highly colored derivatives known as azo compounds”.
History of dyes
The history of dyes is divided into two periods.
- Pre-aniline period
- Post-aniline period
In past, a limited range of colors was present that was produced using animals and plants.
The main dyes were extracted from Indian madder (Rubia cordifolia) producing red dye. The leaves of the indigo plant (Indigofera tinctoria) were used to produce the blue dye which is still used to dye jeans. The most famous dye ‘Tyrian purple’ was produced from the mucus of several species of murex snail. It is a reddish-purple natural dye.
In 1856, aniline was produced from coal tar and the period of synthetic dye (post aniline) started. William Henry Perkin synthesized Mauveine (aniline purple) by using aniline.
In 1870, Adolph von Baeyer synthesized indigo.
Synthesis of azodyes
Azo compounds contain azo (-N=N-) linkages, this linkage brings the two aromatic rings into conjugation which is the reason why azo compounds are highly colored. Many azo dyes are prepared by coupling reactions of naphthols and naphthylamines. Azo dyes with particular colors allow absorption of light in the visible region.
Azo dyes are prepared in two steps.
- The diazonium salt is produced
- It reacts with highly reactive aromatic compounds such as phenol.
The reactions in which diazonium ion is coupled with a ring of phenol or aromatic amines are known as diazo coupling reactions.
Synthesis of phenyl Azo-β-naphthol (Red dye)
Phenyl Azo-β-naphthol is an orange-red dye, also known as the red dye.
The synthesis of phenyl Azo-β-naphthol consists of two steps:
In the first step, aniline is reacted with hydrochloric acid (HCl) and sodium nitrite (NaNO2) in the presence of water to produce an intermediate known as diazonium salt.
In the second step, the β-naphthol solution is prepared with the 10％ solution of NaOH. After this, the solution of diazonium salt and β-naphthol is mixed by keeping the temperature at less than 5 ºC. The solution is then left for 30 minutes. After some time, the red color appears, and the crystal of red dye gets formed.
Crystals of phenyl azo-β-naphthol are filtered and purified by recrystallization with a mixture of hot water and rectified spirit.
The chemical equation for this reaction is as.
Mechanism of synthesis of phenyl Azo-β-naphthol
Formation of diazonium salt, starting with an acid and sodium nitrite (NaNO2) in the presence of water.
Formation of phenyl azo-β-naphthol, by reacting beta-naphthol, diazonium salt prepared in step 1, and a base (NaOH).
Synthesis of Methyl Orange dye
Methyl orange, a pH indicator, is a dye mostly used in titrations. It shows red color in acidic and yellow in basic mediums.
The synthesis of methyl orange consists of two-step.
- First of all, sulphanilic acid (C6H7NO3S) is mixed with sodium carbonate solution (Na2CO3).
- The solution is taken into a conical flask and some water is added to it.
- The solution is now cooled, sodium nitrite (NaNO2) is added and stirred.
- When dissolution is complete, the solution is poured into a beaker that contains ice and concentrated hydrochloric acid (HCl).
- After a few minutes, the precipitates of diazonium salt will be seen.
- N,N-dimethylaniline is reacted with glacial acetic acid, forming N,N-dimethylaniline acetate salt.
- The solution of N, N-dimethylaniline acetate is added to the diazonium salt, the precipitation occurs and the color of the solution gets changed.
- Sodium hydroxide (NaOH) solution is added to prepare orange sodium salt.
- The solution is stirred and heated. When all contents are mixed, a clear solution gets formed.
- The temperature is now decreased by placing the conical flask into the beaker of ice and water.
- The precipitates of methyl orange are formed.
- Filteration and drying of the precipitates is the final step.
Synthesis of Phenyl orange dye
The synthesis of phenyl orange dye consists of two steps.
In the first step, phenol is treated with sodium hydroxide (NaOH) to form a solution of sodium phenoxide.
In the second step, the solution is cooled in ice and a solution of diazonium salt is added. After the reaction between the diazonium salt and the phenoxide ion, orange precipitates are formed.
The whole reaction for the synthesis of phenyl orange dye is;
Mechanism of synthesis of Phenyl orange
Synthesis of azo dyes by Gewald reaction
Gewald’s reaction is the condensation of a ketone or an aldehyde (except formaldehyde) with an alpha-cyano ester to give polysubstituted 2-aminothiophene. This 2-aminothiophene is a key intermediate for the diazotization reaction or the synthesis of azodyes.
According to Ram W. Sabnis, the condensation of benzo-thiophene with ethyl cyanoacetate is diazotized by using nitrosyl sulphuric acid and coupled with substituted aryl amines synthesizes azo dyes.
According to the same author, azo disperse dyes can be synthesized from 2-phenyl-5,4H-oxazolone. They diazotized with nitrosyl sulphuric acid, coupled with substituted aryl amines, and formed 5-azo-thieno ⦍2, 3-d⦎ oxazole dye.
Factors affecting the rate of diazotization reaction (Dye formation)
- Diazotization is carried out between the temperature of 0 to 5℃. If the temperature is increased, the diazonium salt decomposes.
- Acidic concentration
- The pH of sodium nitrite (NaNO2)
The use of azo dyes as Indicators
In the laboratory, some azo compounds are used as indicators for acid-base titrations.
1. Methyl orange indicator
Methyl orange is a pH indicator that is mostly used in titrations of acid and base neutralization reactions. At different pH, it shows different colors.
- At [pH < 3.1] → Red color
- At [pH > 4.4] → Yellow color
- At [pH between 3.1 and 4.4] → Orange color
The pKa value of methyl orange is 3.47
2. Methyl red indicator
Methyl red, an azo dye is used as a pH indicator for acid-base titrations especially. At different pH, it shows different colors.
- At [pH < 4.4] → Red color
- At [pH > 6.2] → Yellow color
- At [pH between 4.4 and 6.2] → Orange color
The pKa value of methyl red is 5.1
3. Congo red indicator
Congo red contains two azo groups, it is used as a pH indicator for titrations. It changes color with a change in pH.
- At [pH < 3.0] → Blue color
- At [pH > 5.0] → Red color
4. Complexometric indicators
A complexometric indicator is an azo dye that undergoes a change in color in the presence of metal ions. They form a weak complex with metal ions.
Some examples of the complexometric indicators are shown below:
Ca and Mg
Al, Cd, Zn, Ca and, Mg
Further uses (Applications) of azo dyes
Azo dyes are the most diverse group of synthetic dyes having the following applications.
- Azo dyes are widely used in the pharmaceutical industry for coloring pharmaceutical agents.
- They are used in hypnotic medicine (for the treatment of insomnia).
- They are used in thermal transfer printers.
- Azo groups containing cyclized-cyanine derivatives (ACC1 and ACC2) are used in two-photon photodynamic therapy (used for the treatment of cancer).
- They are used as the complexometric indicator, undergo a color change in the presence of metal ions ( like calcium and magnesium).
- They are widely being used in the textile industry for dyeing purposes.
- They are used in cosmetics, paper, plastics, and food, etc.
Classification of azodyes
Monoazodyes have the general formula
- If (Z) and (Y) are benzene or heterocyclic derivatives.
Chrysoidine belongs to the monoazo family that dyes cotton with its orange color.
- If (Z) benzene and (Y) naphthalene.
The red dye with mordant and blue dye belongs to this group of dyes.
- If (Z) and (Y) are naphthalene.
These dyes are used for dyeing polyamide substrate in black (Mordant black 17) and give a chromium complex. It is also called calcon.
Monosulphonated aromatic azo compounds are widely used in textile and dyeing industries.
Diazo dyes contain two azo groups.
Blue direct dye is a diazo dye.
Polyazo dyes are complex dyes. They contain three or more azo groups in the molecule. They belong to the group of direct dyes. They dye leather with red, brown, and black colors.
How are azo dyes synthesized?
Azo dyes are prepared in two steps. The diazonium salt is produced in the first and, it reacts with highly reactive aromatic compounds such as phenol in the second step. Different pathways are followed to obtain the desired color properties of the azodyes.
What are examples of azo dyes?
Some examples of azo dyes are the following:
- Methyl orange
- Methyl red
- Congo red
- Phenyl orange dye
- Mordant dye
- Mordant Black 17 (Calcon)
- Blue direct dye
- Direct red dye
- 5-azo-thieno ⦍2, 3-d⦎ oxazole dye
- Phenyl Azo-β-naphthol (Red dye)
Why azo dyes are banned?
The European Union and the state of California banned the use of azo dyes because of the use of carcinogenic aromatic amines in the synthesis of azo dyes. Many azo dyes are carcinogenic due to their cleaved product such as benzidine.
How does pH affect the color of azo dyes?
Azo dyes show different colors as pH changes. It is due to the change in the conjugation of π-bonds. Therefore, they are used as indicators.
For example, methyl orange shows different colors at different pH:
- pH under 4.4 → Red color
- pH over 6.2 → Yellow color
- pH between 4.4 and 6.2 → Orange color
Why do azo dyes have colors?
Azo dyes contain a conjugated system. They contain a high area of delocalization which lowers the gap between electron energy levels. So, a less frequency of light is needed to excite the electrons which means that light absorbed by the compound is in the visible part of the spectrum.
What are azo dyes in food?
Azo dyes are widely used for food colors, derived from petroleum. For example, tartrazine is a lemon yellow azo dye that is used for food coloring.
Which reaction prepares azo dyes?
Azo dyes are prepared by diazo coupling reaction. It is a reaction in which diazonium ions are coupled with rings of phenol or aromatic amines. It is an electrophilic aromatic substitution reaction in which diazonium ion acts as an electrophile while aromatic compounds act as nucleophiles.
What happens when benzene diazonium chloride reacts with para nitrophenol?
The reaction between benzene diazonium chloride and p-nitrophenol is a coupling reaction in a basic medium (pH 9 to 10). It forms an orange color azo dye (ortho-azocompound). Normally coupling occurs at the para position but in this reaction, the para position is blocked by the nitro group so that coupling occurs at the ortho position.
Why are aliphatic diazonium salts unstable?
Aliphatic diazonium salts are unstable because of the absence of resonance. The positive charge on the nitrogen atom is highly unstable and hence they remove nitrogen gas to form alcohol.
What is the preparation process of methyl orange?
Methyl orange is prepared from sulphanilic acid and N, N dimethylaniline. First, we prepare a salt of sulphanilic acid with sodium carbonate and then add sodium nitrite. Add the solution into a beaker containing acid and ice. After a few minutes, precipitates of the diazonium salt appear. In the second step, N,N dimethylaniline is reacted with glacial acetic acid and the mixture is added into the diazonium salt. Now methyl orange is prepared when a base (NaOH) is added.
What is a diazonium salt?
Diazonium salt is an organic compound that contains a common functional group (R-(X–N+)≡N). R can be any alkyl or aryl group and X can be organic or inorganic anion such as halogens. When aniline reacts with NaNO2 in the presence of HCl diazonium salts are prepared.
What is the importance of diazonium salt?
Diazonium salts are important organic reagents. They are used to prepare aryl derivatives. They are also used in the synthesis of azo dyes that have larger applications. They are also used in copying papers because they break down upon exposure to visible (violet region) and ultraviolet radiations.