Key Reaction Mechanism and Uses in Chemistry Practicals
2- Naphthol aniline dye is an azo-compound. It is a red-colored dye. It is mainly used for dyeing textiles. Azo compounds have an extended conjugation scheme and they are also colored and used as colors. These compounds are prepared by a reaction known as the coupling reaction. 2-Aniline naphthol dye is made from aniline. In this article, we will study the preparation of 2 naphthol aniline dye in detail.
Preparation of 2 Naphthol Aniline Dye - Practical Experiment
Aim
To prepare 2-Naphthol Aniline Dye from aniline, sodium nitrite, hydrochloric acid and alkaline solution of β-naphthol also called 2-naphthol.
Theory
2- Naphthol aniline dye can be prepared by coupling reaction. In the presence of hydrochloric acid, aniline reacts with sodium nitrite and forms benzene diazonium chloride. A bright orange 2-naphthol aniline dye is formed by additional benzene diazonium chloride reacting with 2-naphthol.
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Azo compounds are prepared under alkaline conditions by the reaction of diazonium salts with phenol. To give a diazonium salt, primary aromatic amines react with nitrous acid at 0oC. In exchange, nitrous acid is formed by a sodium nitrite reaction with hydrochloric acid. Nitrous anhydride or dinitrogen trioxide is an active reagent. Nitrous anhydride reacts with aniline to provide an unstable nitroamine derivative and isomerizes to form a diacetic acid, which is converted into a diazonium salt in turn. Finally, in the presence of sodium hydroxide, this diazonium salt reacts with 2-naphthol to give 2-naphthol aniline, which is an aniline dye.
Materials Required
Aniline
Sodium nitrite
Hydrochloric acid
2-Naphthol
Sodium hydroxide solution
Beaker
Test tube
Buchner funnel
Pipette
Capillary tube
Thermometer
Distilled water
Hot air oven
Ice bath
Procedure
5ml of aniline was dissolved in a mixture of hydrochloric acid and water.
In an ice bath, cool the solution at 0-5 oC.
With constant shaking and monitoring the temperature below 5oC, add a solution of 4gm sodium nitrite in 15ml of water dropwise.
Take another flask to dissolve 8gm of 2-naphthol in a solution of 5 gm sodium hydroxide solution in 50ml of water.
In the ice bath, cool the solution to 0-5oC.
Now mix the two cold solutions dropwise with regular stirring.
Continue stirring for at least half an hour without raising the temperature above 10oC.
It is isolated by an orange azo dye called 2-naphthol aniline.
Filter and wash the crude sample with cold water.
From ethyl alcohol or glacial acetic acid, dry and recrystallize it.
Observations
Result
The 2- Naphthol aniline dye yield is ______ gm.
Precautions
It is important to cool the solution to 5oC. Do not let the temperature increase.
Do not touch the dye as it will stick to your palms. Do not touch
Do not touch the concentrated acids as it can cause irritation.
In order to eliminate soluble impurities, wash the crude sample frequently with cold water.
Keep the pH between 4-5.
Did You Know?
Coupling reaction refers to the class of organic reactions involving the interaction of two chemical species (usually with the help of a metal catalyst). The reaction of an organic halide with an organometallic compound having the general formula R-M that facilitates the formation of a new carbon-carbon bond is an important type of coupling reaction. If the organic halide has the general formula R'-X in this reaction, the compound produced as a result would have the R-R 'formula.
The Coupling Reaction is of Two Types
Homo-coupling reactions
Hetero-coupling reactions (also known as cross-coupling reactions)
Importance of Practical Experiments in Chemistry
Chemistry is an experimental science that needs to be understood more in practical terms than theoretical terms. Practical experiments have a significant role to play as the students learn the different chemical compositions through these. All schools and teachers believe in the benefits of practical experience as seeing something makes you more convinced in terms of understanding its concept. It assists in general evaluation and learning which helps students secure higher marks in their exams. Chemistry is a didactic subject that can only be understood once it is witnessed. Experiments form the core of a subject like Chemistry so that students can pick up whatever they learn in theory faster than usual.
FAQs on Step-by-Step Preparation of 2 Naphthol Aniline Dye
1. What is 2-naphthol aniline dye and what are its primary uses?
2-Naphthol aniline dye is a synthetic organic compound known as an azo dye. It is characterised by a vibrant scarlet or orange-red colour. Its primary application is in the textile industry for dyeing fabrics like cotton, silk, and wool due to its bright hue and ability to bind to fibres. It is also known as 1-phenylazo-2-naphthol.
2. What are the two main stages in the preparation of 2-naphthol aniline dye?
The preparation involves two critical chemical stages as per the CBSE curriculum for the 2025-26 session:
- Diazotization: In this first step, a primary aromatic amine (aniline) is treated with nitrous acid (generated in-situ from NaNO₂ and HCl) at a low temperature (0-5°C) to form a benzene diazonium chloride salt.
- Coupling Reaction: The freshly prepared diazonium salt is then reacted with an activated aromatic compound, in this case, 2-naphthol (β-naphthol), in an alkaline medium. This forms the final 2-naphthol aniline dye, which precipitates as a coloured solid.
3. What is the final colour and physical state of the 2-naphthol aniline dye produced in the experiment?
The 2-naphthol aniline dye synthesised in the lab appears as a bright orange-red or scarlet solid. It typically precipitates from the reaction mixture and can be collected as crystals after filtration and drying.
4. Why must the diazotization reaction be carried out at a very low temperature (0-5°C)?
Maintaining a low temperature is crucial because the benzene diazonium salt formed is highly unstable at higher temperatures. If the temperature rises, the diazonium salt will readily decompose and react with water to form phenol (C₆H₅OH). This side reaction would significantly reduce the yield of the desired azo dye and introduce impurities.
5. What is the role of the azo group (-N=N-) in the structure of 2-naphthol aniline dye?
The azo group, -N=N-, acts as a chromophore in the dye molecule. It connects the two aromatic rings (from aniline and 2-naphthol) and creates an extensive system of delocalised pi-electrons, known as a conjugated system. This specific structure allows the molecule to absorb light in the visible spectrum, which is what gives the compound its intense colour.
6. How does the coupling reaction work to form the final dye?
The coupling reaction is an electrophilic aromatic substitution. The benzene diazonium ion (C₆H₅N₂⁺), formed during diazotization, acts as a weak electrophile. The 2-naphthol molecule, especially in an alkaline medium, becomes highly activated and electron-rich. The electrophilic diazonium ion then attacks the electron-rich position on the 2-naphthol ring, forming a stable azo linkage (-N=N-) and creating the final coloured dye molecule.
7. Why is an alkaline solution of 2-naphthol used for the coupling reaction?
An alkaline medium, typically achieved using sodium hydroxide (NaOH), is essential for the coupling reaction. The hydroxide ions (OH⁻) deprotonate the hydroxyl group (-OH) of 2-naphthol, forming the more reactive phenoxide ion. This ion is significantly more electron-rich and thus a much better nucleophile, making it more susceptible to attack by the weak electrophile (the diazonium salt). This greatly increases the rate and success of the coupling reaction.
8. What are the key safety precautions when preparing 2-naphthol aniline dye?
When performing this experiment, several safety measures are critical:
- Always wear safety goggles and a lab coat.
- Handle concentrated hydrochloric acid (HCl) with care, preferably in a fume hood, as its vapours are corrosive.
- Avoid direct skin contact with aniline and 2-naphthol as they can be toxic and cause irritation.
- Do not touch the final dye with bare hands as it is a potent dye that can stain skin for days.
- Maintain the reaction temperature strictly below 5°C during diazotization to prevent uncontrolled decomposition.
