Preparation of Aniline Yellow - Practical Experiment
Aim
To prepare Aniline yellow (also called as para amino azobenzene) from benzene diazonium chloride and aniline.
Apparatus Required
Aniline
Sodium nitrite
Hydrochloric acid
Distilled water
Glacial acetic acid
Bunsen burner
Round bottom flask
Stirrer
Beaker
Funnel
Conical flask
Water bath
Theory
Aniline yellow is an unstable compound that is used as an intermediate for preparing other dyes. Aniline yellow (para amino azobenzene) is prepared by the coupling reaction between benzene diazonium chloride and aniline in an acidic medium. The dye has no industrial significance because it is an acid-sensitive dye.
The Chemical Reaction Involved
Step 1: When aniline is treated with sodium nitrite in the presence of acid.
Ar-NH2 + NaNO2 + HCl → Ar-N2+ -Cl-
Aniline Sodium Hydrochloric Benzene
Nitrite Acid diazonium chloride
Step 2: Ar-N2+ - Cl- + Ar-NH2 → Ar-N=N-NH-Ar
Step 3: Ar-N=N-NH-Ar → Ar-N=N-Ar-NH2
Aniline Yellow
In the first step, aniline acts as a nucleophile and reacts with benzene diazonium chloride to form a C-N bond. The product loses H+ and gives the azo dye.
Procedure
First of all, take a 10ml concentrated hydrochloric acid (HCl) and 20ml water in a beaker and add4.5ml aniline to the mixture. Dissolve them thoroughly to get a clear solution.
Cool the solution in an ice bath around 5°C. Mix 4gsodium nitrite with 20ml water and add this sodium nitrite solution to the above mixture by doing the constant stirring.
After some time, Diazotization will take place and the formation of benzene diazonium will occur.
Now, Prepare 4ml aniline in 4ml HCl and mix the aniline solution with benzene diazonium chloride solution by slow constant stirring.
After some time, a yellow precipitate of para amino azobenzene which is also called aniline yellow is formed. Filter the yellow precipitate with a Buchner funnel and dry the crude sample in between the folds of paper.
The crude sample can be recrystallized by dissolving in carbon tetrachloride to get the pure dye.
Setup
(Image to be added soon)
Observations
Result
Aniline yellow is produced in _____ gm.
Precautions
Wear lab coats, gloves, and goggles.
Try to maintain a pH of reaction between 4-5.
The temperature of the experiment should range between 40°C to 50°C.
The crude sample should be washed repeatedly by water.
Fun Fact
Preparation of Aniline yellow can be done in the home also. In place of the Buchner funnel, an alternate home-designed suction funnel and plastic bottles can be used. can be used. To maintain the temperature approx 5°C , ice cubes can be used to settle down the solution.
FAQs on Preparation of Aniline Yellow
1. What is the core chemical principle behind the preparation of Aniline Yellow?
The preparation of Aniline Yellow is based on an electrophilic substitution reaction known as a coupling reaction. In this process, benzene diazonium chloride, acting as a weak electrophile, attacks the electron-rich benzene ring of aniline (a nucleophile) at its para position. This reaction forms a stable azo compound, p-aminoazobenzene, which is a yellow dye.
2. What is diazotization and why is it a critical first step in this synthesis?
Diazotization is the process of converting a primary aromatic amine, such as aniline, into a diazonium salt. This is achieved by reacting aniline with nitrous acid (generated in-situ from sodium nitrite and a strong acid like HCl) at a very low temperature (0-5°C). This step is critical because it forms the benzene diazonium chloride, the essential electrophilic reactant required for the subsequent coupling reaction to produce Aniline Yellow.
3. What is the chemical formula and IUPAC name for Aniline Yellow?
The chemical formula for Aniline Yellow is C₁₂H₁₁N₃. Its IUPAC name is 4-aminoazobenzene or p-aminoazobenzene. It belongs to the class of azo dyes, characterised by the presence of an azo group (-N=N-) connecting two aromatic rings.
4. Why must the temperature be strictly maintained at 0-5°C during the diazotization step?
The temperature must be kept between 0-5°C because the product of diazotization, benzene diazonium chloride, is highly unstable at warmer temperatures. If the temperature rises, the diazonium salt will readily decompose into phenol, nitrogen gas, and hydrochloric acid, which prevents the formation of the desired azo dye. The ice bath is essential to ensure the stability of this key intermediate.
5. What are the essential reagents and apparatus required for preparing Aniline Yellow in a lab?
The key materials needed for this experiment are:
- Reagents: Aniline, Sodium Nitrite (NaNO₂), concentrated Hydrochloric acid (HCl), and distilled water.
- Apparatus: Beakers, a conical flask, a funnel, a glass rod for stirring, an ice bath to maintain low temperatures, and filtration equipment like a Buchner funnel.
6. Is Aniline Yellow considered an acidic or a basic dye?
Aniline Yellow is a basic dye. This is because of the presence of a free amino group (-NH₂) on the benzene ring, which can accept a proton. However, it is not used commercially because it is acid-sensitive; the dye's colour fades in acidic environments, making it unstable for industrial applications like textiles.
7. What would happen if the coupling reaction was performed in a strongly acidic or basic medium instead of a mildly acidic one (pH 4-5)?
The pH is critical for the coupling reaction. If the medium is too acidic, the aniline's amino group gets protonated to form the anilinium ion (-NH₃⁺). This deactivates the ring and prevents it from participating in the electrophilic substitution. If the medium is basic, the diazonium salt gets converted into a less reactive diazohydroxide, which also inhibits the reaction. Therefore, a mildly acidic pH of 4-5 is optimal.
8. What safety precautions are important when preparing Aniline Yellow?
When performing this experiment, the following safety measures are crucial:
- Always wear a lab coat, safety goggles, and chemical-resistant gloves.
- Handle concentrated hydrochloric acid and aniline with extreme care, as they are corrosive and toxic.
- Ensure the reaction is performed in a well-ventilated area or a fume hood.
- Carefully manage the reaction temperature using an ice bath to prevent the unstable diazonium salt from decomposing.
9. How does the coupling of two colourless compounds, aniline and benzene diazonium chloride, result in a coloured product?
The formation of colour is due to the creation of an azo group (-N=N-) that links the two aromatic rings. This new structure creates a large, extended conjugated system of delocalised π-electrons across the entire molecule. This extended conjugation system is known as a chromophore. It absorbs light energy from the visible region of the spectrum and reflects the complementary colour, which in this case is yellow.
