To prepare Aniline yellow (para amino azobenzene) from benzene diazonium chloride and aniline.
Aniline yellow, also called para amino azobenzene,and is prepared by the coupling reaction between benzenediazonium chloride and aniline in an acidic medium. It is the simplest and the first azo dye obtained by a coupling reaction. This dye has no industrial significance because it is an acid sensitive dye. The first step involved in the preparation of benzene diazonium chloride from aniline. When aniline is treated with sodium nitrite in the presence of acid.
The chemical reaction involved is given below.
Step-1: Ar-NH2 + NaNO2 + HCl → Ar-N2+-Cl–(benzene 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)
The structure of dye in acidic medium is given below.
Aniline yellow is an unstable compound used as an intermediate in the preparation of other dyes. It is prepared by a coupling reaction between benzene diazonium chloride and phenylamine which is also called aniline. The first step involved in the preparation of benzene diazonium chloride from aniline is the treatment of aniline with sodium nitrite in the presence of acid.
- Hydrochloric acid
- Sodium nitrite
- Glacial acetic acid
- Distilled water
- Round bottom flask
- Bunsen burner
- Water bath
- Conical flask
- Take 10 ml of concentrated hydrochloric acid and 20 ml of water in a beaker and add 4.5 ml of aniline to the mixture.
- Dissolve the mixture thoroughly to get a clear solution.
- Cool the solution by placing it in an ice bath at 5 oC.
- Mix 4 g of sodium nitrite to 20 ml of water. Add this sodium nitrite solution to the above mixture with constant stirring.
- Diazotization takes place and benzene diazonium is formed.
- Prepare 4 ml of aniline in 4 ml of hydrochloric acid.
- Mix the aniline solution to benzene diazonium chloride solution slowly by stirring constantly.
- A yellow precipitate of para amino azobenzene also called aniline yellow is formed.
- Filter the yellow precipitate through a Buchner funnel and dry the crude sample in between the folds of paper.
- Recrystallize the crude sample by dissolving it in carbon tetrachloride to get the pure dye.
|Colour of the crystals||Yellow|
|Expected yield||4 gm|
|Melting point||123 to 126 °C|
Results and Discussion:
The yield of aniline yellow is _____ gm.
- Do not touch the dye otherwise it will stick on to the hands.
- Maintain the temperature of the experiment between 40 oc to 50 oc.
- Wash the crude sample repeatedly with water.
- Maintain the reaction pH between 4-5.
Viva Questions for Preparation Of Aniline yellow
1. How can phenol and aniline be distinguished chemically?
Phenol and aniline can be differentiated by a simple litmus paper test. Aniline is basic in nature so it turns red litmus paper blue. Phenol is slightly acidic so it turns blue litmus red. One more test to differentiate is the ferric chloride test. Phenol develops a violet colour with ferric chloride while aniline does not give any colour.
2. Is aniline a weaker base than ammonia?
Yes, Aniline is a weaker base than ammonia in solution. Because in aniline the phenyl ring is a mild electron-withdrawing and it has drastic effects on the availability of the electrons present on the Nitrogen atom.
3. Usually, diazonium chloride is soluble in water and why?
Diazonium chloride is readily soluble in water because of its tendency to form hydrogen bonds readily with water molecules.
4. What is the formula for aniline and what does it look like?
The formula for aniline is C6H5NH2. Aniline is an aromatic organic compound that look like a yellow liquid.
5. What is meant by diazotization?
The treatment of aniline with nitrous acid produces a diazonium salt in a reaction called diazotization.