To prepare Aniline yellow (para amino azobenzene) from benzene diazonium chloride and aniline.
Aniline yellow is also called para amino azobenzene is prepared by the coupling reaction between benzenediazonium chloride with aniline in acidic medium. It is the simplest and the first azo dye obtained by coupling reaction. This dye has no industrial significance because it is 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. In the first step, aniline acts as a nucleophile and reacts with benzene diazonium chloride to form a C-N bond. The product loses the H+ to reform an aromatic ring and gives the azo dye.
- Hydrochloric acid
- Sodium nitrite
- Glacial acetic acid
- Distilled water
- Round bottom flask
- Bunsen burner
- Water bath
- Conical flask
- Take 10ml of concentrated hydrochloric acid and 20ml of water in a beaker add 4.5ml 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 5oC.
- Mix 4g of sodium nitrite to 20ml of water. Add this sodium nitrite solution to the above mixture with constant stirring.
- Diazotization takes place and benzene diazonium is formed.
- Prepare 4ml of aniline in 4ml of hydrochloric acid.
- Mix the aniline solution to benzene diazonium chloride solution slowly by constant stirring.
- A yellow precipitate of para amino azobenzene also called as aniline yellow is formed.
- Filter the yellow precipitate through a buchner funnel and dry it the crude sample in between the folds of paper.
- Recrystallize the crude sample by dissolving in carbon tetrachloride to get the pure dye.
|Colour of the crystals||Yellow|
|Melting point||123 to 126°C|
Results and Discussion:
The yield of aniline yellow is _____ gm.
- Not to touch the dye otherwise it will stick on to the hands.
- Maintain the temperature of the experiment between 40oc to 50oc.
- Wash the crude sample repeatedly by 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 violet colour with ferric chloride while aniline does not give any colour
2. Is aniline 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 n-electrons.
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 how it looks like?
The formula for aniline is C6H5NH2. Aniline is an aromatic organic compound looks like a yellow liquid.
5. What is meant by diazotization?
The treatment of aniline with nitrous acid produces a diazonium salt in a reaction is called diazotization.