Preparation of 2-Naphthol Aniline Dye

2-Naphthol aniline dye is a red dye with an azo compound. It is primarily used for textile dyeing. Azo compounds have an extended system of conjugation and are often coloured and used as colours. The reaction known as the coupling reaction prepares these compounds.2-Aniline naphthol dye is made from aniline.

Aim:

To prepare 2-Naphthol Aniline Dye from aniline, sodium nitrite, hydrochloric acid and alkaline solution of β-napthol also called 2-naphthol.

Theory:

2- Naphthol aniline dye is a scarlet dye can be prepared by coupling reaction. Aniline reacts with sodium nitrite in the presence of hydrochloric acid forms benzene diazonium chloride. Further benzene diazonium chloride reacts with 2-naphthol forms a bright orange colour 2-naphthol aniline dye.

Preparation of 2-Naphthol Aniline Dye

Azo compounds are prepared by the reaction of diazonium salts with phenol under alkaline conditions. Primary aromatic amines react with nitrous acid at 0oC to give a diazonium salt. Nitrous acid is in turn formed by the reaction of sodium nitrite with hydrochloric acid. The active reagent is nitrous anhydride or dinitrogen trioxide. Nitrous anhydride reacts with aniline to give nitroamine derivative which is unstable and isomerizes to form a diacetic acid which in turn converted to a diazonium salt. Finally, this diazonium salt reacts with 2-naphthol in the presence of sodium hydroxide give 2-naphthol aniline which is an aniline dye.

Materials Required:

  1. Aniline
  2. Sodium nitrite
  3. Hydrochloric acid
  4. 2-Naphthol
  5. Sodium hydroxide solution
  6. Beaker
  7. Test tube
  8. Buchner funnel
  9. Pipette
  10. Capillary tube
  11. Thermometer
  12. Distilled water
  13. Hot air oven
  14. Ice bath

Apparatus Setup:

Preparation of 2-Naphthol Aniline Dye

Procedure:

  1. Dissolve 5ml of aniline in a mixture of concentrated hydrochloric acid and water.
  2. Cool the solution in ice bath between 0-5oC.
  3. Add a solution of 4gm sodium nitrite in 15ml of water dropwise with continuous shaking and controlling the temperature below 5oC.
  4. Take another flask to dissolve 8gm of 2-naphthol in a solution of 5 gm sodium hydroxide solution in 50ml of water.
  5. Cool the solution in the ice bath to 0-5oC.
  6. Now mix the two cold solutions slowly dropwise with constant stirring.
  7. Continue the stirring at least half an hour without allowing the temperature to rise above 10oC.
  8. An orange colour azo dye called 2-naphthol aniline separates out.
  9. Filter the crude sample and wash it with cold water.
  10. Dry and recrystallize it from ethyl alcohol or glacial acetic acid.

Observations:

Colour of the crystals Orange crystals
Expected Yield 13gm
Melting Point 131oC

Results and Discussion:

The yield of 2- Naphthol aniline dye is ______ gm.

Precautions:

  1. The solution must be cooled to 5oC. Do not raise the temperature.
  2. Not to touch the dye otherwise it will stick to hands.
  3. Not to touch the concentrated acids otherwise it will cause irritation.
  4. Wash the crude sample repeatedly with cold water in order to remove soluble impurities.
  5. Maintain the pH between 4-5.

Viva Questions:

1. What is coupling reaction?

Ans: Coupling reaction of diazonium salts with highly activated because of rings such as a phenolic ring. The reaction involves electrophilic substitutions and the product obtained is generally dye.

2. Give the formula of 2-naphthol aniline dye.

Ans: The formula for 2-naphthol aniline is C16H12N2O and it is a scarlet red dye.

3. What is diazotization reaction?

Ans: Diazotization reaction is the reaction of primary aromatic amines with nitrous acid to form diazonium salt. Usually, this reaction is carried out at a very low temperature.

4. What is the colour of pure aniline?

Ans: Pure aniline is star yellow in colour but turns red-brown on exposure to air and light.

5. Mention the uses of azo dyes.

Ans: Azo dyes are used to dye textile fibres, especially cotton, as well as silk, wool, viscose and synthetic fibres.

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