Azo dyes are an important class of dyes. They are colored due to the presence of –N=N- group. In their preparation, aromatic primary amines are diazotised using sodium nitrite and an acid, like HCl (see eq.1). The diazonium salts are coupled with active aromatic compounds to obtain azo dyes (see eq.2).

Ar-NH2  +  NaNO2 + 2HCl    Ar-N2+Cl  + NaCl  + 2H2O …………1

Ar-N2+ Cl +  Ar-OH   Ar-N=N-Ar-OH  + HCl ….……… 2

In the current experiment you will be synthesisizing an azo dye commonly known as Sudan I.

Sudan I-IV are synthetically produced azo dyes which do not occur naturally in food. Sudan‐I is a powdered substance with an orange‐red appearance. It is mainly used to color waxes, oils, petrol, solvents and polishes.

Prerequisite: (Theory)

Students should be familiar with different principles involved in diazotisation reaction

Coupling reaction.

Factors affecting the reaction or reaction conditions

Stoichiometric calculations involved in organic reactions


Slow addition of one solution to another with stirring

Filtration of product using suction

Thin Layer Chromatorgraphy (A-guide-to-thin-layer-chromatography-technique)

Further reading

Systematic Experiments In chemistry page no 483-484


Ice bath1
Glass rod2
Measuring cylinder (10mL)3
Filter papers2
Wash bottle1
Pre weighed butter paper1
TLC chamber1
Capillary tubes3
TLC plates1
Fusion tubes3


NaNO2 solution         10 mL
Conc. HCl             3 mL
10 % NaOH      7 mL
Aniline     1 mL
β-napthol   0.75 g
Starting compound for TLC
Acetone for dissolution,
Eluent for TLC (hexane :Ethyl acetate)



Hazard Symbols

Corrosive to skin 




Harmful to environment 


The history of dyeing is divided into two great periods, the “pre-aniline,” extending to 1856 and the “post-aniline” period. The former was characterized by a rather limited range of colors that were based on dye-producing animals and plants. The main vegetable dyes available were extracted from roots of madder plant (a perennial climbing plant with evergreen leaves and small pale yellow flowers found in Asia and Europe), producing a brilliant red and leaves of the indigo plant (India), yielding the blue dye still used today in jeans.

Aniline became available from coal tar in the 19th century and in 1856, William Henry Perkin at the age of 17 used it in the synthesis of “Mauveine”. Since then, synthetic dyes started replacing natural dyes.

This laboratory task involves preparation of an organic dye, namely, Sudan-I from aniline. The first step of reaction involves diazotization of aniline using sodium nitrite solution (eq 1), whereas the second step involves coupling of the diazonium salt with β-naphthol (eq 2).

Ar-NH2  +  NaNO2  + 2HCl  ®  Ar-N2+Cl  + NaCl  + 2H2O …………1

Ar-N2+ Cl  +  Ar’H ®  Ar-N=N-Ar’  + HCl                           ….……… 2


a) Preparation of diazonium salt

  1. A vial containing 1 mL of aniline is supplied to you. Transfer the entire content of the vial to a clean 100 mL beaker. Using a measuring cylinder, add 2.5 mL of conc. HCl and 5 mL of distilled water to this beaker. Stir the solution with a glass rod to obtain a clear solution. Cool this solution in an ice-bath for 5 to 10 minutes.
  2. Chilled sodium nitrite solution is supplied to you on your table. Add 5 mL of the sodium nitrite in a dropwise manner to the above aniline solution with constant stirring. The addition should be done in cold condition only.

b) Coupling Reaction

  1. In another clean beaker transfer the entire content of the vial containing β-naphthol. Add 5 mL of NaOH solution and 5 mL of distilled water. Stir well with a glass rod to obtain a clear solution. Cool this solution in an ice-bath to 0°C.
  2. Add dropwise the ice cold diazotised solution (prepared in Part a) into the ice cold solution of β-naphthol with constant stirring.
  3. Filter the precipitate using a Buchner funnel and under suction. Inform the laboratory expert when the filteration is over. After filtration, the precipitate should be handed over to laboratory expert for drying.
  4. The precipitate will be handed back to you after it is dried. Carefully transfer the product on a pre-weighed butter paper supplied to you. Take the product for weighing to the laboratory expert.
  5. Record the TLC of the final product after it is weighed.

Procedure for TLC:

Dissolve a small crystal of Sudan I, Aniline and β-naphthol in a small quantity of acetone in a sodium fusion tube respectively. Develop the TLC Plate using the solvent and visualize the spots under UV light. Calculate the Rf value using the formula given below and record the result in the answer sheet.

  1. The mass of the product.
  2. Calculate theoretical yield (based on aniline) in g :
  3. The yield obtained as a percentage of the theoretical yield:
  4. Colour of the product obtained:  Dark Brown     Yellow        Orange red     Red      Any other 
  5. Mechanism of reactions involved in synthesis of Sudan-I is given below. Draw the structures of intermediates and sudan-I.
  1. Azo compounds can be reduced to amines by a variety of reagents including SnCl2/HCl is one of them.

This reduction can be useful in the synthesis of phenacetin (an analgesic). Give the structure of phenacetin and the intermediates A,B and C.

  1. Draw the structures of the products obtained when the following compounds are treated with NaNO2/HCl at 0oC.

8.Treatment of the following amino compound with NaNO2/HCl at 0 to 5oC gives compound E which gives a positive 2,4-DNP test. Draw structure of E

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Synthesis of Dye