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Demonstration of Microscale Projection Experiments - Chemistry en miniature

Basic and acid Azo Dyes

Objective: Azo Coupling

Peter Keusch




German version




Chemicals:
sulfanilic acid
8 mmol phenol / ethanol 96 %  (0.753 g / 20 mL)
6 mmol 1-naphthol / ethanol 96 %  (0.865 g / 20 mL)
4 mmol 2-naphthol / ethanol 96 %  (0.577 g / 20 mL)
0.2 mmol 1-naphthylamine / ethanol 96 %  (0.029 g / 20 mL)
20 mmol N,N-dimethylaniline / ethanol 96 %  (2.5 mL / 17.5 mL)
40 mmol aniline / ethanol 96 %  (3.6 mL / 16.4 mL)
2 N acetic acid
NaNO2
concentrated HCl
2 N NaOH

Reagent solution (Diazo component): 20 mL 0.5 % aqueous NaNO2 solution + 20 mL 0.5 % solution of sulfanilic acid in 2 N acetic acid

Hazards and safety precautions:

Azo dyes are toxic and may cause genetic mutations.
Aniline is highly toxic. Possible carcinogen. Possible mutagen. Possible sensitizer. Cyanotic reagent. Readily absorbed through the skin.
N,N-dimethylaniline is highly toxic. May be fatal if inhaled, swallowed or if absorbed through the skin. May cause serious eye damage. May act as a carcinogen. Danger of cumulative effects. Note very low toxic doses below. Skin and respiratory irritant.
1-Naphthylamine: at high concentrations poisining symptoms similar to aniline toxication.
Phenol is acute poisoning by ingestion, inhalation or skin contact may lead to death. Phenol is readily absorbed through the skin. Highly toxic by inhalation.
NaNO2 is toxic if swallowed. Severe eye irritant. Respiratory and skin irritant. May act as a carcinogen for chronic exposure.
1-Naphthol and 2-Naphthol are harmful if swallowed or inhaled. May be harmful in contact with the skin. Eye, skin and respiratory irritant.
Conc. hydrochloric acid is extremely corrosive. Inhalation of vapour can cause serious injury. Ingestion may be fatal. Liquid can cause severe damage to skin and eyes.
Ethanol is highly flammable.

Safety goggles, protective gloves and a well ventilated room required. The preparation of the ethanolic solutions of aniline, 1-naphthylamine,
N,N-dimethylaniline, and phenol should be performed under a laboratory fume hood!


Experimental procedure:

1. First the simplest azo dye, the 4-(phenylazo)-aniline, is presented. Ethanolic aniline solution is pipetted into each of two tubes (T1, T2). NaNO2 solution is added to each of the aniline solution. The two solutions are mixed with conc. HCl.

Test tube 1   0.5 mL of ethanolic aniline solution    0.5 mL of NaNO2 solution    2 drops of conc. HCl  
Test tube 2   0.5 mL of ethanolic aniline solution    0.5 mL of NaNO2 soultion    4 drops of conc. HCl


2. The ethanolic solutions of aniline, N,N-dimethylaniline und 1-naphthylamine are mixed with the reagent soution. Hydrocloric acid is added to the dye solutions.

Test tube 1   1 mL of ethanolic aniline solution    3 drops of reagent solution    2 drops of conc. HCl  
Test tube 2   1 mL ethanolic of N,N-dimethylaniline solution    3 drops of reagent solution    2 drops of conc. HCl  
test tube 3   1 mL of ethanolic 1-naphthylamine solution   3 drops of reagent solution    2 drops of conc. HCl  


3. The ethanolic solutions of phenol and of the naphtholes are mixed with the reagent solution. Afterwards sodium hydroxide solution is added.

Test tube 1   1 mL of ethanolic phenol solution    1 drop of reagent solution    3 drops of NaOH  
Test tube 2 1 mL of ethanolic 2-naphthol solution    1 drop of reagent solution    3 drops of NaOH
Test tube 3 1 mL of ethanolic 1-naphthol solution   1 drop of reagent solution    3 drops of NaOH
























Results:





1. The ethanolic solution of diazotized aniline in T1 becomes immediately yellow after adding 2 drops of conc. HCl. When 4 drops of conc. HCl are added, the solution in T2 turns orange.
2. Coupling of diazotized sulfanilic acid with aniline, N,N-dimethylaniline and 1-naphthylamine:



Color of the solution
 after addition of HCl
(Photo 1)
Test tube 1 lemon-yellow gold-yellow
Test tube 2 gold-yellow red
Test tube 3 orange-red violet



 Photo3
Photo 1


3. Coupling of diazotized sulfanilic acid with phenol and the naphtholes:



Color of the solution
(Photo 2)		 after addition of NaOH
( Photo 3)
Test tube 1 lemon-yellow gold-yellow
Test tube 2 orange orange-red
Test tube 3 intensively orange red-violet


Photo4
Photo 2
Photo5
Photo 3



Discussion:

·   In the first experiment the formed diazoaminobenzene rearranges itself to 4-(phenylazo)-aniline. "Aniline Yellow" is the simplest azo dye. In contrast to the red azobenzene, "Aniline Yellow" has an auxochrome group and is able to color textiles.

·   Diazotized sulfanilic acid  (1)  reacts with phenol and the naphtholes forming acid azo dyes  (2).




·   The rate of the coupling reaction depends on the substituents attached to the benzene-rings. Electron donating groups (- NH2, - N(R)2, - OH) of the coupling component and electron withdrawing groups (- NO2, - SO3H) of the diazo component promote the coupling reaction. On the one hand the electron density in the ring of the coupling component is increased. On the other hand a nitrogen atom of the diazo component becomes positive.

·   When aqueous NaOH is added to the solutions acid azo dyes, a color change can be observed (3). The conversion of the phenolic or naphtholic hydroxy group to the phenolate or naphtholate group causes a strong bathochromic effect  (Photo 3).  When acid is added to the solution of a basic azo dye, a bathochromic effect can be observed. In acidic medium the yellow color of "Methyl Orange" turns red  (4).


·   2-Naphthol Orange  (Orange II) is a often used mono azo dye, since it colors wool directly as well as with metal hydroxides pickled cotton. The sulfonic groups does not have an effect on the color. They render the dye however soluble in water. The dye is suitable for dyeing textiles.

· Methyl Orange is used as an color indicator in acidimetry. Because of acid sensitivity it is not used in the dye works.


Reference:
Video: Acidic Azo Dyes


General experimental instructions and index of experiments






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