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Demonstration Experiment on Video

Azo Dye Formation

Objectives: Diazotation, Coupling

Peter Keusch






German version



Chemicals:
sulfanilic acid
1-naphthylamine
2-naphthol
sodium nitrite
2 N NaOH
2 N H2SO4

The following solutions are prepared:

Sulfanilic acid solution:   1.7 g of sulfanilic acid in 50 mL in of 2 N H2SO4
Sodium nitrite solution:   0.6 g of NaNO2 in 10 mL of dist. H2O
1-Naphthylamine solution:   0.7 g of 1-naphthylamine in 50 mL of 2 N H2SO4
2-Naphthol solution:   0.7 g of 2-naphthol in 50 mL of 2 N NaOH


Glass wares:
crystallizing dish 190 × 90
3 beakers 250 mL
beaker 100 mL
3 volumetric pipets 50 mL
volumetric pipet 10 mL
2 petri dishes
3 pipet bulbs
3 glass stirring rods


Hazards and safety precautions:

Azo dyes are toxic and may cause genetic mutations.
1-Naphthylamine is absorbed through skin and the respiratory and intestinal tract. It is toxic and carcinogenic.
Sulfanilic acid may act as a skin, eye or respiratory irritant. May act as a sensitizer.
2-Naphthol is harmful if swallowed or inhaled. May be harmful in contact with the skin. Eye, skin and respiratory irritant.
Sodium nitrite is toxic if swallowed. Severe eye irritant. Respiratory and skin irritant. May act as a carcinogen for chronic exposure.

Safety goggles and protective gloves must be worn. Good ventilation required! The 1-naphthylamine solution of phenol and the sodium nitrite solution should be made under a fume hood!


Experimental procedure:

A crystallizing dish is half filled with ice. Four beakers containing the following solutions are placed in a ice bath:
beaker 1:   50 mL of an acidic solution of sulfanilic acid
beaker 2:   10 mL of an aqueous solution of sodium nitrite
beaker 3:   50 mL of an acidic solution of 1-naphthylamine
beaker 4:   50 mL of an alkaline solution of 1-naphthol

The sulfanilic acid solution is mixed with the aqueous solution of sodium nitrite while stirring. Equal volumes of the mixture are poured into the beakers 3 and 4.


Results:

The solutions in the beakers turn red.


   

Video clip (Download RealPlayer .rm file )


Discussion:

Diazotation:  When primary aryl amines are reacted with nitrous acid (generated from NaNO2 in combination with H2SO4 ) a reaction occurs which makes a diazoniumion  (1).  The reaction takes place under freezing conditions.



Coupling reaction:  The diazotized sulfanilic acid reacts with 2-naphthol and 1-naphthylamine to produce an acidic and basic azo dye alternatively  (2).  The azo coupling represents an electrophilic aromatic substitution. The diazonium cation is a relatively weak electrophile. So the aromatic ring which it attacks must have a activating group such as
 - OH and  - NH2. Electron withdrawing groups on the aromatic ring of the diazonium ion facilitate the substitution reaction. Electrophilic substitution of 2-naphthol occurs preferentially at the 1-position. The NH2 group in 1-naphthylamine activates the 2- and 4-position.



In azo compounds the  - N = N -  group is part of an extended delocalized electron system involving the aromatic rings, called a chromophore. Groups such as  - OH and  - NH2 attached to chromophores modify the colors of the dyes.


References:
  Demonstration Experiment on Video   Azo Coupling
Microscale Projection Experiments   Basic and acid Azo Dyes


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