| Demonstration Experiment on Video Objectives: Diazotation, Coupling Peter Keusch |
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
Hazards and safety precautions:
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 Index of Lecture Experiments 
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