| Demonstration of Microscale Projection Experiments -
Chemistry en miniature Objective: Comproportionation Peter Keusch |
Hazards and safety precautions:
Safety glasses, protective gloves and good ventilation required. The preparation of bromine water should be performed under a laboratory fume hood! Experimental procedure: 1 mL of tetramethyl-p-phenylendiamine solution is pipetted into each of three test tubes. 1 drop of bromine water is added to the second and third test tube. The solutions are colored blue violet (Photo 1). Afterwards, the solution in the third test tube is mixed with 10-15 drops of bromine water, until it is again decolorized. A drop of this colorless solution is added to the colorless solution in the first test tube. Result: The colorless tetramethyl-p-phenylendiamine solution in the first test tube turns to blue (Photo 2).
Discussion: · The diamine loses an electron by the addition of 1/4 mol equivalent bromine. The blue solution contains a resonance stabilized radical which proves to be stable. When the solution is mixed again with 1/4 mol equivalent bromine, a dication is formed. The chromophore resembles that of the p-benzoquinone. Due to the low concentration, the solution is colorless. (1).  · When the colorless solution of the quinone diammonium salt is added to the tetramethyl p-phenylendiamine solution, a comproportionation takes place. The position of the equilibrium lies to the right (2). 
|  References: Wurster's original paper of 1808 (in German!) C. Wurster und R. Sendtner Zur Kenntmis des Dimethylparaphenylendiamins - I. Einwirkung von Brom au Dimethylparaphenylendiamin  Demonstration Experiment on Video
Wurster's Blue
General experimental instructions and index of experiments 
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