| Demonstration Experiment on Video Objectives: Light-induced Reaction, Isomerization Peter Keusch |
Hazards and safety precautions:
Safety goggles and protective gloves must be worn. The experiment should be performed under a portable fume cupboard giving all-round visibility! Preparation: Saturated solution of maleic acid: 94.8 g of maleic acid dissolved in 120 mL of dist. H2O Saturated bromine water: 3.5 g of bromine in 100 mL of dist. H2O Experimental procedure: Three volumetric flasks are prepared as follows: Flask A and B: 40 mL of aqueous maleic acid solution are mixed with 10 mL of saturated bromine water. Flask C: 40 mL of aqueous maleic acid solution are diluted with 10 mL of dist. H2O. The volumetric flask A is covered by a black paperboard cylinder. The solutions B and C are simultaneously exposed to the light of a 500 watt day light bulb until a precipitate is formed in flask B. After the light is turned off the paperboard cylinder is removed. Result: The red color of bromine disappears from the flask B exposed to light and a white precipitate is formed. The flask not exposed to light retains the color of bromine. Also the solution in volumetric flasks C remains unaltered.    Video clip (Download RealPlayer .rm file)
|  Discussion:  · Maleic acid is isomerized to fumaric acid by light in the presence of bromine.  · Light initiates the reaction by generating bromine atoms (free radicals) from bromine molecules 1. · The bromine radical is added to maleic acid. In the addition compound the partial single bond character allows rotation around the central C - C single bond 2. · After rotation around the C - C single bond and the elimination of the bromine radical fumaric acid is formed 3. Index of Lecture Experiments 
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