Cis/trans Isomerization of Maleic Acid

Demonstration Experiment on Video

Cis/trans Isomerization of Maleic Acid

Objectives: Light-induced Reaction, Isomerization

Peter Keusch

German version

Chemicals:
maleic acid
bromine

Apparatus and glass wares:
500 watt day light bulb
3 volumetric flasks 100 mL
volumetric pipet 20 mL
2 measuring pipets10 mL
2 erlenmeyer flasks 50 mL

Hazards and safety precautions:

Bromine is highly toxic if inhaled, ingested or comes in contact with the skin.

Bromine water is harmful if ingested or inhaled. Prolonged skin contact can cause burns. Eye irritant - lengthy contact will lead to eye damage.

Maleic acid is harmful if swallowed. Corrosive - causes irritation or burns.

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 CC bond (2).

· After rotation around the CC-bond and the elimination of the bromine radical fumaric acid is formed (3).

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