Projection Experiment: Tautomerism of Ethyl Acetoacetate

Demonstration of Microscale Projection Experiments - Chemistry en miniature

Keto-Enol-Tautomerism of Ethyl Acetoacetate

Objective: Test for Enol Form, Fe(III) Complex

Peter Keusch

German version

Chemicals:
ethyl acetoacetate
ethanol abs.
aqueous FeCl₃ solution (20 mg / 3 mL H₂O)
2 % bromine water
2-naphthol
10 % aqueous KI solution
0.5 % aqueous starch solution

Hazards and safety precautions:

Ethanol is highly flammable.

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.
2-naphthol is harmful if swallowed or inhaled. May be harmful in contact with the skin. Eye, skin and respiratory irritant.

Safety glasses, gloves and effective ventilation required. Bromine water should be prepared in a laboratory fume hood!

Experimental procedure:

The test tubes T1 and T2 are filled with 1 mL water. 3 drops of ethyl acetoacetate and 6 drops of the FeCl₃ solution are added to each test tube. The solution in T2 is mixed with 4 drops of bromine water.

Results:

The red-violet color of the solution in T2 fades to colorless (Photo 1). After a short time, the red color appears again. The solution can be decolorized again by adding bromine water.

Photo 1

Discussion:

· Ethyl acetoacetate involves both the keto form and the enol form (1).

· Like phenol, the enol form of ethyl acetoacetate reacts with Fe³⁺ forming a red-violet color Fe(III) complex (2).

· When elementary bromine is added to the red-violet colored solution, a spontaneous decolorization takes place. The enol form is converted by bromine to a -bromo ethyl acetoacetate (3).

· When bromine is completely consumed, the color of the Fe(III) complex slowly appears again. The enol form has been regenerated from the keto form.

· According to an analysis method developed by Meyer, the experimental enol percentage is 7.3 %. Slightly more than the needed quantity of bromine is allowed to react with the ester solution at 0 °C. Afterwards a solution of 2-naphthol is added rapidly in order to bind an excess of halogen. Then, in presence of potassium iodide one acidifies in order to reduce a -bromo aceto acetic acid ester by the formed iodine hydrogen. The content of enol can be determined indirectly by titration of free iodine (4).

On the basis of the following experiment the above mentioned analytical method can be illustrated:

0.3 mL ethyl acetoacetate and 0.5 mL ethanol are pipetted into a test tube. 0.2 mL of bromine water are added to the mixture. The orange color of bromine is immediately decolorized. After 4 drops of potassium iodide solution are added the solution turns to a orange-yellow. Drops of the yellow solution are added to a test tube containing 1 mL of starch solution. Intensely blue colored 'schlieren' appear mehrfach betonten (Photo 2).

Photo 2

Reference:
Demonstration Experiment on Video Tautomerism of Ethyl Acetoacetate

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