Keto-Enol-Tautomerism of Ethyl Acetoacetate

Demonstration Experiment on Video

Keto-Enol-Tautomerism of Ethyl Acetoacetate

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

Peter Keusch

German version

Chemicals:

ethyl acetoacetate
phenol
ethanol 95 %
bromine
FeCl₃ · 6 H₂0

Fe(III)-chloride solution: 1 g FeCl₃ · 6 H₂0 dissolved in 150 ml of dist. water
Bromine water: 6 g of bromine in 300 mL dist. water

Glass wares:
	2 conical measures, graduated, 500 mL
	2 glass stirring rods
	2 beakers 300 mL
	2 snap-cap vials 20 mL
	volumetric pipet 10 mL
	volumetric pipet 2 mL
	pipette bulb
	2 measuring cylinder 100 mL

Hazards and safety precautions:

Phenol is acute poisoning by ingestion, inhalation or skin contact may lead to death. Phenol is readily absorbed through the skin. Highly toxic by inhalation.
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.

Ethanol is highly flammable.

Safety goggles and protective gloves must be worn when handling bromine. The experiment should be performed under a laboratory fume hood!

Experimental procedure:

Two conical measures are set up as follows:

conical measure 1	2 g of phenol dissolved in 500 mL of dist. water
conical measure 2	400 mL of ice water, 60 mL of ethanol, 2 mL of ethyl acetoacetate

The phenol solution is mixed with 10 mL of aqueous FeCl₃, the ethyl acetoacetate solution with 20 mL of aqueous FeCl₃.
Bromine water is added to the solution in conical measure 2 while stirring.

Results:

The solutions in the two conical measure show a blueviolet and redviolet color, respectively.
The redviolet color in conical measure 2 disappears after bromine water is added. The solution turns yellow in color (bromine excess!). After approx. 70 seconds the solution turns to a redviolet again. This process can be repeated two or three times.

Keto-Enol

Video clip (Download RealPlayer .rm file)

Discussion:

Tautomerism is a special kind of structural isomerism. One of the most common examples of a tautomeric system is the equilibrium between a ketone and its enol form. This type of tautomerism is called keto-enol tautomerism. In many aliphatic aldehydes and ketones, such as acetaldehyde, the keto form is the predominant one; in phenols, the enol form is the major component. An intermediate situation is represented by ethyl acetoacetate, which at room temperature contains about 92.4 percent keto and 7.6 percent enol. The interconversion of the two forms involves the migration of a proton and the shifting of bonding electrons (1). The enolic form of ethyl acetoacetate is stabilized due to intramolecular H - bonding while the enolic form of phenol is stabilized due to resonance. If the enolic form is removed from the equilibrium mixture by another chemical reaction, the more of the keto form changes into the enolic form or if the ketonic form is removed, then the enolic form changes to the ketonic form to maintain equilibrium.

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

iron complex

· 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.

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