Reaction of Benzil with Benzoin

Demonstration Experiment on Video

Reaction of Benzil with Benzoin

Objective: Radical Anion

Peter Keusch

German version

Chemicals:
benzil
benzoin
EtOH 96 %
2 N NaOH

Glass wares:
magnetic stirrer hotplate
magnetic stirring bar
stirring bar remover
gas washing bottle 250 mL (without head), fitted with a stopper
beaker 250 mL
measuring cylinder 100 mL
pipette, graduated, 2 mL
pipette bulb
snap-cap vial 10 mL

Hazards and safety precautions:

Benzil may be harmful, causes irritation (skin, eye, inhaled).

Ethanol is highly flammable.

Safety glasses and protective gloves should be worn.

Preparation of a solution of mixed benzil and benzoin: O.525 g (2.5 mmol) of benzil and 0.53 g (2.5 mmol) of benzoin are dissolved in 150 ml of EtOH 96% while heating the mixture.

Experimental procedure:

The yellow solution of benzil and benzoin (cooled to room temperature) is poured into a gas washing bootle. 1.5 mL of 2 N NaOH are added. The bottle is stoppered and shaken vigorously.

Results:

The solution soon acquires a purplish color. If the bottle is shaken the color momentarily disappears. When the solution is then let stand, the color reappears. The color change can be repeated several times.

Video clip (Download RealPlayer .rm file )

Discussion:

· The ESR spectrum shows that the red solution contains a radical. This is consistent with the fast reaction with oxygen.

· Benzoin undergoes a deprotonation to give a dianion (1). The radical anion formed by reaction of benzil with benzoin is oxidized by oxygen (2). As oxygen is consumed in the solution, the radical anion is generated again until an equilibrium concentration is reached. The colorless solution turns red again.

· The radical concentration is relative high, due to the resonance stabilization of the radical anion. The light absorption in the visible spectrum is based on the extended pi-system. The solution appears colored.

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