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BACKGROUND
Principle: When an ethanolic solution containing acetone and its two equivalents of benzaldehyde is made alkaline with sodium hydroxide, rapid condensation occurs with the formation of dibenzal-acetone, or dibenzylidene-acetone. This is a particular example of Claisen Reaction as Claisen showed that aldehyde under the influence of sodium hydroxide condenses with (i) another aldehyde, or (ii) a ketone, with the elimination of water. Thus benzaldehyde condenses with (i) acetaldehyde to give cinnamic aldehyde, and with (ii) one equivalent of acetone to give (mono) benzal-acetone.
Aim: To prepare dibenzal acetone from benzaldehyde.
Reaction:
Mechanism:
Use:
It is used as a component in sunscreens and as a ligand in organometallic chemistry. It is also used in the production of palladium radiopharmaceuticals, or identification of medicinal plants and their constituents.
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REQUIREMENTS
Chemicals: Benzaldehyde – 10 ml
Acetone – 4 ml
Methylated spirit – 10 ml
10% aqueous sodium hydroxide solution- 2 ml
Apparatus: Conical flask or wide mouthed bottle
Beaker
Measuring cylinder
Buchner funnel etc
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PROCEDURE
About 10 ml (10.4 gm) of benzaldehyde and 4 ml of pure acetone is thoroughly mixed with 10 ml of methylated spirit in a conical flask or wide mouthed bottle. 2 ml of 10% aqueous sodium hydroxide solution is diluted with 8 ml of water, and this dilute alkali solution is added to the former solution. The mixture is shaken vigorously in the securely corked flask for about 10 minutes (releasing the pressure from time to time if necessary) and then allowed to stand for 30 minutes, with occasional shaking and finally cooled in ice-water for a few minutes. During the shaking, the dibenzal-acetone separates at first as a fine emulsion which then rapidly forms pale yellow crystals. It is filtered at the pump, washed well with water to eliminate traces of alkali, and then drained thoroughly.1 The product is recrystallised from hot methylated or rectified spirit.
Calculation of yield:
212 g of benzaldehyde yields dibenzal-acetone = 234 g
10.4 g of benzaldehyde shall yield dibenzal-acetone = (234 / 212) × 10.4 = 11.5 g
Therefore, Theoretical yield of dibenzal-acetone = 11.5 g
If reported Practical yield = 6 g
Then, Percentage Practical yield = (Practical yield / Theoretical yield) × 100
= 6 / 11.5 × 100 = 52.17%
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CONCLUSION
The percent yield of dibenzal-acetone is 52.17% obtained as pale yellow crystals, m.p. 112°.
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REFERENCES
- Mann F. G., Saunders B. C., Practical Organic Chemistry, 4th Edition, Dorling Kindersly. India Pvt. Ltd., New Delhi, Page No. – 231.
