When acetone is added to a basic solution of cinnamaldehyde in ethanol and water, two sequential aldol condensations occur. The second condensation product is bright yellow and insoluble in ethanol. During the induction period the solution supersaturates in this product. It then precipitates suddenly.
Curriculum Notes
This demonstration can be performed in an organic chemistry class when carbonyl or carbanion chemistry is being presented. This demonstration takes about five minutes to perform.
One day of lead time is required for this project.
Discussion
The reaction mixture is 0.8M in cinnamaldhyde, 0.6M in acetone, and 1.4M in KOH. The hydrogens on the carbons adjacent to the carbonyl carbon on the acetone are slightly "acidic" in the sense that they can be removed by the hydroxide ion, resulting in the formation of a carbanion on the alpha carbon. This "nucleophilic" carbon then attacks the carbonyl carbon of the cinnamaldehyde, resulting in a new carbon-carbon bond. The process the repeats with the formation of a carbanion on the alpha carbon that was originally on the other side of the acetone carbonyl group. The final product is 1,9-diphenyl-1,3,6,8-nonatetraen-5-one (dicinnamalacetone). For the entire explanation of this demo see Bassam Shakhashiri. Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 4, pp 65-69. U. of Wisconsin: (1992).
Materials
- A 400 mL beaker containing a basic solution of cinnamaldehyde in ethanol and water.
- A stoppered 25 mL Erlenmeyer flask containing acetone.
- Stirring rod.
- Blue background box (optional).
Procedure
Add acetone to cinnamaldehyde solution and stir. After about 45 seconds, a bright yellow precipitate forms.
Safety Precautions
Cinnamaldehyde is toxic if taken internally. Wear goggles.
Prep. Notes
To prepare the cinnamaldehyde solution, dissolve 20.0 mL of cinnamaldehyde in 125 mL of 95% ethanol and add 75 mL of 2M KOH. Stir. Avoid getting cinnamaldehyde on skin or breathing concentrated vapors. Wear gloves and goggles.