Synthesis of Dibenzalacetone

Sabrina Liao

Purpose: The purpose of this experiment is to synthesize dibenzalacetone (trans,trans-1,5-diphenyl-1,4-pentadien-3-one) using a crossed aldol condensation.

Reaction:

Mechanism:

Table of reactants and reagents:

Reactant/reagent	Amount (g/ml)	Moles (mol)	Molar mass (g/mol)	Density (g/ml)	bp/mp (C)	comments
KOH	3.502 g	0.0624 mol	56.1056 g/mol	2.04 g/ml	1327C/ 360C	reagent
H2O	25 ml	1.38 mol	18.015 g/mol	1 g/ml	100C/0C	reagent
CH3CH2COOH	45 ml	0.771 mol	46.07 g/mol	0.789 g/ml	78.4C/-114.1C	reagent
Benzaldehyde	2.667 g	0.0245 mol	106.12 g/mol	1.04 g/ml	79 C/-26 C	reactant
Acetone	0.735 g	0.013 mol	58.08g/mol	0.79 g/ml	56.5 C/-94.9 C	reactant

Calculations:

Procedure

1. 3.502 g of KOH was added to a 125ml Erlenmyer flask that contained the mixture of 25 ml of water and 20 ml of ethanol. A magnetic stir bar was also placed inside the flask. 

2. 2.667 g of Benzaldehyde and 0.735 g of acetone was combined into a 50ml beaker. 

3. The Erlenmyer flask was then placed on a stir plate and the mixture inside was stirred thoroughly.

4. The Benzaldehyde-acetone mixture was gradually obtained inside the Erlenmyer flask in 15 mins. (a few drops of the mixture was added into the Erlenmyer flask every 2 minutes)

5. The mixture was stirred at room temperature for an additional 30 minutes.

6. The product was then extracted using vacuum filtration. The product was then washed with 35 ml of water. The product was dried under the vacuum for a few minutes. The vacuum product was placed inside an Erlenmyer flask.

7. After a stir bar was added to the Erlenmyer flask that contained the product, 25 ml of ethanol was added to a 50 ml beaker. 

8. The 50ml beaker was then placed on a heating plate, followed by the Erlenmyer flask that contained ethanol also put on the plate after a few drops of ethanol were added into the Erlenmyer flask that had the product inside. As the ethanol was heated, several drops of warm ethanol were added into the  Erlenmyer flask until the product inside the flask was fully dissolved.

9. Both the  Erlenmyer flask and 50ML beaker were removed from the heating plate and were cooled down to room temperature.

10. The Erlenmyer flask was then placed inside the ice bath. After a few minutes, the product crystallized.

11. After it was fully crystallized, the product was purified using vacuum filtration again.

12. The final product weighed 1.267 g and the melting point was measured as 113.2 C.

Table of products

product	Yield (g)	yield(mol)	Theoretical yield(g)	Percent yield	Molar mass	Actual mp (C)	literature mp (C)
dibenzalacetone	1.267 g	0.054 mol	1.26705 g	99%	234.29 g/mol	113.2 C	110-111C

Calculation

Discussion

1. Why is this crossed aldol condensation possible?

This crossed aldol condensation is possible because both benzaldehyde and acetone have carbon that can be deprotonated adjacent to their carbonyl group. This allows the ion in parts of reactants to connect together

2. Which compound forms the enolate during the initial aldol condensation?

Acetone forms the enolate during the initial aldol condensation.

3. Both acetone and benzaldehyde are colorless, while the product is yellow. Why is this?

The product is yellow because another benzene because another benzenring and carbonyl group was added to the original structure. This extended part leads to an absorbing visible light in the yellow region of the color spectrum, leading to the yellow color.

4. Dibenzalacetone is sometimes used in sunscreen products. Explain why, referring specifically to your spectrophotometry results.

Dibenzalacetone can absorb uv light radiation effectively therefore sunscreens containing dibenzalacetone can prevent the skin from absorbing UV lights,. The spectrophotometry results shows the wavelength of maximum absorbance which is 330.2 nm (is attached below), which indicates the specific wavelengths of light absorbed by dibenzalacetone. 

References

PubChem. “PubChem.” Nih.gov, National Library of Medicine, 2022, pubchem.ncbi.nlm.nih.gov/.