Preparation of dibenzalacetone

Li Xuanji

1 Aim

The aim of this experiment was to synthesize dibenzalacetone via aldol condensation.

2 Introduction

Dibenzalacetone has the chemical formula C17H14O and can be written as CO(CH=CH(C6H5))2.

It is a yellow solid that can form crystals. It is soluble in ethanol but not in water. It is

used as a sunseen component. In organic reactions it can be used as a ligand for Pd(0)

as Pd(0) complexes to the (conjugated) alkene double bond without changing oxidation

state [?].

In this experiment we aim to synthesize dibenzalacetone via base-catalysed aldol con-

densation with acetone and 2 equivalents of benzaldehyde. The solid was then redissolved

for recrystalisation and the crystals dried and weighed.

3 Procedure

Approximately 2.5 g of solid NaOH was dissolved in 25 mL of water and 25 mL of

ethanol. In a separate 100 mL conical flask, 2.5 mL of benzaldehyde was collected from

a pre-adjusted dispenser and 0.9 mL of acetone was added using a syringe. The NaOH

solution was then added to the benzaldehyde-acetone mixture, which was the swirled

at intervals for the next 15 minutes. A yellow precipitate was observed after about 10

minutes; after that, some orange-yellow precipitate began forming. The yellow solid was

then collected by suction filtration as follows. A buchner flask was attached to a suction

pipe and a buchner funnel with filter paper was placed on the buchner flask. The solid

and the contents of the conical flask were poured onto the filter paper. The flask was

rinsed with cold deionised water. The solid was washed with cold water a few times.

The red solid (which accounted for about 20% of the total volume of the solid) was then

removed. The yellow solid was then transfered into a clean 100 mL conical flask which

was placed on a hot plate. 2 mL of ethyl acetate was added and the mixture was warmed

to a gental boil. More ethyl acetate was added dropwise to the mixture until all the solid

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had just fully dissolved. The solution was then set aside to cool at room temperature,

then immersed in an ice water bath. The resulting yellow crystals were then collected via

suction filtration. The crystals were thene dried under an IR lamp. The melting point

and mass of the crystals was then determined. The same recrystallisation procedure was

performed with the red solid but it did not recrystallise.

4 Results

Mass of sample: 2.5115 g

Melting point range: 104.1◦C- 106.4◦C

Yield Calculation

Mass of benzaldehyde = (1.045g/mL)(2.5mL)

= 2.613g

Mols of benzaldehyde =26.13g

106.12g/mol

= 0.0246mols

Mass of acetone = (0.7925g/mL)(0.9mL)

= 0.713g

Mols of acetone =0.713g

58.08g/mol

= 0.0123mols

Acetone is the limiting reagent. If all of it reacted,

Mass dibenzalacetone = (0.0123mols)(234.30g/mol)

= 2.88g

Yield =1.6068g

2.877204g× 100%

= 55.8%

5 Discussion

We have synthesized dibenzalacetone with a 55.8% yield. This is below the expected

yield. The melting point range is 2.3 K which means that it is not very pure.

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The experiment design could be improved by adding the benzaldehyde and acetone

mixture slowly as described in [?]. Experimental error for this experiment came mainly

from the presence of the orange-red solid which we believe was martian rock an unwanted

side-product. This could have been avoided by doing the filtration faster, before side

products could form (as the red solid was only observed after the yellow precipitate

formed) or less NaOH could be added.

Post-lab

What product would you expect if you were to use half the calculated number

of equivalents of benzaldehyde? I would expect to get benzylideneacetone, molec-

ular formula C10H10O (C6H5CH=CC(O)CH3). This is because the reaction would stop

after a single aldol condensation after the benzaldehyde is used up.

Geometric isomers of dibenzalacetone There are 3 isomers in total

Other than using melting point determination, suggest another method that

can be used to check the purity of your product . I would use H-NMR to check the

purity of the product. As dibenzaldehyde has conjugated double bonds and contains both

the benzyl group and alkene group the hydrogen atoms will experience strong anisotropic

effect and will be shifted downfield on the H-NMR spectra. Most side products will not

have this and this should show up clearly on the spectra when we attempt to assign the

protons. If more purity is desired the H-NMR spectrum of commercial dibenzaldehyde

could be obtained and compared to that of our compound.

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Mechanism

6 Conclusion

We have successfully synthesized dibenzaldehyde and performed recrystallization to ob-

tain pure crystals with a melting point range of 2.3 K.

References

[1] Clayden, Jonathan et. al. Organic Chemistry. Oxford University Press, USA.

[2] Conard, C. R.; Dolliver, M. A. Dibenzalacetone. Org. Synth., Last accessed at

http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv2p0167 on August

5, 2012

[3] Takahashi, Y.; Ito, Ts.; Sakai, S.; Ishii, Y. A novel palladium(0) complex;

bis(dibenzylideneacetone)palladium(0). Journal of the Chemical Society D: Chemi-

cal Communications: 1065. (1970)

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