DIBENZALACETONE: PROPERTIES, REACTION MECHANISM, USES, RISKS - CHEMISTRY - 2024

The dibenzalacetone (dba) is an organic compound whose molecular formula is C ₁₇ H ₁₄ O. It is a yellowish solid which, depending on its purity, it may present as crystals. It is used in sunscreen and organometallic syntheses in which palladium is used as a catalyst.

Although its synthesis is a relatively simple process, quite recurrent in teaching laboratories to explain aldol condensation, its mechanism is somewhat extensive, and several factors must be taken into account. The benzaldehyde that is used, which will condense with acetone, has to be freshly distilled to guarantee its low oxidation in contact with air.

Sample of dibenzalacetone in glass container. Source: Stephanb

Likewise, a basic ethanol-water medium is used to solubilize the reagents and, at the same time, promote the final precipitation of dibenzalacetone, a hydrophobic and insoluble compound. So far, it is not known what negative effects dibenzalacetone can have on the body or the environment, other than being an irritant.

Properties

Physical appearance

Dusty or crystalline-looking yellowish solid.

Molar mass

234.29 g / mol

Isomers

Dibenzalacetone occurs as three geometric isomers: trans-trans, trans-cis, and cis-cis. The trans-trans isomer is the most stable of all and, therefore, the one that is most produced during synthesis.

Melting point

110-111 ° C. This range varies depending on the degree of purity of the synthesized solid.

Water solubility

Insoluble.

Structure

Molecular structure of dibenzalacetone. Source: Benjah-bmm27

The upper image shows the molecule of the trans-trans dibenzalacetone isomer, represented by a model of spheres and bars. In the center of it we have the carbonyl group, and on its sides, some double bonds and two aromatic benzene rings.

Dibenzalacetone is essentially apolar and hydrophobic, since its entire structure is practically composed of carbon and hydrogens atoms. The carbonyl group gives it only a small dipole moment.

The structure can be compared to that of a leaf, since all its carbon atoms have sp ² hybridization; therefore, they rest on the same plane.

Photons of light interact with the π-conjugated system of dibenzalacetone; especially those of UV radiation, being absorbed to excite delocalized electrons. This property makes dibenzalacetone an excellent absorber of ultraviolet light.

Mechanism of reaction of dibenzalacetone

Mechanism of aldol condensation in the synthesis of dibenzalacetone. Source: Izmaelt

In the upper image we have represented the mechanism of aldol condensation between benzaldehyde and acetone, to thus originate dibenzalacetone; specifically, its trans-trans isomer.

The reaction begins with acetone in a basic medium. OH ^- deprotonates an acidic proton of either of its two methyl groups, -CH ₃, giving rise to an enolate: CH ₃ C (O) CH ₂ ^-, which delocalizes its negative charge by resonance (first row of the image).

This enolate then acts as a nucleophilic agent: it attacks the carbonyl group of a benzaldehyde molecule. Its incorporation into benzaldehyde generates an alkoxide, which, because it is very basic, deprotonates a water molecule and becomes an aldol (second row). The aldol or β-hydroxyketone is characterized by having the groups C = O and OH.

The basic medium dehydrates this aldol and a double bond is formed in its structure, which generates benzylideneacetone (third row). Next, the OH ^- also deprotonates one of its acidic hydrogens, repeating another nucleophilic attack on a second benzaldehyde molecule. This time the attack takes place at a slower speed (fourth row).

The product formed deprotonates another water molecule and undergoes dehydration again to eliminate the OH group and establish a second double bond (fifth and sixth rows). Thus and finally, dibenzalacetone is produced.

Synthesis

Reagents

The reagents to carry out the synthesis of dibenzalacetone are the following:

- 95% ethanol.

- Benzaldehyde freshly distilled from bitter almond oil.

- NaOH as a basic catalyst in distilled water.

The amounts to be used depend on how much dibenzalacetone is to be synthesized. However, it is sought that there is an excess of benzaldehyde, because part of it is oxidized to benzoic acid. It is also guaranteed that the reaction takes less time and that the undesirable benzylideneaketone is produced to a lesser extent.

Ethanol acts as a solvent for benzaldehyde, otherwise it would not dissolve in the basic NaOH medium.

Process

In a large beaker, mix the ethanol with the benzaldehyde. Then the basic NaOH medium is added during constant magnetic stirring. In this step, the Cannizzaro reaction occurs to a lesser degree; that is, two molecules of benzaldehyde disproportionate into one of benzyl alcohol and the other of benzoic acid, easily recognizable by its characteristic sweet odor.

Finally, add the acetone and wait half an hour for the solution to turn cloudy and yellow-orange in color. Dibenzalacetone will precipitate due to water, so a considerable volume of water is added to promote its complete precipitation.

The dibenzalacetone is filtered off under vacuum and its yellowish solid is washed several times with distilled water.

Recrystallization

A recrystallized sample of dibenzalacetone should glow similar to the crystals in this image. Source: Smokefoot

To purify the dibenzalacetone, 95% ethanol or hot ethyl acetate is used, so that crystals of greater purity are obtained each time the recrystallization is repeated. Thus, the initial yellowish powder will be transformed into small yellow crystals of dibenzalacetone.

Applications

Dibenzalacetone is a compound that does not have many uses. Due to its ability to absorb ultraviolet light, it is used in the formulation of sunscreens, or any other product that seeks to mitigate the incidence of UV rays, be it coatings or paints.

On the other hand, dibenzalacetone is used in the organometallic syntheses of palladium. It acts as a binder that coordinates to the metallic palladium atoms, Pd ⁰, to form the tris (dibenzylideneacetone) dipalladium (0) complex.

This organometallic compound provides Pd ⁰ atoms in different organic syntheses, so it behaves as a homogeneous catalyst, since it dissolves in many organic solvents.

Also, dibenzalacetone as a binder is easy to replace by other organic binders, which allows the organometallic syntheses of palladium to develop rapidly.

Risks

Regarding the risks, there is not much information available that reports on the possible impacts on health or the environment that dibenzalacetone can cause. In its pure state, it is a solid irritant by ingestion, breathing, or by direct contact with the eyes or the skin.

However, it is apparently not sufficiently irritating that it cannot be part of sunscreen formulations. On the other hand, being so insoluble in water, its concentration in it is negligible, presenting itself as a solid contaminant. In this sense, it is unknown how harmful the turbidity it causes is to marine fauna or soils.

Until proven otherwise, dibenzalacetone will be considered a relatively safe compound, as its low reactivity is not a reason for risk or greater precautions.

References

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. (10th edition.). Wiley Plus.
4. Wikipedia. (2020). Dibenzylideneacetone. Recovered from: en.wikipedia.org
5. National Center for Biotechnology Information. (2020). Dibenzylideneacetone. PubChem Database., CID = 640180. Recovered from: pubchem.ncbi.nlm.nih.gov
6. Organic Syntheses. (2020). Dibenzalacetone. Recovered from: orgsyn.org
7. Dibenzalacetone by Aldol Condensation. Recovered from: web.mnstate.edu