The dibenzalacetone (dba) is an organic compound whose molecular formula is C17H14O. It is a yellowish solid that, depending on its purity, can appear as crystals. It is used in sunscreens 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 the acetone, has to be freshly distilled to guarantee its low oxidation in contact with air..
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 that of being an irritant substance.
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Dusty or crystalline-looking yellowish solid.
234.29 g / mol
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.
110-111 ° C. This range varies depending on the degree of purity of the synthesized solid..
Insoluble.
The upper image shows the molecule of the trans-trans isomer dibenzalacetone, represented by a model of spheres and bars. In the center of it we have the carbonyl group, and at its sides, some double bonds and two aromatic benzene rings.
Dibenzalacetone is essentially nonpolar and hydrophobic, since its entire structure is practically composed of carbon and hydrogen 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 hybridizationtwo; 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..
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. The OH- deprotonates an acidic proton of either of its two methyl groups, -CH3, giving rise to an enolate: CH3C (O) CHtwo-, 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- it 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.
The reagents to carry out the synthesis of dibenzalacetone are the following:
- 95% ethanol.
- Freshly distilled benzaldehyde 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 benzylideneacetone is produced to a lesser extent..
Ethanol acts as a solvent for benzaldehyde, otherwise it would not dissolve in the basic NaOH medium.
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 under vacuum and its yellowish solid is washed several times with distilled water..
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..
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 palladium metal atoms, Pd0, to form the tris (dibenzylideneacetone) dipalladium complex (0).
This organometallic compound provides Pd atoms0 in different organic syntheses, so it behaves as a homogeneous catalyst, since it dissolves in many organic solvents.
Furthermore, dibenzalacetone as a binder is easy to replace by other organic binders, which allows the organometallic syntheses of palladium to develop rapidly..
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 irritating enough 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..
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