Stopping the Invisible
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The following article was originally published in the journal for educators Chemia w Szkole (eng. Chemistry in School) (4/2019):
With summer now in full swing, at least at the time of writing, it seems appropriate to pause and consider how solar radiation affects the human body. Setting aside the other components of sunlight, this article will focus on the band most commonly regarded as the most harmful.
Radiation with wavelengths shorter than what our eyes can see yet longer than X-rays, approximately 10–400 nm, is called ultraviolet (UV). Some sources place the lower UV limit at 100 nm. The name is Latin in origin (ultra meaning "beyond") [1]. Ultraviolet light was discovered in the 19th century, and as so often happens, it was identified independently by two researchers: physicist Johann Wilhelm Ritter and chemist William Hyde Wollaston.
Because UV carries relatively high photon energy, it has distinct effects on living systems. For practical purposes, it is subdivided into conventional ranges:
- UV-C, λ = 00–280 nm
- UV-B, λ = 280–315 nm
- UV-A, λ = 315–380 nm
UV-A lies closest to the visible band, and we might say figuratively that it is only slightly “more violet than violet.” It is also comparatively the least harmful. That does not make it harmless: prolonged exposure damages collagen fibers in the skin, accelerating photoaging. Large doses of UV-A may induce lenticular opacification, resulting in cataract formation. UV-B and UV-C typically do not reach the lens because they are absorbed by the cornea, the eye’s protective outer layer.
We know UV-B exposure in skin produces cholecalciferol C27H44O, vitamin D3, which the body needs. Only very small doses of this light are required, however. Higher doses of UV-B and UV-C are dangerous: beyond causing erythema, they increase cancer risk (including malignant melanoma arising from pigment cells) and inflict DNA damage. High-energy UV radiation is therefore mutagenic [2].
In short, despite the long-fashionable allure of tanning, moderation is essential, and we should shield our skin from UV’s harmful effects. The market offers many protective formulations such as creams, oils, and sprays, all of which must incorporate actives that block deleterious radiation while remaining safe for human use.
A representative UV-blocking compound, or more precisely a strongly UV-absorbing compound, is dibenzylideneacetone. Its synthesis is straightforward enough to carry out in a modestly equipped university, school, or even hobbyist lab. Demonstrations with dibenzylideneacetone are not only instructive but visually striking and provide an excellent invitation to hands-on experimentation.
Reagents for the aynthesis
For the synthesis we will need the following:
- benzaldehyde C7H6O,
- acetone C3H6O,
- sodium hydroxide NaOH.
The first reagent, benzaldehyde, is the simplest aromatic aldehyde (Fig.1).
Benzaldehyde is a liquid with a characteristic almond aroma and is naturally present in almonds Prunus dulcis. Although several compounds that share this scent, such as hydrogen cyanide HCN and nitrobenzene C6H5NO2, are highly toxic, benzaldehyde itself is not considered poisonous. It has multiple uses in the chemical and fragrance industries and in food applications, for example as a component of almond oil used in marzipan production. It oxidizes readily, even in air, so it should be stored in tightly sealed amber glass. Fresh, pure benzaldehyde is colorless but yellows over time. Older, more strongly colored samples should be purified (for example by distillation) before use.
Acetone is the simplest aliphatic representative of the ketone class (Fig.2).
Acetone is relatively volatile, with a sharp and easily recognized odor that is often associated with nail-polish remover, although most modern removers rely on ethyl acetate C4H8O2 instead. Small amounts of acetone are normally found in human blood and urine, yet in uncontrolled advanced diabetes its levels in the body may increase sharply.
Although benzaldehyde and acetone are not classified as highly toxic, they can pose health risks if ingested, inhaled, or absorbed in sufficient quantities. Both substances may cause irritation to skin and eyes, and contact should therefore be avoided. Sodium hydroxide solutions are strongly caustic and can inflict severe injury, making the use of eye protection imperative. Acetone and common alcohols such as ethanol and isopropanol are flammable. Appropriate personal protective equipment must be used at all times.
The reagents are shown in Photo.1.
As solvents in the synthesis we will use water H2O and approximately 95% ethyl alcohol C2H6O.
Synthesis
The method described below is not the only viable route, but I have verified it experimentally and can recommend it as straightforward and high-yielding [3].
Prepare an alcoholic benzaldehyde solution by dissolving 16 cm3 (≈0.54 fl oz) of benzaldehyde in 125 cm3 (≈4.23 fl oz) of ethanol. The solution should be colorless or only faintly yellow and clear; any cloudiness likely indicates impurities. Because benzaldehyde oxidizes readily, prepare this solution immediately before proceeding.
Separately, prepare a solution of 9 g (≈0.32 oz) sodium hydroxide in 75 cm3 (≈2.54 fl oz) of distilled water. Dissolution is strongly exothermic and heats the liquid substantially.
Before the next step, cool all solutions to room temperature, ideally below 20°C (68°F), with particular attention to the sodium hydroxide solution. This is important because the forthcoming reaction is exothermic, and excessive temperature rise can lower the yield.
Place the alcoholic benzaldehyde solution in a suitably sized beaker, preferably on a magnetic stirrer (Photo.2). If a stirrer is unavailable, vigorous manual stirring is required.
Work quickly at this stage. With vigorous stirring, add the pre-cooled sodium hydroxide solution in one portion. After only a few seconds of mixing, add 5 cm3 (≈0.17 fl oz) of acetone.
The initially colorless mixture should turn turbid and yellow within tens of seconds, and then deepen toward orange as the reaction proceeds (Photo.3).
When technical-grade acetone is used, transient other colors may appear. This effect is usually short-lived and does not materially affect the outcome. The most likely culprits are impurities in acetone sold for solvent use, for example in painting. Impurities can, of course, also be introduced by other reagents.
Monitor the temperature and keep it within 20–25°C (68–77°F). If it rises above that range, cool the beaker in a water or ice bath.
After some time, an abundant yellow precipitate of dibenzylideneacetone forms (Photo.4). From the first appearance of the solid, continue stirring for another 30–60 minutes to maximize conversion.
After the allotted time, add about 100 cm3 (≈3.38 fl oz) of distilled water. At this stage only part of the product has precipitated, while a substantial fraction remains dissolved in the alcoholic phase. Adding water drastically reduces solubility and causes more solid to separate.
After thorough mixing and removing the stir bar, allow the solid to sediment. Wash it by decantation with water several times until the washings are close to neutral pH, and then filter. This process is time-consuming, so if equipment is available, another method is preferable.
Vacuum filtration with on-filter washing is more efficient. The setup consists of a sintered-glass Büchner funnel seated in a stopper on a side-arm Büchner flask (Photo.5). Protect the frit with a wetted filter paper of suitable diameter and connect the side arm to a vacuum source such as a water aspirator or pump.
Transfer the reaction mixture into the funnel in one portion if the vessel volume allows, or otherwise in several portions (Photo.6). Then apply vacuum.
After the first filtration, cover the cake with fresh distilled water, gently stir with a glass rod, and filter again. Repeat this several times until the filtrate approaches neutral pH. Then leave the vacuum on for a few minutes to partially dry the solid (Photo.7).
Remove residual water by drying at room temperature, ideally in a desiccator.
While the crude product is adequate for simple demonstrations, purification is preferable and requires little extra effort.
Purification
Recrystallization offers a straightforward route to purity. Dissolve the crude dibenzylideneacetone in the minimum amount of hot ethanol or another suitable alcohol. I successfully used isopropanol. Remember that alcohols are flammable, so heating must be carried out with caution on an electric hotplate or equivalent and never over an open flame.
Use just enough solvent to obtain a hot, saturated solution. Evaporate a small portion of the solvent, then remove heat and allow the clear yellow solution to cool slowly (Photo.9A).
Within minutes, needle-like crystals typically appear (Photo.9B). Soon, large amounts of product crystallize (Photos 9C and 9D). After the solution has cooled to room temperature, refrigerate for a few hours, for example overnight, to maximize recovery.
After decanting the residual solution, you can admire the well-formed crystals, rewarding both as a synthetic achievement and as an aesthetic sight (Photo.10).
Rinse the crystals with a small amount of the cold ethanol and dry them. Store the purified material for future experiments in a tightly sealed amber bottle, clearly labeled (Photo.11).
Demonstrations
To investigate the optical behavior of dibenzylideneacetone, prepare an ethanolic solution near room-temperature saturation. In contrast to the comparison sample of ethanol without the compound (Photo.12A), the solution is distinctly yellow (Photo.12B). It remains stable when stored in a tightly sealed amber vial kept in the dark.
Place the vessels on a sheet of white printer paper. Such paper contains optical brighteners that fluoresce strongly under UV, which allows us to assess the UV transmittance of each solution. In visible light, whether viewed from the side or from above, both liquids are transparent and differ only in color (Photos 13A and 13B). We may therefore infer that they transmit most visible light.
C – UV light (top view). Left vessel: ethanol; right vessel: ethanol with dibenzylideneacetone
Under UV illumination the picture changes (Photo.13C). The paper fluoresces bright blue, and pure ethanol does not fluoresce, so the paper beneath it glows as brightly as the surroundings. In contrast, the vessel with dibenzylideneacetone appears dark, almost black, when viewed from above, and the paper beneath it shows almost no fluorescence. The effect is visible even with layers only a few millimeters deep (Photo.14).
A – visible light; B – UV light
This confirms that the compound strongly absorbs UV light. We can demonstrate this in another way as well.
Cyanotype, an iron-based photographic process rather than a silver-based one, offers a convenient test. In a previous issue of Chemistry… I described synthesizing potassium tris(oxalato)ferrate(III) K3[Fe3(C2O4)3]·3H2O as a light-sensitive complex and its use in noble photographic processes [4]. Although the compounds employed are not highly toxic, they can be irritating, and hexacyanoferrates in strong acids can release toxic hydrogen cyanide.
To prepare light-sensitive paper, make an aqueous solution containing equal parts of the oxalate complex and potassium ferricyanide K3[Fe(CN)6], at concentrations up to a few percent, optimized empirically. Soak filter-paper disks and dry them. Prepare, impregnate, and dry the paper in darkness or under a red safelight to prevent early exposure. The prepared disk appears green (Photo.15).
Cover the paper with a glass plate, in this case a large Petri dish, crossed with black adhesive tape strips to divide the paper into four sectors (Photo.16). This arrangement allows one disk to test multiple conditions by applying different materials onto the glass:
- sector 1: no coating (control),
- sector 2: glass coated with ethanol plus a few drops of glycerol,
- sector 3: glass coated with ethanolic dibenzylideneacetone plus a few drops of glycerol,
- sector 4: glass coated with sunscreen.
Glycerol slows drying, but for longer exposures the liquids on sectors 2 and 3 should be reapplied periodically. In sector 4, a transparent sunscreen labeled SPF 30 was used, which blocks about 97% of UV-B [5].
Expose the assembly to direct sunlight, adjusting the exposure time experimentally. Artificial light sources may also be used, although they usually require longer illumination. Stop the exposure once the control sector 1 has turned distinctly blue. Disassemble the setup, gently rinse the paper in cool running water to remove unreacted species, and allow it to dry. The result is shown in Photo.17.
Neither the bare glass in sector 1 nor the ethanol in sector 2 provides protection, as shown by the formation of Prussian blue on the paper [6]. This reaction is especially efficient under high-energy UV light. The areas covered with tape remain uncolored. In contrast, the sections treated with the dibenzylideneacetone solution (sector 3) and with sunscreen (sector 4) appear almost clear to the eye but show only faint exposure. Under these conditions, our solution worked nearly as well as the commercial product in blocking UV.
A simpler demonstration is also possible. Deposit a drop of the dibenzylideneacetone solution on fluorescent paper and let it dry (Photo.18). The dried spot is faintly yellow and barely visible in normal light.
Under UV illumination, the spot appears dark against the paper’s bright blue fluorescence (Photo.19).
One more observation can be made by covering part of the spot with black paper and then irradiating with UV light (Photo.20A).
Explanation
The synthesis is an example of an aldol condensation, a base-catalyzed reaction that forms 3-aldols (aldehydes bearing a hydroxyl group at the third carbon relative to −CHO) from two carbonyl substrates. At least one of the substrates must contain an α-hydrogen on the carbon adjacent to the carbonyl group. The medium is typically basic. Substrates may consist of two aldehydes, two ketones, or one aldehyde and one ketone. In this context, ketones are generally less reactive [7].
Aldol condensation can also occur between identical molecules, which yields mixtures. To avoid this, one substrate can be chosen without α-hydrogens, such as benzaldehyde, and conditions can be adjusted accordingly.
Why is it important to add acetone immediately after mixing benzaldehyde with NaOH? In a strongly basic medium, benzaldehyde undergoes the Cannizzaro reaction, which disproportionates it into benzyl alcohol C7H8O and benzoic acid C7H6O2 [8]. Delaying the addition of acetone allows this side pathway to consume benzaldehyde, thereby reducing the yield of dibenzylideneacetone.
The main reaction between benzaldehyde and acetone in a basic aqueous–alcoholic medium (NaOH) is:
Dibenzylideneacetone can in principle form three geometric isomers. Under these conditions the trans-trans isomer dominates, as it is more stable than the cis-cis and cis-trans forms (Fig.3).
In my experiments, the yield was about 75% of the theoretical value, which is a respectable outcome. Literature methods report yields of about 80% or higher under optimized conditions [9].
Dibenzylideneacetone is a yellow crystalline solid. It dissolves well in alcohols but is essentially insoluble in water. Beyond the UV-shielding properties demonstrated here, it also serves as a ligand in organometallic chemistry.
Molecules absorb electromagnetic radiation at wavelengths determined by their electronic structure. The absorbed energy is not lost; it is converted into other forms. In phosphorescence, a portion of the energy is re-emitted at longer wavelengths (Stokes shift), as seen with UV-excited fluorescent whitening agents in paper.
For dibenzylideneacetone, most of the absorbed UV energy dissipates as heat.
The yellow color of the solution indicates that the compound absorbs invisible UV light as well as, to a smaller degree, visible light from the blue region of the spectrum.
Because UV light carries higher energy (shorter wavelength) than visible light, many photosensitive systems respond more strongly to it, as our cyanotype experiment demonstrated. It also showed that even a thin layer of the dibenzylideneacetone solution provides substantial UV attenuation, comparable to commercial products.
There is, however, a caveat. The very efficiency with which dibenzylideneacetone absorbs UV light makes it photolabile. Upon UV excitation, molecules become reactive and undergo cycloaddition, producing a mixture of products. Prolonged irradiation therefore diminishes UV absorption (see Photo.20). Many sunscreen actives share this limitation, which is why the practical advice is to reapply sunscreen at regular intervals. Modern formulations, of course, include additional components that strengthen UV protection.
References:
- [1] Wojtusiak R., Rozróżnianie barw u zwierząt a barwy kwiatów, Kosmos B, 62, 1936, pp. 259-284 back
- [2] Skórska E., Oddziaływanie słonecznego promieniowania ultrafioletowego na organizm człowieka, KOSMOS. Problemy Nauk Biologicznych, 65 (4), 2016, pp. 657-667 back
- [3] Making Sunscreen, w serwisie: https://www.youtube.com/, dostępne online: https://www.youtube.com/watch?v=gavq_sZZ8B0 [dostęp: 27.07.2019] back
- [4] Ples M., Światłoczułe związki w fotografii, Chemia w Szkole, 1 (2018), Agencja AS Józef Szewczyk, pp. 35-41 back
- [5] Wskaźnik ochrony przeciwsłonecznej, w serwisie: https://pl.wikipedia.org/, dostępne online: https://pl.wikipedia.org/wiki/Wska%C5%BAnik_ochrony_przeciws%C5%82onecznej [dostęp: 27.07.2019] back
- [6] Stulik D. C., Kaplan A., Cyanotype, w: The Atlas of Analytical Signatures of Photographic Processes, dostępne online: http://www.getty.edu/conservation/publications_resources/pdf_publications/pdf/atlas_cyanotype.pdf [dostęp: 27.07.2019] back
- [7] Morrison R., Boyd R., Chemia organiczna (tom 1), wyd. II, Wydawnictwo naukowe PWN, 1990r., p. 804 back
- [8] Cannizzaro S., Ueber den der Benzoësäure entsprechenden Alkohol, Liebigs Annalen, 88, 1853, pp. 129-130 back
- [9] Conard Ch. R., Dolliver M. A., Dibenzalacetone, Organic Syntheses, 12, 1932, p. 22 back
All photographs and illustrations were created by the author.
Marek Ples