by Marek Ples
| * |
|
| Johann Wolfgang von Goethe |
| poet, playwright, novelist, scientist, statesman, theatre director, critic (1832) |
| 4-Bromophenylmagnesium bromide (o.) | |
| Green tea polyphenols (o.) | |
| Lophine or |
|
| Lucygenin or |
|
| Luminol or |
|
| Manganese(II) (ao.) | |
| Nitrogen (ao.) | |
| Oxalyl chloride (o.) | |
| Safranin or |
|
o. - organic compound; ao. - anorganic compound |
|
Of course, these are not all of the chemiluminescent reactions I'm working on. I will be adding more descriptions here soon. |
A Grignard reagent, also known as a Grignard compound, is a type of chemical compound characterized by the general formula R−Mg−X, where X represents a halogen and R denotes an organic group, typically an alkyl or aryl group.
Grignard compounds are widely utilized reagents in organic synthesis for the formation of new carbon-carbon bonds. When combined with another halogenated compound R'−X' in the presence of a suitable catalyst, they typically produce R−R' and magnesium halide (MgXX') as a byproduct. The resulting magnesium halide is insoluble in commonly used solvents.
Pure Grignard reagents are highly reactive solids and are typically handled as solutions in solvents such as diethyl ether or tetrahydrofuran. These solvents offer stability to the compounds. In such solvent media, a Grignard reagent exists in the form of a complex.
In 1906, Wedekind made the initial discovery of luminescence connected to a Grignard reagent. His observations revealed a striking green glow when a solution of phenylmagnesium bromide or iodide in ether reacted with chloropicrin [1]. Five years later, Heczko made a significant advancement by demonstrating that Grignard reagents emit light when exposed to oxygen from air. He described a demonstration using this reaction to produce light visible in a large auditorium [2].
During my experiments with various Grignard compounds, I decided to investigate the chemiluminescent capabilities of these substances. To do so, I placed 1,4-dibromobenzene C6H4Br2 and metallic magnesium Mg turnings in a round-bottom flask, using anhydrous diethyl ether (C2H5)2O as the reaction medium. To initiate the reaction, a few small crystals of iodine were added to the mixture, followed by gentle heating of the flask. The reaction is sensitive to the presence of water, so the reflux condenser should be equipped with a tube containing a drying agent, such as anhydrous calcium chloride CaCl2. The result was a cloudy solution of the Grignard reagent, specifically 4-bromophenylmagnesium bromide C6H4Br2Mg (Fig.1).
Fig.1 - 4-bromophenylmagnesium bromide in diethyl ether
The synthesized compound is highly reactive. Simply placing the solution in an open container (Fig.2A) and allowing air to flow in will result in the emission of bright blue light (Fig.2B) [3].
Fig.2 - Chemiluminescence of the Grignard reagent; A - solution in container, B - emmission of light
Bright glow can also be observed after applying a few drops of the solution onto a filter paper using a Pasteur pipette.
Tea, the second most consumed beverage worldwide after water, is classified into three types based on different processing methods: green tea (nonfermented), oolong tea (partially fermented), and black tea (fully fermented).
Numerous studies have demonstrated the potential disease prevention properties of tea consumption, which can be attributed to its polyphenol content. Polyphenols are secondary metabolites that plants produce to defend against environmental stresses like UV radiation and pathogens. The primary polyphenols found in tea are flavonoids, specifically flavanols (catechins), and phenolic acids. Tea is also a significant source of gallic acid. Catechins make up 30-42% of the water-soluble solids in brewed green tea. The four major tea catechins are epicatechin C15H14O6, epicatechin gallate C22H18O10, epigallocatechin C15H14O7, epigallocatechin-3-gallate C22H18O11.
To conduct the described experiment, readily available substances are required, including green tea sencha (Fig.3). Additionally, paraformaldehyde OH(CH2O)nH (n = 8 - 100), sodium carbonate Na2CO3, and hydrogen peroxide H2O2 are needed. Caution must be exercised when working with these substances, as formaldehyde is toxic, sodium carbonate irritates the eyes, and hydrogen peroxide in higher concentrations is highly corrosive.
Fig.3 - Sencha tea
The experiment involves preparing a tea infusion, with further dissolving paraformaldehyde and sodium carbonate in it (Fig.4A). After cooling, hydrogen peroxide is added to the solution, resulting in the emission of red-colored light (Fig.4B), foaming, and a change in the color of the liquid to yellow [4].
Fig.4 - Chemiluminescence of the green tea; A - green tea, B - chemiluminescence
The chemiluminescence effect sparks great interest and often surprises students. As you can see, a teacup can be used as a vessel in this experiment, which provides a beautiful visual effect.
The combination of polyphenol and alkaline hydrogen peroxide, along with the presence of formaldehyde, has been found to produce a remarkably vibrant red chemiluminescence. This particular reaction, referred to as the Trautz-Schorigin reaction, is characterized by its distinct red emission, which is thought to be generated through the production of excited singlet oxygen molecular species, specifically 1O2* [5].
Lophine C21H16N2 can be prepared by heating hydrobenzamide C21H18N2 to about 300°C. The reaction involves ring closure to form an intermediate heterocyclic compound, amarine C21H20N2 at about 130°C. On further heating this dehydrogenates to form lophine [6].
Lophine can be synthesized with higher efficiency from benzaldehyde C6H5CHO, benzil C14H10O2, and ammonium acetate C2H7NO2 as a donor of ammonia NH3. The reaction is conducted in boiling acetic acid C2H4O2 [7].
As a historical sidelight, these reactions were investigated by the Russian organic chemist, Alexander Borodin (1833-1887) in his scientific paper. Today Borodin is better known as a composer than a chemist, although he always regarded chemistry as his vocation and music as his hobby [8].
I have tried both described methods of lofin synthesis, and each of them yielded good results.
Lophine, under normal conditions, appears as a white crystalline substance (Fig.5) [9].
Fig.5 - Crystals of lophine
lophine emits a yellow light when it reacts with oxygen in the presence of a strong base (Fig.6, Vid.1).
Fig.6 - Chemiluminescence of lophine solution in a bottle
Vid.1 - Chemiluminescence of lophine
Lucigenin C28H22N4O6 is perhaps the most commonly used chemiluminescent probe for the detection of superoxide in the cells and tissue. It is important to note that lucigenin is subject to redox cycling in the presence of endogenous cellular reductases, which can result in direct superoxide generation by the probe itself [10].
The synthesis of lucigenin is highly interresting as it encompasses various processes such as nucleophilic and electrophilic aromatic substitutions, nucleophilic aliphatic substitution, reductive coupling, and oxidation reactions. The key techniques utilized in this works include steam distillation, charcoal decolorization, Soxhlet extraction for recrystallization, and the application of an aprotic solvent to enhance ionic reactions [11].
The lucigenin synthesized by me has the form of orange, flake-like crystals (Fig.7).
Fig.7 - Crystals of lucygenin
Lucigenin is relatively soluble in water, and its diluted solution has a yellow-orange color (Fig.8).
Fig.8 - Solution
Lucigenin exhibits its strong chemiluminescent properties during oxidation by hydrogen peroxide in an aqueous alkaline solution (Fig.9) [12].
Fig.9 - Chemiluminescence of lucygenin; A - initial stage, B - late stage
Interestingly, the color of light emitted through lucigenin chemiluminescence changes over time. Initially, its color is yellow-green (Fig.9A), and then it transitions to a noticeably more blueish (Fig.9B). This is associated with the molecular mechanism of the described process. Chemiluminescence occurs when the product of an exothermic reaction is formed in an excited electronic state. Both lucigenin and the major product of the reaction, N-methylacridone, display strong fluorescence. According to the most plausible reaction mechanism, lucigenin is oxidized to an unstable peroxide, which then undergoes a thermally forbidden -[2 + 2] cleavage reaction, resulting in the formation of electronically excited N-methylacridone. Initially, energy transfer takes place, leading to the emission of light with the color of fluorescent lucigenin. However, as the reaction progresses, the emitted light gradually shifts towards a bluish shade, characteristic of fluorescent N-methylacridone.
Luminol C8H7N3O2 exists in normal condition as a solid with a white-to-pale-yellow color (Fig.10) and can dissolve in most polar organic solvents but remains insoluble in water [13].
Fig.10 - Luminol
Described compound displays chemiluminescence, producing a blue glow, when combined with a suitable oxidizing agent, eg. hydrogen peroxide H2O2 in presence of catalizator such as potassium ferricyanide K3[Fe(CN)6] (Fig.11) [14].
Fig.11 - Ferricyanide-catalyzed chemiluminescence of luminol; A - solutions (left/colorless - luminol and hydrogen peroxide, right/yellow - ferricyanide), B - chemiluminescence after mixing both solutions
A beautiful effect also occurs when small crystals of ferricyanide are added to an alkaline solution of luminol and hydrogen peroxide - we can then observe luminous blue trails (Fig.12).
Fig.12 - Glowing trails
Forensic professionals utilize luminol to identify small quantities of blood at crime scenes by reacting with the iron present in hemoglobin (Fig.13) [15].
Fig.13 - Blood testing with luminol; A - hemoglobin smeared on blotting paper, B - after spraying the stain with an alkaline solution of luminol with oxidizer, visible blue chemiluminescence
An alternative demonstration of luminol chemiluminescence can be catalysed by a range of transition metal compounds, including those of cobalt Co and copper Cu (Fig.14) [16].
Fig.14 - Chemiluminescence of luminol catalyzed by copper wire
The production of light during the oxidation of luminol is even more efficient when the reaction is conducted in a solvent other than water, such as dimethyl sulfoxide DMSO C2H6OS and dimethylformamide DMF C3H7NO. In such cases, a special oxidiser agent and catalyst is not required - oxygen from the air is sufficient. The color of the generated light is slightly more greenish compared to the reaction takes place in an aqueous environment (Fig.15).
Fig.15 - Chemiluminescence in DMSO
The color of luminol chemiluminescence can be altered using fluorescein C20H12O5 [17]. A solution of luminol with the addition of this substance is bright orange (Fig.16A), whereas the color of chemiluminescence changes to yellow-green instead of blue due to the transfer of excitation energy from the oxidation products of luminol to the fluorescein molecule (Fig.16B) [18].
Fig.16 - Green chemiluminesccence with luminol; A - alkaline solution of luminol with the addition of fluorescein, B - chemiluminescence
Biologists use luminol in cellular assays to detect various substances such as copper, iron, cyanides, and specific proteins through western blotting.
Potassium permanganate KMnO4 has fascinated chemists since its discovery in the middle of XVIIth century. Surprisingly, its potential as a chemiluminescence reagent has been largely overlooked. One of the simplest chemiluminescent reactions involves the reaction between potassium permanganate and sodium borohydride in an aqueous solution [19].
Potassium permanganate appears purplish-black in its solid form and exhibits intensely pink to purple colors in solution (Fig.17). It's caused by its permanganate anion, which gets its color from a strong charge-transfer absorption band caused by excitation of electrons from oxo ligand orbitals to empty orbitals of the manganese(VII) center [20].
Fig.17 - Potassium permanganate solution
To begin the demonstration, the room lights should be turned off. Sodium borohydride NaBH4 solution is then poured slowly into a 500 mL beaker containing solution of potassium permanganate and hexametaphosphate Na6[(PO3)6]. An intense and rapid red-orange chemiluminescence emission, filling the entire beaker, is observed then (Fig.18).
Fig.18 - Chemiluminescence of manganese(II)
The presented chemiluminescent reaction most likely proceeds according to the equation (1).
| (1) |
As you can see, the substrates of the reaction are permanganate (VII) MnO4- and borohydride BH4- anions, along with hydrogen cations H+. Sodium hexametaphosphate acts as an auxiliary component. In addition to water H2O and boric acid H3BO3, the reaction also produces manganese (II) Mn2+. These ions play a crucial role in the chemiluminescence process as they initially exist in an excited state with high energy. However, this situation is unstable, and the ion quickly transitions to a stable ground state following (2).
| (2) |
According to the conservation law, the energy difference between the excited and the ground state is transferred to the environment. In most cases, this occurs in the form of heat, but in chemiluminescence, a portion of that energy is converted into electromagnetic radiation in the form of visible light. In this case, the wavelength at the peak emission is approximately 690 nm, corresponding to red light [21].
Apart from more or less well-known chemiluminescent reactions, there are also those that are heard and read about much less frequently. In many cases, the reason is the limited amount of information available on this topic in literature. It may also be due to the less visually striking effect of the considered reaction or the unavailability and often high cost of the required reagents. To change this situation a bit, I would like desrcribe a reaction, which - despite the clearly noticeable light produced during this process and the fact that the required reagents are very inexpensive - is almost unknown [22]. However, I must admit that although the mentioned reaction is conceptually straightforward, conducting it safely requires careful preparations.
Nitrogen N is a nonmetal and the lightest member of group 15 on the periodic table. It is the seventh most abundant element in the universe and makes up about 78% of Earth's atmosphere as diatomic N2 gas. Nitrogen is essential to all organisms, found in amino acids, nucleic acids, and so on. It is the fourth most abundant element in the human body. Nitrogen compounds like ammonia NH3, nitric acid HNO3, organic nitrates and many more have industrial importance.
Interestingly, through a chemical reaction, we can easily generate excited state gaseous nitrogen that emits light easily observable to the naked eye. To do this, it is necessary to build the appropriate apparatus. It consists of a set of laboratory vessels forming a chlorine Cl gas generator (Fig.19a), a reaction chamber (Fig.19b), and an absorber (Fig.19c), where any remaining chlorine will be neutralized. Gas washing bottles were used as vessels b and c.
Fig.19 - Apparatus
To produce chlorine, I used a reaction in which hydrochloric acid HCl(aq) reacts with calcium hypochlorite Ca(ClO)2. This substance is commonly used for disinfecting swimming pools and for bleaching paper, fabrics, and other materials. The mentioned reaction can be described by the equation (3).
| (3) |
This reaction is the most economical in terms of the ratio of acid consumed to chlorine produced. Based on my experience, by carefully controlling the rate of acid addition, it is easy to achieve a suitable and uniform flow of generated chlorine [23].
The actual chemiluminescence reaction occurs in the second vessel while passing the stream of chlorine gas through concentrated ammonia solution. This results in the emission of yellow-orange light.
Fig.20 - Chemiluminescence of excited nitrogen; A - ammonia solution in the reaction chamber, B - chemiluminescence
In alkaline solution, two main chemical reactions are likely to occur. The first one is the direct reaction between ammonia and chlorine (4), while the second one involves the hypochlorite anion (5) [24].
| (4) |
| (5) |
There is evidence that the mechanism of chemiluminescence in this case is based on the excitation of nitrogen formed in the reaction [25]. Nitrogen is likely formed in a highly excited triplet state, T2. The yellow light hν may originate due the transition from the excited triplet state T2, to the triplet ground state T1, followed by a nonradiative transition to the singlet ground state, S0.
| (5) |
| (6) |
Oxalyl chloride is an organic chemical compound with the formula C2O2Cl2. This colorless, sharp-smelling liquid, the diacyl chloride of oxalic acid C2H2O4, is a useful reagent in organic synthesis. Oxalyl chloride was first prepared in 1892 by the French chemist Adrien Fauconnier, who reacted diethyl oxalate C6H10O4 with phosphorus pentachloride PCl5 [26]. It can also be prepared by treating oxalic acid with phosphorus pentachloride.
Oxalyl chloride can be used for the presentation of sensibilized chemiluminescence. For this purpose, I prepared three Erlenmeyer flasks and placed 10 cm3 of 30% hydrogen peroxide H2O2 and 20 cm3 of
Fig.21 - Solutions;
from left to right: rhodamine B, 9,10-diphenylanthracene,
2-chloro-9,10-bis(phenylethynyl)anthracene
To initiate the reaction, 10 cm3 of a 5% solution of oxalyl chloride in dioxane was added to each flask, which resulted in the emission of bright light in various colors (Fig.22).
Fig.22 - Chemiluminescence with oxalyl chloride;
from left to right: rhodamine B, 9,10-diphenylanthracene,
2-chloro-9,10-bis(phenylethynyl)anthracene
As observed, the addition of
Safranin
It is a little-known fact that safranin is also a useful as an example of a chemiluminescent compound. To observe this, we need to prepare a low concentrated solution of this dye in isopropyl alcohol
Fig.23 - Safranin; A - prepared solution, B - chemiluminescence of dye reacting with ozone
To observe the chemiluminescence of safranin, we need to pass a stream of bubbles of air enriched with ozone O3 through its alcohol-based alkaline solution. We can then see the emission of bright green light (Fig.23B).
For more information, please visit www.weirdscience.eu. You can also contact me by email (moze.dzis@gmail.com).