Abstract

Photochemistry is an important topic which is relevant regarding the usage of solar light as clean energy source. By employing photoreactions, a broad variety of molecular transformations can be conducted which is different from classical thermal chemistry. Ketones provide advantages and thus can be used as model compounds to develop central aspects and principles of photochemistry for teaching purposes. A photochemical experiment for university teaching, dealing with the basic reaction modes of ketones with γ-C-H-bonds, can be introduced and adjusted to the learners capability and pre-knowledge. Different basic characteristics of photochemistry and radical reactions as well as advanced topics and working techniques can be accessed in a hands-on-way using the model reaction. Depending on the equipment and the time frame available, the experiment can be conducted on preparative or on micro scale.

1. Introduction

Photochemistry is a current facet of chemistry and has experienced a renaissance in the recent years ¹ against the background of renewable and clean energy sources and of sustainable syntheses and processes using solar light ^{2, 3}. Its inclusion into university study courses ^{4, 5, 6, 7} and school teaching ^{8, 9, 10} is important because of its relevance in nature, everydays life, industry and education ^{4, 9, 10, 11} as well as because of its interdisciplinarity ^{4, 11}. The Sustainable Development Goals of the United Nations also imply and emphasize its very recent relevance. Photoreactions, however, are based on concepts which are partly different from those of the so-called thermal “dark reactions” ¹¹.

In general, different reaction types can be conducted photochemically, namely by irradiation ^{4, 11, 12}:

- Isomerizations resp. rearrangements (including electrocyclizations and sigmatropic reactions) leading to changings of the constitution or the configuration of the molecules

- Dissoziations (homolysis, heterolysis or cycloreversions) resp. fragmentations and cleavages of chemical species

- Addition resp. covalent bond formation between reaction partners (radical recombinations, radical additions to unsaturated systems and cycloadditions), e.g. starting of radical polymerizations

- Oxidations and reductions

In the course of these reaction types, also strained cyclic compounds can be generated, e.g. by additions and isomerizations.

Ketones are a fundamental class of compounds in organic chemistry. Simple ketones are also known to pupils from everyday life (e.g. acetone as solvent and nail polish remover). The photoreaction modes of the substance class are well studied and include most of the typical photoreactions of organic molecules ^{2, 12, 13}, amongst them (i) the photoreduction of benzophenone and other ketones in the presence of H-atom donors, (ii) the photoenolization of ortho-methyl phenylketones, (iii) the α-cleavage (Norrish type I reaction) and decarbonylation, (iv) the Norrish type II reaction and Yang cyclization of ketones with γ-C-H-bonds, (v) the Paternó-Buechi reaction (oxetane formation) between an excited ketone and an alkene, and (vi) the photosensitization of other species by excited ketones via energy transfer resp. triplet transfer. Due to the nπ*-transition, simple aromatic keto compounds absorb already in the UVA range ⁴ and do readily undergo photochemistry. For both reasons, they are suitable as model substances for developing the topic of photochemistry in laboratory teaching in university settings ¹³.

Butyrophenone has been chosen as model compound for teaching purposes (experimental and theoretical) ^{11, 12, 13, 14}. We have discovered, scientifically investigated and published another reaction pathway which occurs in connection and in competition with the Norrish type II reaction ^{15, 16} (Scheme 1, bottom) and will show in this article how it can be utilized didactically in teaching in university laboratory education.

We propose an laboratory experiment which allows to retrace typical characteristics of ketone photochemistry in a hands-on-, low-cost- and most important up-to-date-way. It allows to be adjusted to (i) the learners pre-knowlegde and abilities, to (ii) the equipment and budget available, and (iii) to the time frame possible.

2. Model Reactions and Didactical Simplification

After photochemical excitation via light absorption and Inter System Crossing (ISC), butyrophenone deactivates mainly via intra- and intermolecular H-atom-transfer (HAT) ¹⁶, shown in Scheme 2.

Via the Norrish type II reaction, a 1,4-biradical is formed by intramolecular HAT, which afterwards undergoes fragmentation to main products ethene and acetophenone enol ^{12, 17, 18, 19, 20, 21, 22, 23, 24}. Acetophenone enol tautomerizes uncatalyzed on the seconds to minutes-scale to acetophenone ^{12, 25, 26}. As a by-product, 1 phenylcyclobutanol is formed via Yang cyclization of the 1,4-biradical ^{12, 17, 27, 28}.

Our investigations show that in competition to intramolecular HAT, products generated by intermolecular HAT arise via the photoredox coupling ^{15, 16}. Excited keto forms attack the O-H-bond of the intermediate acetophenone enol ^{15, 16, 17}, forming phenacyl and ketyl radicals ²⁹. These then react by phenacyl radical addition ^{30, 31} to acetophenone enol, followed by H-atom loss via HAT or via ketyl radical transfer to ground state ketones ³², and by radical recombination to give 1,2-dibenzoylethane ^{31, 33} and pinacols ^{2, 12, 18, 29, 34}.

On the university level, the topic could and should be taught as outlined, e.g. in organic chemistry or photochemistry courses and laboratory teaching settings. It provides the opportunity to develop and deepen the following concepts strongly interlinked with each other ¹⁵:

- Basic radical reactions modes (Scheme 2): In essence, most of the reaction steps considered are simply radical follow-up chemistry of photochemically generated radicals. The topic therefore offers the opportunity to address several important radical reactions of organic molecules ^{12, 35}: (i) Radical substitution (e.g. radical HAT), (ii) radical addition to double bonds, and (iii) radical recombination. The reverse reactions are in each case (i) back-HAT, (ii) fragmentation (analogy to the Mc Lafferty fragmentation in mass spectrometry), and (iii) homolytic cleavage (also a fragmentation).

- Synthetic accessibility of tensed systems via photoreactions, exemplified by 1phenylcyclo-butanol

- The principle of capturing an intermediate species can be strengthened (acetophenone enol scavenged by excited ketones or by phenacyl radicals)

- Keto-enol-tautomerism: A portion of the acetophenone enol is captured before its tautomerization, giving rise to the intermolecular products. Enols may have lifetimes on the minute scale. The Norrish type II reaction is a standard method for the generation of acetophenone enol ^{24, 36, 37}.

- A core characteristic of photochemistry becomes succinctly clear: There is typically a competitive situation ⁷ between several deactivation pathways. The fastest path leads to the main event or product. ^{4, 12}

- The principle behind the intramolecular deactivation pathways can be deepened (Scheme 3): By observing the energetics of the intermediates and products, the fundamental principle becomes clear that nature strives for the energetic minimum. Depending on the pre-knowledge of the learners, reasons regarding molecular structure can be discussed.

In general, the teacher should decide (adapted to the respective learning group) which of the concepts is a learning objective. For teaching the photophysical deactivation processes occuring after excitation (if desired in both cases), a proposal can be used that has already been made available to the community ³⁸.

3. Experimental Setups and Observations

The experiment ¹⁵ can be conducted on micro scale. Advantages are the low costs and the most safe conditions (1 mL reaction volumes). As an option, the whole experiment can be conducted as experimental demonstration by the teacher, in parallel to the ongoing lecture.

By analyzing and comparing thin layer chromatography (TLC) product bands obtained by irradiation of a butyrophenone and a acetophenone solution, the photochemical reactions of butyrophenone can be retraced.

Experiment: The students irradiate 1 mL-HPLC borosilicate vials, containing the butyrophenone or the acetophenone solution respectively (0.05 M in acetonitrile each) and a small magnetic stirrer bar under ventilator cooling, with UVA-LED-, UVA bulb- or UVB bulb light sources using published low-cost equipment for photochemical reactions ³⁹ or LED-solutions commercially available for teaching purposes ⁴⁰ (Figure 1). An exemplary setup is schematically shown in Figure 2.

Prior to irradiation, the reaction vials are shortly flushed with inert gas (nitrogen or argon) by employing needles piercing the septum ³⁹. For pressure compensation, one syringe piercing the septum can be kept during irradiation. The shielding of the experimentalists from the UV-light can easily be realized using the published approach ³⁹. Depending on the light sources used (Table 1), different irradiation times can be compared. Several vials can be irradiated in parallel or for different lengths one after the other.

Results and Observation: After irradiation of butyrophenone, the resulting product mixture can be made visible by fluorecent TLC (e.g. Silica Gel 60 F₂₅₄, Merck, 80%) (Figure 3). Due to the phenyl carbonyl chromophore in butyrophenone, acetophenone and 1,2dibenzoylethane, the compounds are visible under 254 nm TLC-lamp wavelenght.

By comparing the irradiated butyrophenone with the irradiated acetophenone lanes, the products formed from butyrophenone can be observed. The main spot has the same R_f value as acetophenone (assignment of acetophenone). The spot below acetophenone can be attributed to 1,2dibenzoylethane, also by employing a purchased reference (cf. Figure 6 below) or by giving the information to the students.

1-Phenylcyclobutanol can be observed by staining the plate (after photographing) with a diluted potassium permanganate solution in ethanol (Stock solution (based on ⁴¹): KMnO₄ (5 g), K₂CO₃ (33.3 g), NaOH solution (5%, 8.3 mL), 500 mL of bidistilled water. This stock solution was diluted by a factor of 5 with bidistilled water for staining the butyrophenone-photoproduct mixture.). Alcohols can be oxidized by KMnO₄ ^{41, 42}, whereby KMnO₄ is reduced and the spot on the TLC plate loses the purple color. Since the 1phenylcyclobutanol reference is expensive and difficult to purchase, the proof is shown as an example obtained with a purchased reference for comparison (Figure 3).

The acetophenone solution is treated equally, but no visible product bands are observed. This finding can be used to raise the question of the formation mechanism of 1,2dibenzoylethane (“investigative learning”). If the α-C-H-bond would be the donor for an H-atom-transfer (radical H-atom-abstraction), the formation of 1,2dibenzoylethane also from acetophenone should be found, which is not the case (and which has been shown in mechanistic investigations ¹⁶).

Minimal hazards, precautions and disposal of waste: During photochemical experiments, the uninterrupted wearing of UV-intransparent protective goggles is mandatory. The following personal protective equipment is recommended and is assumed to be given in the following: Protective goggles, lab coat, closed footwear and splash protection gloves. The dangers posed by the UV radiation used and the possibility of peroxide formation and the safe control of these have been allready discussed in detail ^{38, 39}; what is said there applies analogously, but an eventual peroxide formation is uncritical and harmless, since only smallest quantities can be formed in the microscale experiment of only 1 mL. The shielding of the experimentalists from the UV-radiation can easily be done by positioning the setup within an enclosure made of cardboard. The setup can be cooled during irradiation using a small ventilator. An open part at the backside of the cardboard housing ensures air exchange and cooling (Figure 2C).

All following dangers and hazards are uncritical and inherently safe due to a smallest volume scale.

- Properties of the organic solvents (highly flammable, toxicity and harmfulness to health).

- Breakage of a reaction vial.

- Ketones used diluted (0.05 M): Butyrophenone (contradiction in the literature: Classified as “harmless” ^{43, 44} or attributed with H317, H318 in older sources), acetophenone (H302, H319 ⁴³), 1,2dibenzoylethane (“harmless” ⁴⁵). Not all the subcomponents formed in smallest traces ¹⁶ are fully toxicologically characterised, so splash guard gloves are advised.

Potassium permanganate is a well known oxidant (H272, H302, H314, H410). Inert gas cylinders should be connected outside the building if possible, otherwise they should be secured against falling over. Care must be taken not to eject much choking gas unnoticed. The usage of small gas cylinders ³⁹ is proposed.

The waste should not be disposed with the normal sewage water but should be collected according to the institutions chemical waste disposal process.

By using the preparative experiment, e.g. in hands-on laboratory organic chemistry courses in universities, in addition to the aspects outlined in the explanation of the microscale experiment, the students can:

- Isolate and characterize 1,2-dibenzoylethane (and optionally acetophenone) via NMR, MS, IR methods if available for teaching purposes in the institution. 1,2-Dibenzoylethane characterization data are given in the Supporting Information. Acetophenone characterization data are available on the internet ⁴⁶. The spectra of both compounds can be compared (similar NMR- and IR-spectra). If a mono-crystal of 1,2dibenzoylethane is obtained, also X-ray characterization can be considered (Supporting Information). Note: The isolation of the small amounts of 1phenylcyclobutanol and of pinacols is difficult and not suitable for teaching purposes.

- Collect the formed ethene resp. gas phase. The alkene presence can be verified by adding a iodine monobromide solution (brown color). The color disappears after shaking the solution in the collected gas phase (halogen addition to the double bond). As a further development of the classical alken detection by means of elemental bromine, the use of iodine monobromide is far less dangerous.

- Learn and practice typical working techniques of preparative photochemistry (oxygene exclusion, irradiation in enclosure).

Experiment: Butyrophenone (0.05 M, 2.25 mL in 300 mL acetonitrile) is filled in a borosilicate 300 mL single-neck Erlenmeyer flask (equipped with a magnetic stirrer bar) and closed with a rubber stopper pierced with two syringe needles (a long needle for the inlet and a short one for the outlet). By flushing with protective gas (Ar or N₂, cf. low-cost equipment ³⁹) for 3 min while stirring, previously dissolved gases are replaced by protective gas. The inert gas flow rate is selected in such a way that a strong “bubble” occurs, but not so strong as to cause solution to leak outwards. After that, both cannulas are quickly closed with two 50 mL syringes, which will serve as low-cost piston samplers to collect the ethene. The closed reaction volume is positioned between the light sources in an enclosure (subject to air exchange), cooled by a small venitlator (Figure 4), and irradiated for 4 hours (Table 2) under stirring with 4 UVB tube lamps and 2 UVA-LED lamps (distance approx. 1 cm to the flask each). If ethene shall not be collected by the students, the irradiation can be paused and samples can be taken using a syringe.

Ethene detection: During irradiation, the pistons of the syringes have been pushed outwards by the successive development of gas. The gas phase is mixed and homogenised by careful one-time flushing it back and forth from one syringe to the other (in the still closed reaction volume). One syringe is completely emptied so that the other syringe is filled. The filled syringe is quickly closed with a luer lock tap (or otherwise). By quick opening, injecting several drops (0.5 mL) of iodine monobromide water (specification: 40 mg IBr in 10 mL bidist. H₂O, solid sediment under the saturated solution), closing again and shaking the closed syringe vigorously, the iodine monobromide water is rapidly decolored by the ethene contained in the gas phase (Figure 5A). This is due to the interhalogen adding to the C-C-double bond ^{47, 48, 49, 50}. Shaking the iodine monobromide solution in an air-filled reference syringe does not cause discoloration.

Solvent removal: Variant 1: Using a rotary evaporator, the solvent is separated in a 500 mL single-neck flask under reduced pressure (60 °C, successively up to 130 mbar). Approximately 1,5 mL of a yellowish and viscous-liquid residue strongly smelling of acetophenone is obtained. The residue is transferred into a small, sealed screw cap glass and allowed to stand for a day or longer (Figure 5B, middle). It becomes darker and crystal formation is observed, consisting of 1,2-dibenzoylethane (as “second main component” besides acetophenone). After separating the supernatant (pipette), the crystals are clearly visible but slightly yellowish (Figure 5B, right). A small part of both the crystals and the supernatant can be dissolved in acetonitrile and subjected to TLC analysis. Optionally, acetophenone (boiling point 202°C ⁴³) can be purified by distillation of the supernatant.

Variant 2: If a rotary evaporator is not available, the following preparative procedure is appropriate: The liquid components of reaction mixture are allowed to evaporate in a large crystallization dish for several days in a fume hood. Afterwards, the residue is completely dissolved twice (each in approximately 20 mL of ethyl acetate) and the ethyl acetate is evaporated again to remove acetophenone rests. As soon as the acetophenone odor no longer appears, it has been approximately separated. The residue is dissolved a third time in 20 mL of ethyl acetate and the crystallization dish is positioned as plain as possible. After the crystallization of 1,2-dibenzoylethane occured, the dish with the still moist content (Figure 5C, left) is placed inclined standing and the supernatant is removed with the pipette. It contains most of the by-products. The crystals are quickly washed with a little cold ethanol (pipette!). Whitish to colorless 1,2dibenzoyl-ethane crystals are obtained (Figure 5C, right) that are purer than those obtained using variant 1, since both acetophenone and other components were removed more thoroughly before crystallization occurred. The TLC examination is similar to that of variant 1, except that no supernatant sample is available because the acetophenone has been evaporated.

The obtained crystals can be characterized depending on the available resources: By spectroscopic methods (NMR, MS, IR, UV-VIS, and X-ray diffractometry), melting point determination (literature values: 142-144 °C ⁵¹, 144-148 °C ⁵², 145-149 °C ⁴⁵ and 145-150 °C ¹¹) or staining (derivatization) on TLC plates with 2,4dinitrophenylhydrazine solution in ethanol (keto group specific reagent, not shown) ^{42, 45}. The crystals can optionally be further purified by (flash) chromatography until colorless crystals are obtained. Alternatively, they can be recrystallized multiple times from ethyl acetate.

Results and Observation: The product mixture can be made visible by fluorecent TLC (e.g. Silica Gel 60 F₂₅₄, Merck, 80% cyclohexane and 20% ethyl acetate as mobile phase, Figure 6), similar to the microscale experiment (Figure 3). In addition, the obtained crystals and the supernatant can be observed separately resp. distinguished. As an option, e.g. if a sample of the reaction mixture has been taken prior to full conversion, a two-dimensional TLC run (2 subsequent runs) can be conducted.

Time required: The preparative experiment requires about 4 hours for the complete butyrophenone turnover; one laboratory day should be planned including assembly, sampling, solvent separation, crystallization and clean-up.

If the solvent is removed without a rotary evaporator overnight, the experiment should be run in parallel with other experiments during the laboratory week.

Risk Assessment of the preparative experiment, hazards and precautions: All aspects mentioned in section 3.1 apply accordingly. Conducting a preparative experiment, the following aspects have to be taken into account. One main hazard is the properties of the organic solvents (highly flammable, possibility of creating an explosive atmosphere, toxicity and harmfulness to health). The irradiation must be done in a housing with high air exchange at room temperature, or similar (cf. Figure 4). In the case of breakage of the flask, the solution can be spilled into a small tray in which the setup is positioned. Solvent evaporation must be done in a fume hood. In the case of rotary evaporator usage, the operating instructions apply. The main hazards are solvent vapours (inhalative) and the possibility of implosion (especially when applying or removing the negative pressure). The rotary evaporator should therefore not be operated openly, but either in a closed enclosure (with sufficient air exchange) or in the fume hood with the front gate closed. Butyrophenone is used diluted in acetonitrile (0.05 M). Not all the subcomponents formed in very small quantities of a few milligrams ¹⁶ are fully toxicologically characterised, so caution is advised when handling (splash guard gloves). The main components are laboratory-standard. The weighing and preparation of the iodine monobromide water can be done by the teacher with protective glasses and gloves. Iodine monobromide is corrosive (eyes, skin) and has specific target organ toxicity (single exposure) category 3 ⁴³. However, it is safer to handle than the very volatile and acutely toxic elemental bromine ⁴³, which is why the teaching experiment was developed using iodine monobromide. Finally, when working with syringe needles, there is a risk of prickling oneself or prick oneself with contaminated needles; syringe disposal containers should therefore be used and recapping is forbidden.

4. Conclusion

Butyrophenone as model system is suitable for the development of advanced concepts through research-based learning ^{53, 54, 55, 56} in universities. The basic reaction modes of ketones with γ-C-H-bonds can be introduced and deepened. The system-immanent HAT donor acetophenone enol creates a competitive situation of intra- and intermolecular reaction possibilities. It is, beside the Norrish type II and Yang cyclization name reactions, a suitable example for dealing with the topics of H-atom transfer, photoreduction resp. pinacolization, side reactions, secondary photochemistry, involvement of intermediates in photoreactions and stabilization of the excited system by different deactivation pathways.

Advantages of this model reaction are the inreactivity, air and oxygen stability and the relative harmlessness of the components of the butyrophenone system.

Several experimental working techniques can be learned: Exclusion of oxygene (for preventing energy transfer to oxygene molecules and subsequent side reactions), irradiation of photochemical setups, detection of gaseous olefins by decoloration of iodine monobromide water, isolation of a compound by crystallization, and staining of thin-film chromatography plates with KMnO₄.

As an option, the model reaction can also be used as microscale experiment in high schools with high-performance learning groups preparing for the A-levels.

Conflict of Interest

The authors declare no conflict of interest.

List of Abbreviations

HATH-atom-transfer

ISCinter system crossing

MEKmethyl ethyl ketone

TLCthin layer chromatography

References

Supporting Information

NMR-spectra were recorded on Bruker DRX 500 (500 MHz), Avance III (300 MHz) or Avance II (300 MHz) spectrometers in the solvents indicated. Chemical shifts (δ) are reported in ppm relative to TMS. Coupling constants (J) are given in Hz. ¹³C spectra were recorded in {¹H}-decoupled manner. Signals were assigned using ¹H, ¹³C, HSQC, DEPT, HMBC, COSY and NOESY experiments.

Mass spectra were measured using the following devices: MS-EI: MAT 95 (Finnigan), MS-APCI and MS-ESI: Impact II (Bruker Daltonik)

IR spectra were recorded on Nexus FT-IR (Thermo Nicolet) or Spectrum-Two-FTIR-ATR (PerkinElmer) at ambient temperature. Wavenumbers () are given in cm⁻¹.

Crystal structure determination was conducted using an AXS Mach3 (Bruker) device. Microscopy for single crystal identification was done using a VHX digital microscope (Keyence), equipped with a VH-Z100R-lens.

Unless otherwise noted, materials obtained from commercial suppliers were used as received without further purification.

The ¹H-NMR, ¹³C-NMR, and IR signals of 1,2 dibenzoylethane obtained have been compared with literature data ⁵⁷. Abbreviations used: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br. = broad signal, overl. = overlapping signals, w. = weak signal. NMR spectra or corresponding images were created and processed by using MestReNova (“Mnova”, version 12.0.4-22023, Mestrelab Research S. L.) and have been partially reprocessed for better clarity.

MS (ESI+): m/z calcd for C₁₆H₁₅O₂ [M+H]+: 239.10720, found: 239.11.

MS (ESI+): m/z calcd for C₁₆H₁₄NaO₂ [M+Na]+: 261.08915, found: 261.09.

MS (ESI+): m/z calcd for C₃₂H₂₈NaO₄ [2M+Na]+: 499.18853, found: 499.19.

MS (APCI+): m/z calcd for C₁₆H₁₅O₂ [M+H]+: 239.10720, found: 239.11.

IR (ATR) cm^-1(relevant parts): 3490 (overlay), 1676 (Carbonyl), 1593 (Ar-C=C), 736, 693.

Crystal structure of 1,2-dibenzoylethane (taken from [59]):