Communication pubs.acs.org/jchemeduc
Bringing Photochemistry to the Masses: A Simple, Effective, and Inexpensive Photoreactor, Right Out of the Box Theint Aung and Charles A. Liberko* Department of Chemistry, Cornell College, Mount Vernon, Iowa 52314, United States S Supporting Information *
ABSTRACT: An inexpensive and commercially available ultraviolet light device, intended for “drying” gel-type fingernail polish, was tested as a photochemical reactor with five organic photochemical laboratory experiments. The device performs satisfactorily and gives product yields comparable to or better than reported results, often in less time. The low cost and ease of use should make photochemical reactions more accessible to undergraduate students.
KEYWORDS: Second-Year Undergraduate, Laboratory Instruction, Organic Chemistry, Laboratory Equipment/Apparatus, Photochemistry
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everal articles1 in this Journal have made the case that, although photochemistry is an important topic in the undergraduate curriculum, it is underutilized because it is often inconvenient or expensive to carry out.2 Suggested sources of ultraviolet light have included specialized or expensive UV sources,3 less expensive “sun” lamps,4 “black lights”,5 standard fluorescent lights,6 or direct sunlight.7 While sunlight is essentially free, it is unreliable and photoreactions in sunlight can take days or weeks to carry out. Some less expensive devices require assembly of electronic parts or a reaction chamber.8 Sun lamps are less expensive than many specialized lamps, but their high wattage bulbs are energy inefficient and produce excess heat that needs to be mitigated.9 In addition, even at a cost of a few hundred dollars per lamp, purchasing enough sources to serve an undergraduate laboratory may prove to be too much for tight budgets. A brief survey of organic laboratory textbooks shows only a small sampling of photochemisty experiments offered in a standard curriculum.10 In the hope of expanding the number of routine photochemical experiments available to undergraduates, a reliable UV photoreactor that would be readily available and inexpensive was sought. A UV light device, intended for “drying” gel-type fingernail polish, was tested (Figure 1) .11 It consists of a highly reflective box (dimensions 14 × 24 × 8 cm) open in front and illuminated by three 18 W UVA fluorescent bulbs with a broad emission peak at λ= 365 nm (Figure 2). The bulbs are arranged with two on top and one in the back of the device so that no place in the chamber is more than 6 cm away from a bulb. The device is intended to expose fingernails to UV light to photocure applied polymer “polish”. During operation, the chamber remains cool to the touch unless direct contact is made with the bulbs. At a cost of approximately $50, it is one of the least expensive devices for a teaching laboratory. © 2014 American Chemical Society and Division of Chemical Education, Inc.
Figure 1. Photographs of the UV light gel nail dryer, Thermal Spa 49135 (top); solid sample (bottom left); liquid samples in holder (bottom center); and liquid samples being irradiated (bottom right).
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EXPERIMENTAL METHODS AND RESULTS The effectiveness of this UV source was tested with five previously reported organic photoreaction experiments, and the results are summarized in Table 1. All of the experiments were performed by an undergraduate researcher with some experience in the laboratory, and the results are considered typical for someone of that skill level. All reagents are commercially available. The reactions were carried out without the use of quartz glassware or special sample holders. Solid samples were sandwiched between two ordinary glass microscope slides held together by paperclips (Figure 1). Liquid samples were placed in stoppered test tubes and held at a 45° angle in the opening of the chamber with an improvised test Published: April 8, 2014 939
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Communication
Dimerization of Anthracene
The photodimerization of anthracene is another fairly wellknown photoreaction and occurs in a variety of solvents. For safety reasons, toluene was chosen rather than benzene for this application. Upon irradiation, a solid began to form within a few minutes, and after three hours of irradiation, an 88% yield of colorless needle-like crystals was obtained. Compared with the literature procedure,14 this method gave a slightly higher yield in a shorter time with a much lower wattage source.
Figure 2. Spectrum of UV drying device, Thermal Spa 49135, fluorescent light source. The spectrum was obtained with an Ocean Optics Jaz spectrophotometer by holding the fiber optic detector cable ∼20 cm from the opening of the device in order to keep the peaks on scale.
Photoreduction of Benzophenone
One of the more commonly used photochemistry laboratories in the undergraduate curriculum is the photoreduction of benzophenone.15 2-Propanol serves as both the solvent and the reducing agent. Irradiation of a solution of benzophenone in 2propanol for 24 h gives a 92% yield of benzopinacol. The yield of benzopinacol is comparable to the literature reaction in sunlight, but the reaction can be carried out in a much shorter time.
tube holder fashioned from a scrap piece of aluminum.12 This particular device can accommodate nine liquid (12 × 120 mm size test tubes) or 15 solid (75 × 25 mm slides) samples (Figure 1). The results from the photochemical experiments show that the low watt UV source compared favorably with the other sources and, in many cases, produced better yields in shorter times. 2 + 2 Photodimerization of Solid trans-Cinnamic Acid
The dimerization of trans-cinnamic acid is interesting because the reaction is known to occur in the solid state as well as in solution. Such a reaction requires that the alkene carbons be aligned and held close enough together in order to undergo a Woodward−Hoffmann thermally forbidden but photochemically allowed 2 + 2 cycloaddition. A thin layer of solid transcinnamic acid (0.020 g) was placed between glass microscope slides. After 3.5 days of irradiation, 1H NMR (DMSO-d) showed the appearance of protons for the cyclobutane ring at δ = 4.24 and 3.80 ppm as well as a nearly complete disappearance in the alkene proton intensities at 7.59 and 6.53 ppm. The ratio of the peaks indicated a near quantitative conversion. This conversion exceeded the reported yield of α-truxillic acid13 from a high pressure UV lamp, even when a shorter exposure time was used. The literature procedure was carried out on a much larger scale (145 g), and the longer reaction time could be attributed to a thicker sample.
Trans−Cis Isomerization of Dibenzoylethylene
In addition to photodimerization and oxidation−reduction reactions, another common type of photoinitiated reaction is the isomerization of alkenes. Canary yellow trans-dibenzoylethylene (freshly recrystallized using decolorizing charcoal) was irradiated for three hours giving 73% yield of colorless flakey crystals. The yield is the same as the procedure carried out in sunlight.10a For this reaction, the decolorizing step was critical to obtaining any product at all. Presumably the colored impurities absorbed UV light, preventing the desired material from reacting.
2 + 2 Addition of BPE-Resorcinol Cocrystal
One of the more recent additions to the undergraduate photochemical repertoire involves the solid-state photodimeriTable 1. Comparison of Photoreactions Carried Out Using the UV Light Gel Nail Dryer to Literature Procedures Photo Reactions 2 + 2 photodimerization of trans-cinnamic acid Dimerization of anthracene Photoreduction of benzophenone Trans−cis isomerization of dibenzoylethylene 2 + 2 addition of BPE-resorcinol cocrystal
Literature Procedure
UV Light Gel Nail Dryer 13a
Suspension in water, 550 W Hanovia lamp, 5 days, 56% yield THF solution, sunlamp 15 days, 85%13b Benzene solution, 125 W Hg high pressure lamp, 5 h, 83% yield14a Benzene solution, sunlight, 7 days, 56%14b 2-Propanol solution, sunlight, 4 days, 95% yield15 Ethanol solution, sunlight, unspecified time, 73% yield10a Solid, 500 W medium pressure Hg lamp, 18 h, 80−100% yield16 940
Solid, 3.5 days, >99% yield Toluene solution, 3 h, 88% yield 2-Propanol solution, 1 day, 92% yield Ethanol solution, 3 h, 73% yield Solid, 12 h, 81% yield; 24 h, >99% yield
dx.doi.org/10.1021/ed4000316 | J. Chem. Educ. 2014, 91, 939−942
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wavelength that would allow for the illumination of fluorescent TLC plates, it may be used for more than just preparative chemistry. Photochemical reactions were tested successfully directly in NMR tubes. This is a particularly convenient way to follow the progress of a reaction and may be useful for performing kinetics experiments on photochemical reactions.
zation of trans-1,2-bis(4-pyridyl)ethylene, BPE, where the molecules are aligned and held in place by hydrogen bonding to the resorcinol template. The BPE-resorcinol cocrystal was placed between glass microscope slides and irradiated. The reaction progress was followed by 1H NMR (DMSO-d6) by monitoring the disappearance of the alkene protons (δ = 7.53 ppm) and the appearance of the cyclobutane protons (δ = 4.64 ppm). By monitoring the change using NMR, the reaction was found to be 24% complete after one hour, 81% after 12 h, and nearly 100% complete after 24 h. The solid state 2 + 2 photoaddition of 1,2-bis(4-bispyridyl)ethylene as a resorcinol cocrystal proceeded at about the same rate and with the same yields as with the 500 W mercury lamp used in the literature.16
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CONCLUSION The performance of the device for carrying out photoreactions was found to be quite satisfactory. For the conjugated compounds examined, special quartz glass or deep UV sources were not required. These reactions proceeded at the longer wavelengths produced by this source. The relatively low wattage bulbs were more energy efficient than higher watt systems, and they had enough intensity to carry out a variety of reactions in a reasonable time. The low cost, commercial availability, and ease of use of this device make photochemical experiments more available to undergraduates. This device should also allow for the possibility of more open-ended experiments where the effects of solvents or reaction conditions can be easily probed.
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ASSOCIATED CONTENT
S Supporting Information *
Reaction details, NMR spectra, and information on lamp stability. This material is available via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
HAZARDS Although no warning is given with the UV device, it should only be operated while wearing UV blocking goggles. Whenever the device was operated, the opening was turned toward the wall. Anthracene, benzophenone, and CDCl3 are carcinogens. Toluene, benzophenone, and resorcinol are toxic. Anthracene, cinnamic acid, DMSO-d6, CDCl3, ethanol, 2propanol, and BPE are irritants. Toluene, ethanol, 2-propanol, and DMSO-d6 are flammable. Toluene is also listed as a teratogen. Gloves should be worn when handling toxic or carcinogenic substances.
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work received financial support from Cornell College. We are grateful to Len MacGillivray and John Sander (U. Iowa) for sharing their expertise in photochemistry. The UV−vis spectrum was obtained by John T. O’Connor (Cornell College).
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DISCUSSION This type of UV source offers several advantages over other sources. Most importantly, it is inexpensive. Although some individual UV bulbs may be less expensive than this device, they require a power source to be purchased or constructed and a reaction chamber to be devised. This particular source has the added advantage of coming in a convenient sized, highly reflective box. The entire device is small enough to fit into a glovebox or glovebag if an inert atmosphere is required. Additionally, its small size means that it can be placed on top of a magnetic stirring motor allowing liquid samples to be stirred during irradiation. The chamber remains cool enough so that reactions can be carried out in low boiling solvents, such as dichloromethane. The bottom panel is also removable, allowing further flexibility of configuration such as placing the device on end to illuminate a refluxing sample. The low wattage bulbs can be turned on and off without the warm-up and cool-down times that restrict the cycle times of higher wattage systems. Furthermore, these low wattage bulbs do not require cooling water. Although this particular device does not operate at a
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