Undergraduate Organic Experiment: Tetrazole Formation by

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Undergraduate Organic Experiment: Tetrazole Formation by Microwave Heated (3 + 2) Cycloaddition in Aqueous Solution Heather DeFrancesco, Joshua Dudley, and Adiel Coca* Department of Chemistry, Southern Connecticut State University, New Haven, Connecticut 06515, United States S Supporting Information *

ABSTRACT: An undergraduate experiment for the organic laboratory is described that utilizes microwave heating to prepare 5substituted 1H-tetrazole derivatives through a (3 + 2) cycloaddition between aryl nitriles and sodium azide. The reaction mixture is analyzed by thin layer chromatography. The products are purified through an acid−base extraction and recrystallization. Characterization is accomplished using melting point, infrared, 1H NMR, and 13C NMR spectroscopy. Students are tasked with determining and confirming the structure of their product based on their assigned nitrile and characterization data. KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Inquiry-Based/Discovery Learning, Hands-On Learning/Manipulatives, Addition Reactions, Green Chemistry, Heterocycles, Catalysis, Laboratory Instruction, Acids/Bases

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INTRODUCTION

The use of microwave heating to accelerate reactions as part of undergraduate experiments has been described.1−3 The authors in these previous reports have discussed the many advantages that microwave heating offers such as fast reaction times and high conversion rates. There are several reports that introduce pericyclic reactions into the organic teaching laboratory.3−8 The most common among these is the Diels−Alder reaction.3,4 Undergraduate students are typically exposed to (3 + 2) cycloaddition reactions during second-year undergraduate organic chemistry through reactions such as the oxidative cleavage of alkenes and alkynes by ozone, potassium permanganate, or osmium tetroxide. Most organic textbooks devote an entire chapter to pericyclic reactions, which includes a discussion about cycloadditions. “Click” chemistry to make triazoles has been previously reported as part of an undergraduate experiment.5−8 The (3 + 2) cycloaddition between benzonitrile oxide and styrene has been described as an experiment for the undergraduate laboratory.9 Herein, the synthesis of tetrazoles through the (3 + 2) cycloaddition between aryl nitriles and sodium azide utilizing microwave heating is described. The reactions are performed in an aqueous isopropanol solvent mixture. To the best of our knowledge, this is the first report that describes the synthesis of tetrazole derivatives as part of an undergraduate experiment. A number of applications have been found for the tetrazole ring including being used as bioisosteres for carboxylic acids.10 In addition, a number of approved drugs that treat hypertension and congestive heart failure contain a tetrazole ring.11 These include losartan (Figure 1), irbesartan, olmesartan, candesartan, valsartan, and fimasartan. Because of the tendency of the tetrazole ring to decompose and release nitrogen gas under high heat, some tetrazole derivatives, such as 5aminotetrazole, have also been studied as propellants in airbags as well as in pyrotechnics and explosives.12 The antibacterial © XXXX American Chemical Society and Division of Chemical Education, Inc.

Figure 1. Structures of some tetrazole-containing compounds.

properties of tetrazole-containing compounds have also been reported.13 The tetrazole ring has been synthesized by a number of methods. The most common route to the tetrazole ring is through the (3 + 2) cycloaddition reaction of sodium azide (NaN3) with an organonitrile (Scheme 1).14−17 This reaction Scheme 1. Cycloaddition Reaction To Form Tetrazole Derivatives

was first reported in 1901, and it is typically catalyzed by a Brønsted or Lewis acid and requires heating for several hours using conventional heating or a few minutes/hours with microwave radiation.18 Received: August 10, 2017 Revised: January 29, 2018

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DOI: 10.1021/acs.jchemed.7b00617 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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Laboratory Experiment

PEDAGOGICAL SIGNIFICANCE In the two-semester, undergraduate introductory organic chemistry laboratory sequence, students first learn many of the basic laboratory techniques and purification methods during the first semester of the course. In the second semester, lab experiments aim to review fundamental organic reactions covered in lecture while at the same time reviewing lab techniques learned in the first semester. This experiment covers a fundamental type of organic reaction (cycloadditions) as well as many important techniques typically covered in the undergraduate organic chemistry laboratory. Given the acidic (similar to carboxylic acids) and crystalline nature of 5substituted aryl 1H-tetrazoles, these compounds are ideally suited for teaching basic organic techniques at the undergraduate level. These compounds can be purified simply through an acid−base extraction and recrystallization. Although column chromatography can also be used to purify the products, it is usually not necessary. Aryl 5-substituted 1Htetrazoles are solids that give sharp melting points. Furthermore, the tetrazole products can be characterized through diagnostic absorptions in IR and 13C NMR spectroscopy. Students are also exposed to microwave heating in this experiment as well as the use of a rotary evaporator. Thus, students gained important hands-on experience with essential laboratory equipment. The pedagogic goals of this experiment are for each student to (1) synthesize an aryl 5-substituted 1H-tetrazole derivative using microwave heating; (2) monitor the reaction via thinlayer chromatography (TLC); (3) purify their product through acid−base extraction; (4) identify their product via melting point as well as infrared (IR), 1H NMR, and 13C NMR spectroscopy; and (5) have a deeper understanding of cycloaddition reactions. After completing this experiment, students should feel more comfortable in performing laboratory experiments involving multiple organic techniques as is often the case in research. In addition, students should be more capable of interpreting spectral data, in particular, 13C NMR and IR spectral data. Lastly, students should be more knowledgeable in difficult lecture topics such as pericyclic reactions, heterocycles, and aromaticity.

The characterization is completed during the following week to allow the products to dry thoroughly. The yield of the product is determined the following week, and the product is characterized by melting point as well as IR, 1H NMR, and 13 C NMR spectroscopy. The tetrazole derivatives are not sufficiently soluble in deuterated chloroform. Therefore, NMR samples are prepared in dimethyl sulfoxide-d6 (DMSO-d6). Students determine the structure of their product based on their assigned nitrile and characterization data. More details can be found in the Supporting Information.

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HAZARDS Goggles and gloves should be worn during this experiment, and the experiment should be performed in a properly ventilated chemical hood. The reaction should be performed in a microwave reactor that is intended for laboratory use. The use of sodium azide in an undergraduate experiment has been previously described, but care should be taken due to its hazards.7,8 Sodium azide is very toxic and should be handled with gloves to avoid contact with skin. Large quantities of sodium azide should not be mixed with acids to avoid formation of large quantities of hydrazoic acid, HN3, which is a toxic, volatile, pungent-smelling, and explosive liquid. Dilute aqueous solutions of hydrazoic acid (less than 5%) are regarded as safe to handle. Waste containing sodium azide should be handled separately from other types of waste. Sodium azide should not be disposed down the sink. Sodium azide should not be mixed with heavy metals to avoid formation of explosive compounds. Hydrochloric acid spills on the bench or on the skin should be thoroughly washed with cold water. 4Nitrobenzonitrile is acutely toxic (orally and to aquatic life). 4-Chlorobenzonitrile is acutely toxic (orally and through inhalation) as well as an irritant (skin and eye). 4Methoxybenzonitrile and 4-acetylbenzonitrile are acutely toxic orally. 2-Nitrobenzonitrile is acutely toxic (orally and through inhalation). 2-Chlorobenzonitrile and iron(III) chloride are both acutely toxic (orally) and skin irritants. 4-Methylbenzonitrile is an irritant (skin and eye) and acutely toxic to aquatic life. Benzonitrile is acutely toxic (orally, through skin, and to aquatic life) as well as a flammable liquid. Zinc chloride is acutely toxic (orally and to aquatic life) and corrosive to skin and eyes. Cerium chloride heptahydrate is an eye irritant and corrosive to skin. Isopropanol and ethyl acetate are eye irritants and flammable liquids. Dimethyl sulfoxide-d6 is a flammable liquid. The hazards of tetrazole compounds are largely unknown but as a general precaution they should be kept away from heat/ sparks/open flames/hot surfaces to avoid possible explosions.

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EXPERIMENTAL OVERVIEW Students work individually on the experiment. They are assigned an aryl nitrile and a Lewis acid catalyst. Each student mixes the assigned nitrile (2 mmol) with sodium azide (4 mmol), cerium chloride heptahydrate, zinc chloride, or iron(III) chloride (0.4 mmol) and a 3:1 isopropanol/water mixture (8 mL) in a 25 mL glass microwave vessel. The nitriles, catalysts, and other reagents used in this experiment were chosen partly because they are all fairly inexpensive. The nitriles used were also chosen to include a variety of secondary functional groups. After capping the microwave vessels, they are placed in a carousel with capacity for 16 vessels and heated in a microwave reactor. The reaction vessels are heated for 30 min at 160 °C plus an additional 5 min to reach this temperature. The power in the instrument is set to a maximum of 1000 W. If time is not an issue, the reactions can be heated for an additional 30 min (1 h) to improve yields. After the reactions are cooled for 10 min, the reaction mixtures are analyzed by thin layer chromatography. The products are purified using acid−base extraction. The organic layer is dried with sodium sulfate, and the solvent is removed using a rotary evaporator.

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RESULTS AND DISCUSSION

The synthesis and purification of the tetrazole derivatives for this experiment were performed in a 4 h lab period (most students completed the experiment within 3 h) as discussed below. The experiment described here was first fully tested by the authors of this paper (two undergraduate students and faculty advisor). It was then performed by approximately 84 undergraduate students in organic chemistry (seven sections of approximately 12 students each) during two different semesters over the past year. The experiment was the penultimate laboratory experiment in the second semester of the organic chemistry sequence to coincide with the lecture coverage of heterocyclic rings and concerted reactions. Typical student yields (excluding outliers) before recrystallization for each B




Table 1. Typical Student Results

a

See ref 14. bSee ref 19. cSee ref 20. dSee ref 21. eSee ref 22. fSee ref 23.

spectra due to the carbon in the tetrazole ring. This absorption is usually broad. The tetrazole products can also be characterized by 1H NMR spectroscopy. The 1H NMR spectra were used to teach typical splitting patterns produced by different types of substituted aryl rings. However, the 1H NMR spectra alone could not be used to distinguish the products from the reactants because the 1H NMR spectra for both are nearly identical. The tetrazole NH was often not observed in the 1H NMR spectra. It typically shows up around 16 ppm.21 All nitriles and tetrazole products were UV-active. The tetrazole products were more polar than the nitriles and generally had a retardation factor (Rf) of about 0.2 or less with ethyl acetate as the developing solvent and using silica TLC plates. The nitriles generally had an Rf of about 0.9 under these conditions. A small amount of unreacted nitrile was seen in most reactions before the extraction. Representative student IR and NMR spectra as well as TLC drawings can be found in the Supporting Information (Figures S1−S36). This was the penultimate experiment in the second semester of undergraduate organic chemistry. The experiment served nicely to review several laboratory techniques that students had previously learned in the course such as acid−base extraction, TLC monitoring, melting point determination, and evaporation through a rotary evaporator. In addition, structure determination through acquired spectral data was reviewed one more time through this experiment while at the same time reviewing concepts from lecture such as pericyclic reactions, heterocycles, and aromaticity. The student learning outcomes for the experiment are listed in Table 2. We set 75% or higher as the benchmark for the percentage of students that should be

tetrazole product are listed in Table 1. The yields after recrystallization were generally 5−20% lower. The products are fairly pure even without recrystallization because the main impurities (aryl nitrile, sodium azide, and catalyst) are removed during the acid−base extraction. The melting point data given in Table 1 and the representative spectra in the Supporting Information were obtained before recrystallization. The students in one lab section recrystallized their tetrazole products, but no significant improvement was observed in the purity of the tetrazole products used in this experiment based on melting point and NMR data. Three catalysts (cerium chloride heptahydrate, zinc chloride, and iron(III) chloride) were used for this experiment, and these were largely chosen based on cost. All three performed well in the reaction, but zinc chloride generally gave higher yields with the fewer reactive nitriles. The yields can be improved approximately 10−15% for most nitriles by heating the reactions for 1 h instead of 30 min. As can be seen in Table 1, substrates with electron withdrawing groups generally gave better yields since electron withdrawing groups make the nitriles more electrophilic. This pattern has been noted previously in the literature.14−17 The reaction could also be performed using dimethylformamide (DMF) as the solvent but the 3:1 isopropanol/water mixture is more environmentally friendly and has less odor than DMF. The products are crystalline and gave melting points fairly close to the literature values. The IR spectra of the products showed a broad NH absorbance around 3300−3400 cm−1. Students looked for the lack of the nitrile absorbance around 2100 cm−1 in their IR spectra. The products also had a resonance at approximately 150−160 ppm in their 13C NMR C




Research Grant, and a Southern Connecticut State University School of Graduate Studies Graduate Assistantship. The authors thank the undergraduate students and lab instructors (Todd Ryder and Jan Pikul) who contributed to improving this experiment.

Table 2. Student Learning Outcomes Learning Outcome 1 2 3 4 5 6 7 8 a

Student Leaning Outcome: Students Should Be Able To

Studentsa Successfully Performing Task, %

Identify product spot in TLC Obtain desired product Identify aromatic carbons in 13C NMR spectrum Label NH absorption in IR spectrum Answer postlab question 2 identifying product by IR spectroscopy Predict product structure Give all product structures in postlab question 1 covering cycloaddition reactions Answer postlab question 3 relating to cycloaddition reactions

59 94 85

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(1) Baar, M. R.; Gammerdinger, W.; Leap, J.; Morales, E.; Shikora, J.; Weber, M. H. Pedagogical Comparison of Five Reactions Performed under Microwave Heating in Multi-Mode versus Mono-Mode Ovens: Diels- Alder Cycloaddition, Wittig Salt Formation, E2 Dehydrohalogenation to Form an Alkyne, Williamson Ether Synthesis, and Fischer Esterification. J. Chem. Educ. 2014, 91, 1720−1724. (2) Reilly, M. K.; King, R. P.; Wagner, A. J.; King, S. M. MicrowaveAssisted Esterification: A Discovery-Based Microscale Laboratory Experiment. J. Chem. Educ. 2014, 91, 1706−1709. (3) Baar, M. R.; Falcone, D.; Gordon, C. Microwave-Enhanced Organic Syntheses for Undergraduate Laboratory: Diels-Alder Cycloaddition, Wittig Reaction, and Williamson Ether Synthesis. J. Chem. Educ. 2010, 87, 84−86. (4) Zelisko, P. M.; Amarne, H. Y.; Bain, A. D.; Neumann, K. Extension of a Basic Laboratory Experiment: [4 + 2] and [2 + 2] Cycloadditions. J. Chem. Educ. 2008, 85, 104−106. (5) Lipshutz, B. H.; Boskovic, Z.; Crowe, C. S.; Davis, V. K.; Whittemore, H. C.; Vosburg, D. A.; Wenzel, A. G. Click” and Olefin Metathesis Chemistry in Water at Room Temperature Enabled by Biodegradable Micelles. J. Chem. Educ. 2013, 90, 1514−1517. (6) Ison, E. A.; Ison, A. Synthesis of Well-Defined Copper NHeterocyclic Carbene Complexes and Their Use as Catalysts for a “Click Reaction”: A Multistep Experiment That Emphasizes the Role of Catalysis in Green Chemistry. J. Chem. Educ. 2012, 89, 1575−1577. (7) Mendes, D. E.; Schoffstall, A. M. Citrus Peel Additives for OnePot Triazole Formation by Decarboxylation, Nucleophilic Substitution, and Azide-Alkyne Cycloaddition Reactions. J. Chem. Educ. 2011, 88, 1582−1585. (8) Hansen, T. V.; Wu, P.; Sharpless, W. D.; Lindberg, J. G. Just Click it: Undergraduate Procedures for the Copper(I)-Catalyzed Formation of 1,2,3-Triazoles from Azides and Terminal Acetylenes. J. Chem. Educ. 2005, 82, 1833−1836. (9) Martin, W. B.; Kateley, L. J.; Wiser, D. C.; Brummond, C. A. Microscale Synthesis of a Diphenylisoxazoline by a 1,3-Dipolar Cycloaddition. J. Chem. Educ. 2002, 79, 225−227. (10) Biot, C.; Bauer, H.; Schirmer, R. H.; Davioud-Charvet, E. 5Substituted Tetrazoles as Bioisosteres of Carboxylic Acids. Bioisosterism and Mechanistic Studies on Glutathione Reductase Inhibitors as Antimalarials. J. Med. Chem. 2004, 47, 5972−5983. (11) Aulakh, G. K.; Sodhi, R. K.; Singh, M. An update on nonpeptide angiotensin receptor antagonists and related RAAS modulators. Life Sci. 2007, 81, 615−639. (12) Sabatini, J. J.; Moretti, J. D. High-Nitrogen-Based Pyrotechnics: Perchlorate-Free Red- and Green-Light Illuminants Based on 5Aminotetrazole. Chem. - Eur. J. 2013, 19, 12839−12845. (13) Feinn, L.; Dudley, J.; Coca, A.; Roberts, E. Antimicrobial Evaluation of 5-Substituted Aryl 1H-Tetrazoles. Med. Chem. 2017, 13, 359−364. (14) Demko, Z. P.; Sharpless, K. B. Preparation of 5-Substituted 1HTetrazoles from Nitriles in Water. J. Org. Chem. 2001, 66, 7945−7950. (15) Coca, A.; Turek, E. Synthesis of 5-substituted 1H-tetrazoles catalyzed by ytterbium triflate hydrate. Tetrahedron Lett. 2014, 55, 2718−2721. (16) Coca, A.; Turek, E.; Feinn, L. Preparation of 5-Substituted 1HTetrazoles Catalyzed by Scandium Triflate in Water. Synth. Commun. 2015, 45, 218−225. (17) Coca, A.; Feinn, L.; Dudley, J. Microwave Synthesis of 5Substituted 1H-Tetrazoles Catalyzed by Bismuth Chloride in Water. Synth. Commun. 2015, 45, 1023−1030. (18) Hantzsch, A.; Vagt, A. Ueber das sogenannte Diazoguanidin. Justus Liebigs Ann. Chem. 1901, 314, 339−369.

85 68 91 82 85

N = 34.

able to successfully perform each task as was done in a previous report.24 Outcomes #1 and 2 assessed students’ ability to perform lab techniques such as extraction and reaction monitoring through TLC as well as using a rotary evaporator. Outcomes #3−5 assessed students’ ability to interpret spectral data, whereas outcomes #6−8 evaluated students’ understanding of cycloaddition reactions. The benchmark was met for six of the eight outcomes. More details of the assessment can be found in the Supporting Information.

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CONCLUSIONS In this experiment, undergraduate organic students synthesized one of several possible aryl tetrazole derivatives using a (3 + 2) cycloaddition. They were then tasked with identifying the structure of their product. The experiment exposed students to several basic organic techniques. Students also analyzed their product by several spectral methods including IR, 1H NMR, and 13C NMR spectroscopy. A microwave reactor and rotary evaporator were also incorporated into this experiment.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00617. Student handout, instructor notes with list of chemicals, sample quiz, representative spectra, representative TLC drawings, and assessment details (PDF, DOCX)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Adiel Coca: 0000-0001-6702-4291 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS These results were obtained in large part thanks to a Connecticut State University-American Association of University Professors Faculty Research grant, a Southern Connecticut State University Faculty Creative Activity Research grant, a Southern Connecticut State University Undergraduate D




(19) Sreedhar, B.; Kumar, A. S.; Yada, D. CuFe2O4 nanoparticles: a magnetically recoverable and reusable catalyst for the synthesis of 5substituted 1H-tetrazoles. Tetrahedron Lett. 2011, 52, 3565−3569. (20) Bonnamour, J.; Bolm, C. Iron salts in the catalyzed synthesis of 5-substituted 1H-tetrazoles. Chem. - Eur. J. 2009, 15, 4543−4545. (21) Aureggi, V.; Sedelmeier, G. 1,3-Dipolar cycloaddition: click chemistry for the synthesis of 5-substituted tetrazoles from organoaluminum azides and nitriles. Angew. Chem., Int. Ed. 2007, 46, 8440− 8444. (22) Aridoss, G.; Laali, K. Highly efficient synthesis of 5-substituted 1H-tetrazoles catalyzed by Cu−Zn alloy nanopowder, conversion into 1,5- and 2,5-disubstituted tetrazoles, and synthesis and NMR studies of new tetrazolium ionic liquids. Eur. J. Org. Chem. 2011, 31, 6343−6355. (23) Bold, G.; Faessler, A.; Capraro, H.-G.; Cozens, R.; Klimkait, T.; Lazdins, J.; Mestan, J.; Poncioni, B.; Roesel, J.; Stover, D.; TintelnotBlomley, M.; Acemoglu, F.; Beck, W.; Boss, E.; Eschbach, M.; Huerlimann, T.; Masso, E.; Roussel, S.; Ucci-Stoll, K.; Wyss, D.; Lang, M. New aza-dipeptide analogues as potent and orally absorbed HIV-1 protease inhibitors: candidates for clinical development. J. Med. Chem. 1998, 41, 3387−3401. (24) Potteiger, S. E.; Belanger, J. M. Phospholipids, dietary supplements, and chicken eggs: an inquiry-based exercise using thinlayer chromatography. J. Chem. Educ. 2015, 92, 896−899.

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Undergraduate Organic Experiment: Tetrazole Formation by

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