Laboratory Experiment pubs.acs.org/jchemeduc
Modified Birch Reduction for the Introductory Undergraduate Organic Laboratory Heba Abourahma,* Lynn Bradley, Nichole M. Lareau,† and Megan Reesbeck†† Department of Chemistry, The College of New Jersey, Ewing, New Jersey 08628, United States S Supporting Information *
ABSTRACT: The Birch reduction is a reaction commonly taught in the second-year undergraduate organic curriculum that involves the reduction of an aromatic compound to an alicyclic product using sodium metal and liquid ammonia. The experimental procedure can be challenging for novice chemists and thus has not been widely performed in introductory organic laboratories. An experimental procedure appropriate for introductory organic students is described that utilizes commercially available sodium-impregnated silica gel that is safer and easier to handle. This modified Birch reduction effectively reduces naphthalene to 1,4-dihydronaphthalene while minimizing safety concerns associated with the use of alkali metals and liquid ammonia.
KEYWORDS: Second-Year Undergraduate, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives, Aromatic Compounds, Gas Chromatography, Metals, Mass Spectrometry, Synthesis
T
he Birch reduction1 is a dissolving metal reduction that is customarily taught in the second-year undergraduate organic chemistry curriculum. The reduction of benzene to 1,4-cyclohexadiene using an alkali metal (typically sodium) in liquid ammonia is the example commonly taught. A laboratory experiment implementing the reaction is not typically offered in the introductory organic laboratory because of the hazards associated with the reaction conditions, in addition to the complexity of the procedure for a classroom setting (e.g., the use of sodium metal, ammonia gas tanks, and cold fingers). Sodium metal reacts violently with moisture and requires special handling that may be challenging for introductory organic students; liquid ammonia is toxic, hazardous, and requires cryogenic temperature for the duration of the experiment. A literature search for “Birch reduction experiments” in chemical education journals returned a single hit for a microscale Birch reduction experiment.2 The experiment, however, is designed for the advanced organic laboratory and still involves an elaborate setup and the use of liquid ammonia. Herein, a modified Birch reduction experiment suitable for the introductory organic laboratory is reported that reflects current trends in research and development and enhances student learning with a hands-on experimental component. The reduction is effected by employing a sodium-impregnated silica gel reagent (stage I), Na-SG (I), in which the sodium metal is incorporated into a nanostructured silica gel.3 The reagent has been shown to reduce a number of aromatic compounds effectively and efficiently at mild temperatures and in the absence of ammonia.4,5 Incorporating the use of Na-SG (I) reagent in a modified Birch reduction of naphthalene to 1,4dihydronaphthalene provides an experiment that is safe and © 2014 American Chemical Society and Division of Chemical Education, Inc.
accessible to introductory organic students without the use of cryogenic temperatures, pyrophoric materials, or liquid ammonia (Scheme 1). Furthermore, the procedure introduces Scheme 1. Naphthalene Reduction to 1,4Dihydronaphthalene in a Modified Birch Reduction Procedure Employing a Sodium-Impregnated Silica Gel Reagent, Na-SG (I)
students to relatively advanced techniques that include performing a reaction under inert conditions, using reagent bottles with Sure-Seal caps, and characterizing the product using gas chromatography−mass spectrometry (GC−MS). Finally, the reagent presents a green alternative to traditional dissolving-metal reducing agents because less hazardous solvents are employed and no hazardous byproducts are formed. A minor product, tetralin, is also observed when naphthalene is reduced using Na-SG (I). This product of the Published: January 10, 2014 443
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Journal of Chemical Education
Laboratory Experiment
allowed for direct product analysis from solution and because of its low detection limits. 1H NMR can be conveniently used as an alternative or supplemental characterization technique (see the Instructor Notes). A typical gas chromatogram of the crude product is shown in Figure 1. There are 3 peaks with retention
over-reduction of naphthalene has previously been observed when metal catalysts, such as RhCl2 or NiO/SiO2−Al2O3, have been used.6
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EXPERIMENT Students work individually or in a group of two students. The experiment is divided into two 3-h lab periods. In the first lab period, students perform the reduction reaction and workup and submit a sample for GC−MS data collection, which is performed outside of lab time. Sodium silica gel, Na-SG (I) (7 mmol), is added to a flask with a stir bar, and the flask is purged with nitrogen. Under a nitrogen atmosphere, anhydrous tetrahydrofuran (THF) is added to the flask, and the flask is placed in an ice bath. Anhydrous ethylenediamine (EDA) is added to the flask and stirred for 15 min. t-Amyl alcohol is added dropwise to the solution. A solution of naphthalene (1 mmol) in anhydrous THF is added to the flask and stirred for 30 min. A solution of methanol/water (9:1) is added in small increments and stirred for 5 min to quench the reaction. The reaction is worked up by filtering the reaction mixture, diluting the filtrate with water, and extracting with ethyl acetate. Crude product, an oil, is obtained by removal of the organic solvent. The crude product is submitted for GC−MS analysis. In the second lab period, students complete a postlaboratory assignment, which includes analysis of the GC−MS spectrum to determine the percent composition of the product mixture and the ratio of the desired 1,4-dihydronaphthalene product to the tetralin byproduct.
Figure 1. Representative gas chromatogram of student product (Group 1 in Table 1) obtained by reducing naphthalene with NaSG(I).
times of 3.71, 3.83, and 3.86 min, corresponding to tetralin (M+ = 132 m/z), 1,4-dihydronaphthalene (M+ = 130 m/z), and naphthalene (M+ = 128 m/z), respectively. The retention time for each component was determined from commercially available samples of the products and starting material. Students identified the peaks in their samples, determined the percent composition of the mixture, and calculated the ratio of product to byproduct; unreacted naphthalene was also present in the product mixture. The use of a greater quantity of Na-SG (I) reduced the amount of unreacted naphthalene; however, it increased the amount of the over-reduced tetralin byproduct. Thus, the optimal molar ratio of Na-SG (I) to naphthalene of 7:1 was used in this procedure. Because naphthalene’s retention time was very similar to 1,4-dihydronaphthalene, in some cases, its peak overlapped with the 1,4-dihydronaphthalene peak and appeared as a shoulder. In these cases, students manually calculated the area under the peak to determine the percent composition of each component. Table 1 shows representative student results from one section where the average ratio of product to byproduct (1,4-dihydronaphthalene to tetralin) was 11:1 and the average GC-yield of 1,4-tetrahydronaphthalene was 73.1%. Complete student results (see Instructor Notes) showed that the average ratio of product to byproduct among all three sections was 10:1, and the range of GC-yield was 59.5−93.2%, with an average yield of 80.0%.
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HAZARDS Na-SG (I) is water-reactive and should be protected from moisture. Avoid formation of dust and aerosols and keep away from sources of ignition. Measures should be taken to prevent the buildup of electrostatic charge. Naphthalene is carcinogenic and can cause skin and eye irritation. Tetrahydrofuran, t-amyl alcohol, and ethyl acetate are flammable and are skin and eye irritants. Ethylenediamine is toxic, flammable, and corrosive. Deuterated chloroform is an eye and skin irritant and hazardous if inhaled or ingested. No hazard information is known about 1,4-dihydronaphthalene, but like any chemical, exposure should be kept to a minimum and personal protective equipment, such as gloves and goggles, must be worn at all times. The reaction must be carried out under a fume hood. Material should be disposed of following the MSDS guidelines for each chemical.
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RESULTS AND DISCUSSION The experiment was incorporated into three sections of a second-year undergraduate organic laboratory course, one in the spring of 2010 and two in the summer of 2012. Students worked independently in the first section and in pairs in the other sections. The number of reactions conducted in each section was nine. A fresh bottle of Na-SG (I) from the same batch was used in each section with consistent results. Each student working independently or student group successfully completed the modified Birch reduction following the procedure. Students in all sections were able to obtain products without exception and determine the composition of the product mixture using GC−MS. A number of characterization techniques were considered, including GC−MS, 1H NMR, UV−vis, and melting point determination (see the Instructor Notes in the Supporting Information). GC−MS was found to be the most suitable technique, particularly because it
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CONCLUSIONS A simple and effective modified Birch reduction experiment that reduced naphthalene to 1,4-dihydronaphthalene using commercially available sodium-impregnated silica gel reagent was developed for the introductory organic laboratory. The experiment reinforced lecture material and illustrated that the reduction of aromatics can be achieved by an alternative green method to the traditional Birch reduction. The reaction conditions eliminated the use of liquid ammonia, cryogenic temperatures, and pyrophoric materials. Students analyzed their 444
dx.doi.org/10.1021/ed4000537 | J. Chem. Educ. 2014, 91, 443−445
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Laboratory Experiment
Table 1. Representative Student Data from the Summer of 2012 composition of product mixture (%) tetralin
1,4-dihydronaphthalene
naphthalene
1,4 dihydronaphthalene/tetralin
1 2 3 4 5 6 7 8 9 Average
5.4 5.5 4.6 8.4 8.5 7.9 7.3 5.6 7.0
72.3 85.6 68.5 79.2 71.7 82.6 64.0 68.1 65.9 73.1%
22.3 8.9 27.0 12.4 19.8 9.5 28.7 26.3 27.1
13:1 16:1 15:1 9.4:1 8.4:1 11:1 8.8:1 12:1 9.4:1 11:1
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products by GC−MS. Overall, students gained experience in advanced laboratory techniques, such as performing a reaction under inert atmosphere and using GC−MS for quantitative product analysis. The primary assessment of student learning was the postlaboratory assignment. Students successfully analyzed the GC−MS spectra to determine qualitatively the composition of their reaction mixture and quantitatively the ratio of product to byproduct. In doing so, the students confirmed the validity of the procedure and its effectiveness in producing the same product that would have been expected from the traditional Birch reduction procedure. Students commented on the ease of the experimental procedure and appreciated the exposure to spectroscopic techniques that they do not typically use.
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REFERENCES
(1) (a) Birch, A. J. Reduction by Dissolving Metals. Part I. J. Chem. Soc. 1944, 430−436. (b) Birch, A. J. The Birch Reduction in Organic Chemistry. Pure Appl. Chem. 1996, 68, 553−556. (2) Fuhry, M. A. M.; Colosimo, C.; Gianneschi, K. A Microscale Birch Reduction for the Advanced Organic Chemistry Laboratory. J. Chem. Educ. 2001, 78, 949−950. (3) Dye, J. L.; Cram, K. D.; Urbin, S. A.; Redko, M. Y.; Jackson, J. E.; Lefenfeld, M. Alkali Metals Plus Silica Gel: Powerful Reducing Agents and Convenient Hydrogen Sources. J. Am. Chem. Soc. 2005, 127, 9338−9339. (4) Costanzo, M. J.; Patel, M. N.; Petersen, K. A.; Vogt, P. F. Ammonia-Free Birch Reductions with Sodium Stabilized in Silica Gel, Na−SG(I). Tetrahedron Lett. 2009, 50, 5463−5466. (5) Nandi, P.; Dye, J. L.; Jackson, J. E. Birch Reductions at Room Temperature with Alkali Metals in Silica Gel (Na2K-SG(I)). J. Org. Chem. 2009, 74, 5790−5792. (6) Kirumakki, S. R.; Shpeizer, B. G.; Sagar, G. V.; Chary, K. V. R.; Clearfield, A. Hydrogenation of Naphthalene over NiO/SiO2−Al2O3 catalysts: Structure−activity correlation. J. Catal. 2006, 242, 319−331.
ASSOCIATED CONTENT
S Supporting Information *
Instructor notes; student handout; GC−MS conditions and spectral data, and NMR data supporting the characterization of the product. This material is available via the Internet at http:// pubs.acs.org.
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ratio of desired product to byproduct
group
AUTHOR INFORMATION
Corresponding Author
*Corresponding Author E-mail:
[email protected]. Present Addresses †
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37232, United States. †† Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Paul Vogt and Brian Bodnar, formerly of SiGNa Chemistry Inc., for providing the Na-SG (I) samples used in this experiment and for their helpful suggestions on the project; David Hunt for the many fruitful discussions during the preparation of this manuscript; Katrina Wunderlich for collecting GC−MS data and CHE332 students (fall 2010 and summer 2012) whose results are summarized here. The authors also acknowledge NSF-MRI grant 1125993 for funding the purchase of a 400 MHz Bruker NMR instrument. 445
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