Laboratory Experiment pubs.acs.org/jchemeduc
Implementation of picoSpin Benchtop NMR Instruments into Organic Chemistry Teaching Laboratories through Spectral Analysis of Fischer Esterification Products Kasey L. Yearty, Joseph T. Sharp, Emma K. Meehan, Doyle R. Wallace, Douglas M. Jackson, and Richard W. Morrison* Department of Chemistry The University of Georgia, 140 Cedar Street, Athens, Georgia 30602-2556, United States S Supporting Information *
ABSTRACT: 1H NMR analysis is an important analytical technique presented in introductory organic chemistry courses. NMR instrument access is limited for undergraduate organic chemistry students due to the size of the instrument, price of NMR solvents, and the maintenance level required for instrument upkeep. The University of Georgia Chemistry Department recently acquired three picoSpin desktop 1H NMR instruments for the undergraduate organic laboratories. These instruments can sit on a standard lab bench, can analyze samples without NMR solvents, and are easily maintained. In this Fischer esterification experiment, students used unknown starting alcohols to synthesize esters through Fischer esterification. Upon completion of the reaction, students identified the unknown starting alcohol via spectral analyses of the products. Over the course of 4 semesters, 704 out of 940 students (75%) correctly identified the starting alcohol and 71% of students surveyed indicated that 1H NMR spectrum was the most helpful identification tool in their analyses. This experiment established for students the utility of NMR spectral analysis and provided them with the opportunity to employ technology commonly used in academic research facilities. KEYWORDS: Second-Year Undergraduate, Laboratory Instruction, Organic Chemistry, Inquiry-Based/Discovery Learning, Esters, NMR Spectroscopy
■
■
INTRODUCTION
EXPERIMENTAL METHODS Prior to 1H NMR analysis, students performed a standard Fischer esterification experiment using acetic acid and an unknown alcohol (either 1-propanol, 1-butanol, or isopentyl alcohol), as shown in Procedure 1 of the Supporting Information.24 The product was transferred into a prelabeled vial for 1H NMR analysis. Students were provided with a tutorial for analyzing product spectra (see Handout 1 of the Supporting Information), including the sample spectrum of ethyl acetate shown in Figure 2. A total of 940 students were evaluated. Our data includes the students’ identification of their starting material from Spring 2014, Spring 2015, Fall 2015, and Spring 2016 semesters. In Fall 2015 and Spring 2016 semesters, a postsurvey was also employed where the usefulness of 1H NMR in the study was measured quantitatively in three ways: (1) by the correct identification of the starting alcohol, (2) by the student responses in the usefulness of the analytical tool, and (3) by student confidence levels in their response using only the 1H NMR spectrum.
1
Proton nuclear magnetic resonance ( H NMR) spectroscopy is taught during the first semester of organic chemistry at the University of Georgia. Students are introduced to the topic in the lecture course. Textbooks commonly provide softwaregenerated or idealized spectra. As a consequence, students infrequently interpret crude spectra taken from student data on an NMR instrument. 1H NMR suitable experiments have been published in this journal, but the advent of benchtop NMR instruments provided an opportunity for students to use recent NMR technology to obtain 1H NMR spectra of products recovered or synthesized in the undergraduate instructional laboratory.1−15 Students are able to see experimental signal integrations, determine signal multiplicity, and learn about solvent and reference signals in spectra. The Fischer esterification reaction and its application in multioutcome experiments has been studied for many years and is commonly included in organic chemistry laboratory curricula across the country.16−23 The reaction produces an ester and water from a carboxylic acid and an alcohol, as shown in Figure 1. Because this reaction is reversible, Le Châtlier’s principle can be utilized to drive the reaction toward the desired product esters. In practice, removal of the ester product through distillation drives the reaction to completion. In this experimental procedure, students were provided with a known carboxylic acid and an unknown alcohol for esterification. Students were then asked to identify the unknown alcohol via analysis of the ester product synthesized. © XXXX American Chemical Society and Division of Chemical Education, Inc.
■
HAZARDS Alcohols and esters are flammable. Acetic acid and sulfuric acid cause severe burns and should be treated with care. Pressure may build up when neutralizing with sodium bicarbonate due to the release of carbon dioxide gas. Perform the experiment in a Received: December 15, 2016 Revised: March 31, 2017
A
DOI: 10.1021/acs.jchemed.6b00972 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Figure 1. Generic Fischer esterification scheme.
Figure 2. Sample 1H NMR spectrum of ethyl acetate from the 45 MHz picoSpin benchtop NMR.
■
DISCUSSION Each analytical technique used to characterize the ester product provided a piece of the puzzle to identify the starting alcohol. Fourier transform infrared (FTIR) spectra indicated that an ester was present but did not indicate which specific ester was formed. However, collecting FTIR spectra of the starting materials and product was beneficial for students because it allowed them to monitor the reaction for completion, observing the disappearance of the alcohol stretch and the appearance of the carbonyl stretch. It was also instrumental in reinforcing the purpose of FTIR; i.e., to determine the functional groups of a molecule. The boiling point determination proved inconsistent, as students sometimes recorded boiling points that were several tens of degrees lower than the literature value of the product ester. Some groups had trouble obtaining enough volume of the product to successfully distill the ester, which was remedied by grouping students that were assigned the same unknown into larger groups and combining their samples to obtain the boiling point using a Hickman still. Students also noted the fruity aroma of the volatile product, which some claimed helpful. As a part of the postlab report, students were asked a series of questions to allow them to rank the identification tools that were most useful in helping them successfully identify the unknown starting alcohol. Figure 3 shows that 71% of students surveyed indicated that 1H NMR was the most helpful characterization technique followed by boiling point and
well-ventilated room or hood away from sources of ignition. Safety glasses and lab gloves must be worn at all times.
■
RESULTS For all 4 semesters studied, 704 out of 940 (75%) students successfully identified the starting alcohol (Table 1). After an Table 1. Combined Student Data by Correct Identification Semester
Students
Correct
Percent (%)
Spring 2014 Spring 2015 Fall 2015 Spring 2016 Total
260 164 214 302 939
233 110 140 221 704
90 67 65 73 75
initial correct identification rate of 90% in Spring 2014, the subsequent semesters correct identification rates were closer in value with an average of 68.3 ± 4.2%. Specific data listed by semester are depicted in the Supporting Information, Tables 1−4 along with data listed by the unknown alcohol used for the reaction shown in the Supporting Information, Table 5. Example student spectra are also provided in the Supporting Information, Figures 2−4. B
DOI: 10.1021/acs.jchemed.6b00972 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Figure 3. Fall 2015 and Spring 2016 surveyed student responses to indicate the most useful tool for identifying the unknown alcohol. n = 516 students. No students selected color.
scent, which were each respectively selected by 11% of students. FTIR spectroscopy was selected by students as the second most useful tool due to its value in determining the formation of an ester even though it does not readily allow for the elucidation of one ester product from another. Surveyed students indicated feeling most confident (34%) or somewhat confident (48%) in correctly identifying the unknown alcohol with only the picoSpin 1H NMR data (Supporting Information, Figure 5). The level of uncertainty illustrated students’ previous reliance on other identification techniques and inexperience with nonidealized spectra. On the other hand, students indicated feeling not confident (46%) or only somewhat confident (35%) in correctly identifying the unknown starting alcohol without the inclusion of the picoSpin 1 H NMR data (Supporting Information, Figure 6). This demonstrated students’ understanding that the other tools for identification were less beneficial for structure elucidation than the 1H NMR data. Overall, both students and teaching assistants (TAs) surveyed provided positive feedback when asked about the incorporation of the picoSpin 1H NMR instrument into the Fischer esterification experiment. One student stated that “1H NMR was the easiest and most efficient method of identification of the unknown alcohol.” Another student stated that “it is important for the students to actually see a 1H NMR spectrum that is not straight out of the textbook. It helps give a little clarity between lab and lecture.” Students also stated that without the 1H NMR data, “it would have been very hard for me to identify my unknown” and that the incorporation of the picoSpin 1H NMR data “should continue to be used for this experiment in the future.” The TAs for the students participating in the survey indicated that the picoSpin 1H NMR data was the most useful tool for students to correctly identify the unknown. Most indicated that students would not be able to correctly identify the unknown alcohol without the picoSpin 1H NMR data and indicated confidence that the students would be able to correctly identify the unknown alcohols with only the picoSpin 1 H NMR data for the product ester.
experimental results with 71% of surveyed students indicating that the 1H NMR spectrum was most helpful in identifying the ester product. All data prior to Spring 2016 were collected on the 45 MHz picoSpin NMR instrument. Future directions include obtaining higher resolution data from the 82 MHz instrument which was recently incorporated into the undergraduate laboratory curriculum. In addition, the promise of obtaining 13C NMR spectra using the picoSpin 82 MHz instrument provides another complementary experiment for structure elucidation. Proper training of graduate laboratory assistants ensures that all students can receive their own spectra during the laboratory period.
CONCLUSIONS This experiment allowed students to gain hands-on experience with 1H NMR spectroscopy and analysis of actual student experimental results as opposed to computer generated or idealized spectra. Overall, 75% of students over four semesters correctly identified their unknown starting alcohol from
(1) Bonjour, J. L.; Pitzer, J. M.; Frost, J. A. Introducing High School Students to NMR Spectroscopy through Percent Composition Determination Using Low-Field Spectrometers. J. Chem. Educ. 2015, 92 (3), 529−533. (2) Erhart, S. E.; McCarrick, R. M.; Lorigan, G. A.; Yezierski, E. J. Citrus Quality Control: An NMR/MRI Problem-Based Experiment. J. Chem. Educ. 2016, 93 (2), 335−339.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00972. Procedure, student handout, and instructor information for preparing and performing the experiment (PDF, DOCX)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Richard W. Morrison: 0000-0002-2807-6379 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This project was financially supported by the UGA Office of STEM Education as a part of the Board of Regents’ STEM Initiative.
■
■
C
REFERENCES
DOI: 10.1021/acs.jchemed.6b00972 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
signed for an Undergraduate Organic Chemistry Laboratory. World Journal of Chemical Education 2015, 3 (5), 111−114. (24) Hubbard, R.; Morrison, R. Modern Organic Chemistry II; Hayden McNeil: Plymouth, MI, 2016.
(3) Williams, T. J.; Kershaw, A. D.; Li, V.; Wu, X. An Inversion Recovery NMR Kinetics Experiment. J. Chem. Educ. 2011, 88 (5), 665−669. (4) Hartel, A. M.; Moore, A. C. Extraction and 1H NMR Analysis of Fats from Convenience Foods: A Laboratory Experiment for Organic Chemistry. J. Chem. Educ. 2014, 91 (10), 1702−1705. (5) Shine, T. D.; Glagovich, N. M. Organic Spectroscopy Laboratory: Utilizing IR and NMR in the Identification of an Unknown Substance. J. Chem. Educ. 2005, 82 (9), 1382. (6) Harmon, J.; Coffman, C.; Villarrial, S.; Chabolla, S.; Heisel, K. A.; Krishnan, V. V. Determination of Molecular Self-Diffusion Coefficients Using Pulsed-Field-Gradient NMR: An Experiment for Undergraduate Physical Chemistry Laboratory. J. Chem. Educ. 2012, 89 (6), 780−783. (7) Seyse, R. J.; Pearce, H. L.; Rose, T. L. Calculation of complex NMR spectra in the undergraduate laboratory. J. Chem. Educ. 1975, 52 (3), 194. (8) Isaac-Lam, M. F. Analysis of Bromination of Ethylbenzene Using a 45 MHz NMR Spectrometer: An Undergraduate Organic Chemistry Laboratory Experiment. J. Chem. Educ. 2014, 91 (8), 1264−1266. (9) Olsen, R. J.; Olsen, J. A.; Giles, G. A. An Enzyme Kinetics Experiment for the Undergraduate Organic Chemistry Laboratory. J. Chem. Educ. 2010, 87 (9), 956−957. (10) Kenny, D. H.; Sandel, V. R.; Osterby, B. R. Relative carbonium ion stabilization energies: an nmr experiment for the undergraduate organic laboratory. J. Chem. Educ. 1974, 51 (4), 253. (11) Sorensen, J. L.; Witherell, R.; Browne, L. M. Use of 1H NMR in Assigning Carbohydrate Configuration in the Organic Laboratory. J. Chem. Educ. 2006, 83 (5), 785. (12) Jacobus, J.; Raban, M. An NMR determination of optical purity: An advanced undergraduate laboratory experiment. J. Chem. Educ. 1969, 46 (6), 351. (13) Kenny, D. H. The disproportionation of an unsymmetrical azine. An NMR experiment for the undergraduate organic laboratory. J. Chem. Educ. 1980, 57 (6), 462. (14) Gonzalez, E.; Dolino, D.; Schwartzenburg, D.; Steiger, M. A. Dipeptide Structural Analysis Using Two-Dimensional NMR for the Undergraduate Advanced Laboratory. J. Chem. Educ. 2015, 92 (3), 557−560. (15) Bodsgard, B. R.; Lien, N. R.; Waulters, Q. T. Liquid CO2 Extraction and NMR Characterization of Anethole from Fennel Seed: A General Chemistry Laboratory. J. Chem. Educ. 2016, 93 (2), 397− 400. (16) Bromfield-Lee, D. C.; Oliver-Hoyo, M. T. An Esterification Kinetics Experiment That Relies on the Sense of Smell. J. Chem. Educ. 2009, 86 (1), 82. (17) Wade, P. A.; Rutkowsky, S. A.; King, D. B. A Simple Combinatorial Experiment Based on Fischer Esterification. An Experiment Suitable for the First-Semester Organic Chemistry Lab. J. Chem. Educ. 2006, 83 (6), 927. (18) Clausen, T. P. Combining a Standard Fischer Esterification Experiment with Stereochemical and Molecular-Modeling Concepts. J. Chem. Educ. 2011, 88 (7), 1007−1009. (19) 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 (10), 1706−1709. (20) Pavia, D.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. A Microscale Approach to Organic Laboratory Techniques; Brooks Cole: Pacific Grove, CA, 2012; Vol. 5. (21) Branz, S. E.; Miele, R. G.; Okuda, R. K.; Straus, D. A. ″Double Unknown″ Microscale Preparation and COSY Analysis of an Unknown Ester: An Introductory 2D-NMR Experiment. J. Chem. Educ. 1995, 72 (7), 659. (22) Mohrig, J. R.; Noring Hammond, C.; Schatz, P. F.; Morrill, T. C. Modern Projects and Experiments in Organic Chemistry: Microscale and Williamson Microscale; W.H. Freeman and Company: New York, New York, 2002. (23) Lampkowski, J. S.; Bass, W.; Nimmo, Z.; Young, D. D. Microwave-Assisted Esterifications: An Unknowns Experiment DeD
DOI: 10.1021/acs.jchemed.6b00972 J. Chem. Educ. XXXX, XXX, XXX−XXX
