Laboratory Experiment pubs.acs.org/jchemeduc
Structure Determination of Unknown Organic Liquids Using NMR and IR Spectroscopy: A General Chemistry Laboratory John T. Pavel, Erin C. Hyde, and Martha D. Bruch* Department of Chemistry, Oswego State University, Oswego, New York 13126, United States S Supporting Information *
ABSTRACT: This experiment introduced general chemistry students to the basic concepts of organic structures and to the power of spectroscopic methods for structure determination. Students employed a combination of IR and NMR spectroscopy to perform de novo structure determination of unknown alcohols, without being provided with a list of possible structures. Students used IR spectra to identify functional groups present, then constructed trial structures and predicted the NMR spectrum, modifying the structure until agreement was obtained. The structure was confirmed by comparison of boiling point and density data, measured in triplicate, to literature values, and the precision and accuracy of student measurements was critically evaluated. KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Problem Solving/Decision Making, Alcohols, IR Spectroscopy, NMR Spectroscopy, Physical Properties, Student-Centered Learning
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adjusted until a match was obtained; an algorithm for doing this is included in Supporting Information. Once a structure consistent with the spectra was determined, students looked up the literature values1 for the density and boiling point. If significant discrepancies existed between experimental and literature values, spectra were reexamined to check for mistakes in interpretation.
raditional general chemistry courses include limited content in organic chemistry due to time constraints. However, including basic concepts of organic structures is desirable as it provides a foundation for those students who will ultimately study organic chemistry, while also providing a more comprehensive survey for students who will not continue beyond general chemistry. Infrared (IR) spectroscopy is a good way to introduce organic functional groups, while nuclear magnetic resonance (NMR) spectroscopy helps students visualize organic structures as composed of building blocks of CH, CH2, and CH3 groups. This report describes a two-week experiment, done during traditional 3-h lab periods. Students performed de novo structure determination of two unknown alcohols, isomers of propanol or butanol, through analysis of IR and 1H NMR spectra, then confirmed the structure by comparison of density and boiling point measurements to literature values.1 The experimental data were obtained during the first week and analyzed during the second week. At the start of week two, a tutorial on interpretation of IR and NMR spectra was presented (see Appendix A in the Supporting Information), along with a discussion of organic functional groups. The spectra were predicted for two examples of known structure, including one containing equivalent CH3 groups. Then, the structure of ethyl acetate was determined from the IR and NMR spectra (see the Supporting Information), modeling the process students’ use for their unknowns. After the tutorial, students analyzed spectra of each unknown, first identifying functional groups from the IR spectrum using a simplified table of expected frequencies (see the Supporting Information), then determining the total number of hydrogen atoms in the molecule from the NMR integrals. Students drew a trial structure and predicted the NMR spectrum for this structure. The trial structure was © XXXX American Chemical Society and Division of Chemical Education, Inc.
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JUSTIFICATION This experiment is similar to one previously reported, where IR and NMR spectra were used to identify an unknown from a list of 15 possible structures.2 In another published experiment, students identified the correct isomer of C3H8O from three possibilities using the 1H NMR spectrum.3 An experiment where students used 13C NMR of neat liquids to identify unknown alcohols has been reported4 and an experiment where students identified an unknown white powder using melting point, thin-layer chromatography, IR, and NMR data was recently presented.5 However, for all these experiments, students were provided with a list of possible structures or formulas. The proposed experiment is unique because it requires de novo structure determination. Neither a list of possible structures nor a molecular formula was provided. Furthermore, students had no knowledge of the number of different unknowns available. The primary goal of this experiment is to use molecular spectroscopy to determine the structure of unknown organic compounds, using a strategy that mimics real-world analysis of unknown samples and emphasizes critical-thinking and problem-solving skills. Specific student learning objectives are to (1) use IR spectra to determine the presence or absence of
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Laboratory Experiment
Figure 1. IR spectra of (A) 1-propanol, (B) 2-propanol, (C) 1-butanol, (D) 2-butanol, and (E) 2-methyl-1-propanol.
Figure 2. NMR spectra of student-prepared samples of (A) 1-propanol, (B) 2-propanol, (C) 1-butanol, (D) 2-butanol, and (E) 2-methyl-1-propanol.
individual functional groups, (2) draw plausible organic structures incorporating specific functional groups, and (3) predict the intensities and splitting patterns expected in the 1H NMR spectrum of a specific organic compound. A secondary
goal is to enhance student understanding of precision and accuracy. Standard deviations for triplicate student measurements of densities and boiling points are used (1) to critically assess accuracy by comparing experimental averages to B
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Table 1. Physical Properties of Alcohols Density/(g/mL) Alcohol 1-Propanol 2-Propanol 1-Butanol 2-Butanol 2-Methyl-1-propanol a
Instructora 0.800 0.782 0.809 0.800 0.794
± ± ± ± ±
0.001 0.003 0.002 0.004 0.004
Boiling Point/°C
Studentb
LIterature
Instructora
Studentb
Literature
± ± ± ± ±
0.8035 0.7855 0.8098 0.8063 0.7982
97.8 ± 0.5 83.1 ± 0.8 120.2 ± 0.8 100.1 ± 0.5 110.0 ± 0.8
99 ± 1 84 ± 1 118 ± 3 101 ± 1 109 ± 3
97.4 82.4 117.2 99.5 108
0.79 0.77 0.80 0.79 0.78
0.01 0.01 0.02 0.01 0.02
Instructor measurements are in triplicate. bStudent results comprise 15−30 trials.
literature values1 and (2) to determine whether the precision is sufficient to distinguish two compounds based solely on physical properties. Each group of 2−3 students was assigned two unknown alcohols: one with a linear structure (1-propanol or 1-butanol) and the other containing branching or an internal hydroxyl group (2-propanol, 2-butanol, or 2-methyl-1-propanol). All five unknowns are inexpensive, have minimal hazards, yield wellresolved spectra, and have easily measured physical properties. Because there are six distinct combinations of unknowns, at most two groups in a lab section of 20−24 students had the same combination, forcing students to focus on interpretation of their own data, without regard for other students’ results.
is small, especially for 2-propanol, some students incorrectly concluded alkanes are not present. After determining the presence of an alcohol, indicative of O−H and C−O bonds in the structure, NMR spectra (Figure 2) were used to determine the total number of hydrogen atoms in each unknown. The smallest integral was assigned to one hydrogen, and the number of hydrogens associated with each peak was found by dividing that integral by the smallest integral, then rounding to the nearest whole number. Numerical integrals for each alcohol, summarized in Supporting Information (Table S1), are reproducible, with all but one ratio within 10% of the expected value. The total number of hydrogens in the molecule was correctly determined by all students without assistance, which enabled students to deduce the number of carbons needed. Students then drew a trial structure and used a worksheet and corresponding algorithm to determine if the trial structure matched the NMR data, and the structure was varied until a match was obtained (see the Supporting Information). Students correctly determined the structure of 95% of the group unknowns; 20% of the initial trial structures were correct, with 2−3 attempts typical. Once the unknown structures were determined, students obtained the literature values1 for the densities and boiling points and compared their experimental results to literature values. Good agreement was observed (Table 1), confirming the identity of the unknown, with discrepancies typically of similar magnitude to the standard deviation for triplicate student measurements, which ranged from 0.001 to 0.03 g/mL for densities (average 0.009 g/mL) and from 0.1 to 2.8 °C for boiling points (average 0.6 °C). Students were asked if they could distinguish the two unknowns based on density and/or boiling point data. Their answer depended primarily on the combination of unknowns they were assigned. For example, differences between the physical properties of 1-propanol and 2-butanol are small compared to typical standard deviations, so these two alcohols could not be distinguished based on physical properties, whereas 1-propanol could be readily distinguished from 2-propanol. Consequently, students were forced to think critically about their own data to answer the discussion questions and could not simply provide a “stock” answer from the textbook.
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EXPERIMENTAL OVERVIEW The alcohols were dried using activated 3 Å molecular sieves (Aldrich), and students received vials containing 6−8 mL of each unknown. During the first week, students rotated through three stations to obtain (1) density and boiling point data, (2) IR spectra, and (3) NMR spectra of their unknowns. Students obtained their own IR spectra under guidance of the instructor. Although students prepared NMR samples by dissolving each alcohol in chloroform-d (Cambridge Isotopes), an experienced operator obtained the spectra using a Varian 300 MHz NMR spectrometer while students watched. Experimental details are in the Supporting Information. This experiment required NMR and IR spectrometers, which may not be available in all departments. However, spectra of all five unknowns are provided in the Supporting Information, and the experiment could be conducted as a one-week dry lab.
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HAZARDS 1-Propanol (CAS # 71-23-8), 2-propanol (CAS #67-63-0), 1butanol (CAS #71-36-3), 2-butanol (CAS#78-92-2), and 2methyl-1-propanol (CAS #78-83-1) are flammable, toxic, and irritants to the skin and eyes; safety glasses and gloves must be worn. Chloroform-d (CAS # 865-49-6) is a carcinogen, so NMR samples must be disposed of as hazardous waste.
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RESULTS AND DISCUSSION This experiment was done in the first semester of general chemistry (annual enrollment of 55−65 students each spring) by 3 sections of approximately 20 students each, working in groups of 2−3 students. Typical student IR spectra of the unknowns are shown in Figure 1, with peak-frequency labels omitted for simplicity; students used labeled spectra provided in Supporting Information. Using the table of expected IR frequencies for various functional groups (Appendix A in the Supporting Information), all students successfully identified alcohols as present, while approximately 80% of students identified alkanes. Since the C−H bend signal near 1370 cm−1
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STUDENT ASSESSMENT Assessment of individual student understanding was done in conjunction with a model-building lab involving both inorganic and organic molecules. Each student was given IR and NMR spectra of one of the five alcohols not yet studied and worked independently to deduce the structure; 95% of students obtained the correct structure, the same percentage as for group unknowns. Furthermore, 65% of students demonstrated good understanding of spectroscopy in response to an extra C
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credit question on the final exam asking students to describe the information obtained from NMR and IR spectra (average question score 64%; final exam average 66%). The difficulty level was considered about right by 94% of students for the group unknowns and by 77% for individual unknowns, and 84% of students felt they were given enough instruction to determine their unknown structures. Over 90% of students surveyed felt that critical-thinking and problem-solving skills were important for this lab (Table S2). Nearly 60% said the lab helped them understand precision and accuracy, and over 75% of students surveyed felt it helped them understand organic structures and spectroscopy (Table S3). Although this lab is unlikely to have a dramatic impact on performance in future chemistry courses, it should provide a foundation for students who will ultimately study organic chemistry (1/3 of the class). Over 85% of these students agreed it would help them understand organic functional groups, organic structures, and spectral interpretation when discussed in the future, even though 60% of these students will wait a year to take organic chemistry (Table S4). Although 44% of students found the lab to be more difficult than other laboratories, 69% felt they learned more (Table S5). Finally, 61% of students thought the lab was more fun than other laboratories, 86% said it was a nice change of pace, and 84% would recommend it to a friend studying chemistry.
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ASSOCIATED CONTENT
S Supporting Information *
Instructor notes; student instructions for group unknowns; summary of integral data for student-prepared NMR samples; student survey results; Appendix A: Tutorial on Determination of Structural Formulas by IR and NMR; Student Instructions for Individual Unknown; NMR and IR spectra. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) CRC Handbook of Chemistry and Physics, 60th ed., CRC Press: Boca Raton, FL, 1979. (2) Baer, C.; Cornely, K. J. Chem. Educ. 1999, 76, 89−90. (3) Davila, R. M.; Widener, R. K. J. Chem. Educ. 2002, 79, 997−999. (4) Alonso, D. E.; Wong, P. A. Chem. Educator 2008, 13, 234−235. (5) Rummel, S. A.; Keiser, J. T.; Dong, J.; Anderson, G. Abstracts of Papers, 241st ACS National Meeting & Exposition, Anaheim, CA, March 27−31, 2011.
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