Laboratory Experiment pubs.acs.org/jchemeduc
Introducing NMR to a General Chemistry Audience: A StructuralBased Instrumental Laboratory Relating Lewis Structures, Molecular Models, and 13C NMR Data Curtis R. Pulliam,* William F. Pfeiffer, and Alyssa C. Thomas Department of Chemistry and Biochemistry, Utica College, Utica, New York 13502, United States; S Supporting Information *
ABSTRACT: This paper describes a first-year general chemistry laboratory that uses NMR spectroscopy and model building to emphasize molecular shape and structure. It is appropriate for either a traditional or an atoms-first curriculum. Students learn the basis of structure and the use of NMR data through a cooperative learning hands-on laboratory experience, and work in groups to assign names and structures to unknown compounds. This laboratory can be successfully run at a number of experimental levels, from students preparing their own NMR samples for analysis to running this as a dry laboratory with spectra provided by the instructor.
KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Collaborative/Cooperative Learning, Hands-On Learning/Manipulatives, NMR Spectroscopy
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INTRODUCTION In the early 1990s two of us (C.R.P. and W.F.P.) were troubled with what we were seeing in our general chemistry classes. It seemed to us that many of our students did not understand molecular structure and bonding; they were struggling to visualize matter at an atomic or molecular level. We wanted our students to understand and believe that a molecule’s 3-D structure was real and predictable and not just a set of terms to be memorized. At that time we made changes in our general chemistry course which moved some traditional early material later in the course so we could focus on structure and bonding. We have organized our first semester course around two stories based on bonding, and it resembles what recently has come to be known to some as an atoms-first approach. Our first story is about atoms, ions, ionic structure, precipitation reactions, acid− base reactions, and simple oxidation−reduction reactions and simple electrochemistry. Our second story involves covalent bonds, molecules, Lewis structures, VSEPR models, molecular shapes, organic molecules, functional groups and transformations of those groups, and polymers, including biopolymers.1 Our second semester course is fairly traditional except we start by picking up the topics we left out of the first semester in order to focus on atoms and molecules.2 To support these changes, we have developed several new laboratory activities which focus on molecular structure. One is a discovery-based lab introducing students to several kinds of structural isomerism using organic and transition-metal examples which relies on the student using Darling Molecular Models.1 In this paper, we describe a cooperative learning hands-on laboratory, © XXXX American Chemical Society and Division of Chemical Education, Inc.
using model kits, which relates NMR data to molecular structure, and is compatible with an atoms-first approach. This laboratory has been run successfully at a number of experimental levels, from students preparing their own NMR samples for analysis to running this as a dry laboratory with spectra provided by the instructor. Over the past 15 years or so, there have been several NMR based experiments/activities suitable for the first year chemistry experience described in the literature. Some focus on the analysis of student prepared samples,3 some relate NMR data to electronegativity, periodic trends, or pH,4,5 some are multiweek, multi-instrument spectroscopy experiences which involve the use of spectra from known compounds,3,6 and some focus on the identification of unknowns following some pre-lab component (handout or tutorial) where students are introduced to spectral identification prior to the actual laboratory activity.7−11 We believe we have developed a unique cooperative learning hands-on laboratory experience relating information acquired from the 13C NMR experiment to molecular structure without the use of known compounds, or without prior student knowledge of NMR details and methods of spectroscopic interpretation.
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LABORATORY DESCRIPTION This is a laboratory about molecular structure which happens to introduce general chemistry students to some of the information you can get from the NMR experiment. It
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DOI: 10.1021/ed500778h J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Constructing the Chemical Shift Table
intentionally covers just some of the structural information available to a chemist from NMR spectra: number of unique carbon atoms and general chemical shift data. Students match acquired spectra to a list of possible compounds after considering the bonding environment for each carbon atom, correlate types of carbon atoms with chemical shifts, predict spectra from given compounds, and identify unknown compounds from given spectroscopic data. This laboratory can be done any time after covering basic organic molecules (which we do toward the end of the first semester) or any time after introducing Lewis structure since the determination of the number of unique carbon atoms in a molecule is based on bonding descriptions. The data analysis portion of the laboratory can be easily run in a 3-h laboratory period with a class size of up to 24 students. We have over 18 years of experience with this laboratory and have involved over 2200 students, ranging from chemistry and other science majors, health science majors including nursing and physical therapy, and nonscience majors taking general chemistry to fulfill their general education laboratory science requirement. The goal of this activity is to relate data that chemists routinely collect to molecular structure in such a way that the students will be able to draw NMR spectra for given molecules and be able to predict chemical structure from spectroscopic data.
Next, the groups are asked to look at the models and the spectra and determine which carbons in the molecule (saturated, aromatic, carbonyl) correspond to which signals in the spectra. Molecular models sitting on or by the spectra and the ability of the group to see all six spectra at once are critical to the success of this part of the activity. Once a group determines where each class of carbon atoms appears in their spectra, they must send representatives to the other two groups to find out about the other chemical shift ranges. This information is used to construct a chemical shift table, which is necessary to complete the rest of the lab. Drawing NMR Spectra from Given Molecules
Once students have developed a table of chemical shifts based on the three different kinds of carbon atoms in their compounds, they are asked to use that information to sketch NMR spectra for three new compounds. We expect the students to show the correct number of carbon signals in approximately the expected places in their sketches. Determining Unknown Structure from NMR Data
Finally, for two new compounds, we give some spectroscopic data and ask the students to determine the identity of the compounds. We include a list of NMR signals and other information, such as the fact that “another instrument (an infrared spectrometer) indicates that this compound contains both a −OH group and a carbonyl, CO, group.” In the introduction to this final section of the laboratory activity, we tell them that this is like a puzzle; they need to determine the kinds of molecular pieces they have and for them to determine how they can put the pieces together. Use of models really helps with this section of the activity.
Acquiring the Unknown NMR and Determining the Number of Carbon Atoms
The laboratory makes use of 18 compounds in three different functional group classes (six alcohols, six ketones and six brominated compounds, listed in the Supporting Information). The compounds used were chosen such that no two compounds in a particular class exhibit the same number of carbon signals and at least one member of each class contains a phenyl ring. The level of NMR instrument use can range from students making their own samples, to students using pre-made samples, to students seeing a demonstration of the instrument acquiring a single data set, to no direct use of an instrument at all. Those without ready access to an NMR spectrometer can also do this experiment as a laboratory activity by handing out the spectra available as Supporting Information. Students have been introduced to functional groups in lecture and are told what class of compound they have at the time they acquire their spectrum. Prior to the next laboratory, each student was asked to determine how many signals they observed in their spectrum by counting the number of nonsolvent peaks seen.
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HAZARDS Compounds used as unknowns may be flammable and toxic, and can be harmful if inhaled, ingested, or comes into contact with the skin. Deuterated chloroform is a possible carcinogen; alkyl halides are possible carcinogens and flammable; alcohols and ketones are all flammable. Hazards are minimized due to the small volumes used (1 mL or less). Care must be taken when handling all chemical compounds, proper safety apparel must be worn at all times including goggles and gloves, and chemicals should be dispensed in a hood.
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RESULTS AND DISCUSSION While this laboratory gives our students their first introduction to NMR spectroscopy, it is about molecular structure and thinking about molecules as 3-D entities and not just a formula, or even a Lewis structure, drawn on a page. It has been successful in relating Lewis structure, molecular models, and instrumental data together and to our global story of visualization of molecular structure. Students are assessed during and after the activity through grading of their lab notebook and answering supplementary questions on the lab final. Looking at their lab notebooks, nearly 80% of the students are able to use their constructed chemical shift table to correctly draw NMR spectra from known compounds as well as determine the structure of unknown compounds from spectroscopic data. We also ask three additional questions on the lab notebook final at the end of the semester (see Supporting Information). Two-thirds of the students are able to explain how NMR can be used to positively identify 3 isomers
Matching Spectra to Compounds
On the day of the NMR-Structure laboratory, the laboratory handout is passed out to the students at the start of the lab. We intentionally omitted any pre-lab activities as we want the class as a group to begin to relate NMR data to structure. The students arrange themselves into groups by compound class. Next, each group draws out the six possible compounds, builds the models, and determines the number of expected signals for each possibility. While we do introduce basic symmetry operations (mirror planes) in the drawings, we have found the use of models to be vital in this process, especially for compounds with phenyl rings in them (see instructor notes in the Supporting Information for more details). Once the group agrees on the number of signals expected for each of their possible compounds, the compound and its molecular model are matched to the corresponding spectrum. B
DOI: 10.1021/ed500778h J. Chem. Educ. XXXX, XXX, XXX−XXX
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Laboratory Experiment
(5) Hagan, W. J., Jr.; Edie, D. L.; Cooley, L. D. Imidazole as a pH probe: An NMR experiment for the general chemistry laboratory. J. Chem. Educ. 2007, 84, 1188−1189. (6) Iler, H. D.; Justice, D.; Brauer, S.; Landis, A. Discovering 13C NMR, 1H NMR, and IR spectroscopy in the general chemistry laboratory through a sequence of guided-inquiry exercises. J. Chem. Educ. 2012, 89, 1178−1182. (7) Parvel, J. T.; Hyde, E. C.; Bruch, M. D. Structure Determination of unknown organic liquids using NMR and IR spectroscopy: A general chemistry laboratory. J. Chem. Educ. 2012, 89, 1450−1453. (8) Alonso, D. E.; Wong, P. A. Neat NMR spectroscopy for general chemistry laboratory. Chem. Educator 2008, 13, 234−235. (9) Arrington, C. A.; Hill, J. B.; Radfar, R.; Whisnant, D. M.; Bass, C. G. Peer mentoring in the general chemistry and organic chemistry laboratories. The pinacol rearrangement: an exercise in NMR and IR spectroscopy for general chemistry and organic chemistry laboratories. J. Chem. Educ. 2008, 85, 288−290. (10) Davila, R. M.; Widener, R. K. Structure and nuclear magnetic resonance: an experiment for the general chemistry laboratory. J. Chem. Educ. 2002, 79, 997−999. (11) Baer, C.; Cornely, K. Spectroscopy of simple molecules. J. Chem. Educ. 1999, 76, 89−90.
of pentane by determining how many carbon signals can be expected for each isomer. An equivalent amount of students could correctly draw and label the unique carbons of at least one of the two unknown ketone compounds. The students had more difficulty with determining the structures of unknown carboxylic acid compounds from NMR data and molecular formula with only 50% being able to correctly identify one of the two unknowns. Student evaluations cite this laboratory as one of the most useful and meaningful experiences of the first semester. We have seen the success of this approach in the second semester of our general chemistry sequence and beyond. For example, in the second semester of our general chemistry course, NMR data is again used to relate the number of carbon signals to molecular structure. NMR data is used to supplement student determined pKa and molar mass in the identification of an unknown weak organic acid. Given the NMR data for the unknown acid, they were almost all able to provide the correct identification. Without this first-semester laboratory, students often struggled with the identification of their unknown weak acid. Additionally, after this NMR laboratory, students were better prepared for organic chemistry, and are able to start using 13C NMR spectra to provide molecular structures their first week of organic laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
The laboratory handout, student data sheets for each group, notes for instructors and 250 MHz spectra of the 18 compounds we used. 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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ACKNOWLEDGMENTS Partial funding for the purchase of the NMR was provided by an NSF Instrumentation and Laboratory Improvement grant (Grant No. DUE-ILI-9351955). Matching funds provided by The Camille and Henry Dreyfus Foundation and Utica College are gratefully acknowledged.
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REFERENCES
(1) Pulliam, C. R.; Pfeiffer, W. F. Putting More Structure in the General Chemistry Laboratory by Using Molecular Models, FT-IR, and Carbon-13 NMR. Abstracts of Papers, 27th Northeast Regional Meeting of the American Society, Saratoga Springs, NY, 1997; American Chemical Society: Washington, DC, 1997; 32. (2) Pulliam, C. R.; Pfeiffer, W. F. Putting More Structure in the General Chemistry Laboratory by Using Molecular Models, FT-IR, and Carbon-13 NMR − Part II, the Second Semester. Abstracts of Papers, 32nd Northeast Regional Meeting of the American Society, Rochester, NY, 2004; American Chemical Society: Washington, DC, 2004, GEN113. (3) Parmentier, L. E.; Lisensky, G. C.; Spencer, B. A guided inquiry approach to NMR spectroscopy. J. Chem. Educ. 1998, 75, 470−471. (4) Everest, M. A.; Vargason, J. M. How does atomic structure affect electron clouds? A guided-inquiry NMR laboratory for general chemistry. J. Chem. Educ. 2013, 90, 926−929. C
DOI: 10.1021/ed500778h J. Chem. Educ. XXXX, XXX, XXX−XXX
