In the Classroom
A Strategy for Incorporating 13C NMR into the Organic Chemistry Lecture and Laboratory Courses Perry C. Reeves and Chris P. Chaney Department of Chemistry, Abilene Christian University, Abilene, TX 79699
The use of spectroscopy in establishing the structure of molecules continues to be given increased emphasis in the first course in organic chemistry. However, the point in the course at which these techniques are best introduced remains uncertain. Most organic textbooks place the chapters on nuclear magnetic resonance (NMR), infrared (IR), and mass spectrometry (MS) near the end of the topics covered during the first semester of the course. Because of time constraints, some instructors teach spectroscopy as a part of the organic laboratory course. While this strategy may be effective, spectroscopy must also be emphasized in the lecture course, since a common student misconception is that material covered in the lab is less important than that covered in the lecture. In this paper we suggest that carbon nuclear magnetic resonance spectroscopy should be introduced at an early stage of the lecture course and used extensively for structure determination. A laboratory activity illustrating the use of carbon NMR for structure determination will then be described. The Approach Alkanes are the first group of compounds discussed in detail in most organic chemistry courses. Emphasis is given to bonding, hybridization, symmetry considerations, and constitutional isomerism. Students are introduced to the fact that several hydrocarbons have identical molecular formulas but different physical properties, and are therefore different compounds. Students are shown how to draw structures for these isomers and then taught how to name them. From a pedagogical point of view, this appears to be an ideal place in the course to introduce carbon NMR spectroscopy as a means of determining organic structure (1). While the details of the NMR experiment are better described in the laboratory portion of the course, basic theory and spectral results can be presented to the students and used almost immediately, even at this early stage of the course. Since proton-decoupled carbon NMR spectra provide a single signal for each magnetically unique carbon atom in a molecule, the important concepts of symmetry and equivalence can be discussed and then demonstrated concretely to the students. For example, the structures of the three constitutional isomers of the C5 system can be established immediately from their protondecoupled carbon NMR spectra (n-pentane, 3 signals; 2methylbutane, 4 signals; 2,2-dimethylpropane, 2 signals). Students are also introduced to the terms “primary”, “secondary”, “tertiary”, and “quaternary” and they begin to use these terms to describe how carbon atoms are connected in various hydrocarbons. Obviously not all isomeric hydrocarbons can be distinguished by the number of signals contained in their carbon NMR spectrum. Specifically, both 3-methylpentane and 2,2-dimethylbutane produce four carbon signals yet possess different properties and structure. To obtain additional information about the magnetic environment of carbon atoms in molecules, some texts describe off-resonance 1006
decoupling NMR experiments that yield various multiplets for quaternary, methine, methylene, and methyl carbon atoms. However, for practical reasons, these experiments are rarely employed in the organic research laboratory. Instead, modern FTNMR instruments utilize complex pulse sequences such as APT (for “attached proton test”) and DEPT (for “distortionless enhancement by polarization transfer”) to distinguish among signals due to CH3, CH2, CH, and quaternary carbons. The latest editions of several widely used organic textbooks describe this technique instead of off-resonance decoupling. (More detailed, yet elementary, discussions of these techniques can be found in references 2– 4.) When beginning students are presented with both proton-decoupled and DEPT carbon NMR spectra of hydrocarbons, structure assignment becomes relatively straightforward. As other functional groups are introduced throughout the course, the relationship of chemical shift to hybridization, electronegativity, and magnetic anisotropy can be shown. In the process students become comfortable using carbon NMR spectroscopy to assign structures to the carbon skeletons of organic molecules. Proton NMR and the concepts of peak integrals and spin–spin splitting are best introduced later in the first semester. This strategy allows students to see early in the course how chemists assign structures to molecules. It also capitalizes on the students’ inherent interest in advanced laboratory instrumentation as well as their interest in the closely related medical technology MRI (magnetic resonance imaging). The Laboratory Experiment
Laboratory Management Plan Since our laboratory sections are small (15–20 students), we are able to provide students hands-on experiences with a variety of instrumentation, even sophisticated research instruments such as NMR spectrometers. The week before the experiment, NMR theory, instrument design, and operation are discussed in detail in the laboratory. Students are provided detailed instructions for sample preparation and insertion, locking, shimming, acquisition, and processing of the data. A guided tour of the NMR laboratory is also given to the students. Before the next laboratory period, students (working in pairs) reserve the instrument for a 30-minute period. Upon their arrival at the laboratory, they are given an unknown and told that it is an aliphatic C6 or C 7 hydrocarbon. They mix the sample with CDCl3, transfer it to an NMR tube, and bring it to the instrument, where the teaching assistant helps them insert the sample into the magnet. The students instruct the Varian Gemini-200 NMR to automatically lock and shim before the data are acquired. Even beginning students can acquire, process, and plot both the proton-decoupled and DEPT spectra within the 30-minute period. As the spectra are being plotted, the students are shown how to identify peaks corresponding to the solvent as well as the internal stan-
Journal of Chemical Education • Vol. 75 No. 8 August 1998 • JChemEd.chem.wisc.edu
In the Classroom
dard. After collection of the data, the students return to a classroom where they draw structures for all isomers of the assigned hydrocarbon, predict the number of signals that each isomer would produce, and on this basis identify their unknown hydrocarbon.
Experimental Details Each of the C6 and C7 isomeric hydrocarbons except 3ethylpentane may be purchased from Aldrich Chemical Company. Sample concentration for NMR analysis was 25% (v/v) hydrocarbon in CDCl3 solution. A Varian Gemini 200 NMR equipped with a broad-band probe was used to obtain the spectra. Good carbon-NMR spectra are obtained using 128 pulses except for compounds containing quaternary carbon atoms, in which case, 256 pulses are used. DEPT spectra (32 pulses) are obtained and processed using a program included in the NMR’s operating software. Spectral analyses of the hydrocarbons are listed in Table 1. An alternative, less desirable, procedure involves collecting the FIDs (free induction decay curves) and storing them on floppy disks before the laboratory period. Students can then retrieve and process the data without having to wait for data collection. Acknowledgments Support for this project was provided by the Abilene Christian University Research Council and the National Science Foundation ILI Program. Literature Cited 1. A similar approach has been suggested previously: Chapman, O. L.; Russell, A. A. J. Chem. Educ. 1992, 69, 779. 2. Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy, 2nd ed.; Harcourt Brace: Fort Worth, TX, 1996; pp 404–419. 3. Macomber, R. S. A Complete Introduction to Modern NMR Spectroscopy; Wiley: New York, 1997; pp 203–210. 4. Abraham, R.; Fisher, J.; Loftus, P. Introduction to NMR Spectroscopy; Wiley: Chichester, 1988; pp 144 –152.
Table 1. Assignment of NMR Carbon Atom Absorption Frequencies (PPM) as Obtained from DEPT Spectra Compound
Carbon Absorption Frequency/ppm Quaternary Methine Methylene Methyl
C6 Hydrocarbons Hexane
–
–
22.7 31.7
14.0
2-Methylpentane
–
27.7
20.5 41.4
14.2 22.5
3-Methylpentane
–
36.2
29.1
11.3 18.6
2,2-Dimethylbutane
30.3
–
36.4
8.7 28.8
2,3-Dimethylbutane
–
33.8
–
19.4
C7 Hydrocarbons Heptane
–
–
22.7 29.2 32.0
14.0
2-Methylhexane
–
28.0
23.0 29.7 38.8
14.1 22.6
3-Methylhexane
–
34.3
20.2 29.6 39.1
11.2 14.2 19.0
2,2-Dimethylpentane
30.3
–
17.7 46.9
15.0 29.4
2,3-Dimethylpentane
–
31.7 40.5
26.8
11.9 14.8 17.9 20.2
2,4-Dimethylpentane
–
25.5
48.7
22.7
3,3-Dimethylpentane
32.6
–
33.6
8.2 26.1
2,2,3-Trimethylbutane
32.7
37.7
–
17.8 27.1
JChemEd.chem.wisc.edu • Vol. 75 No. 8 August 1998 • Journal of Chemical Education
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