Laboratory Experiment pubs.acs.org/jchemeduc
Identification of an Alcohol with
13
C NMR Spectroscopy
Paul H. Chamberlain* Department of Chemistry, George Fox University, Newberg, Oregon 97132, United States S Supporting Information *
ABSTRACT: The identification of an unknown alcohol with 13C NMR is used to introduce students to NMR spectroscopy early in the second-year undergraduate organic chemistry course. Students learn basic concepts of NMR, including an introduction to magnetic resonance and chemical shifts due to electronegativity effects. This experiment also reinforces knowledge from the lecture on organic molecular structures, including functional groups, isomerization, hybridization, and electronegativity. To identify an unknown alcohol, students obtain the boiling point and perform hands-on 13C and DEPT 90 and 135 experiments with a high-field NMR instrument.
KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Alcohols, NMR Spectroscopy
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experiment for the introduction to NMR spectroscopy. A similar approach using 13C NMR spectroscopy was employed for the identification of alkanes.2 However, by using alcohols, the important concept of electronegativity has been added and can now be related to chemical shift. Use of alcohols also has the benefit of expanding the separation of carbon signals for easier interpretation. Finally, use of alcohols allows discussion of the alcohol functional group that is always of interest to students in their everyday life. A brief introduction to nomenclature of alcohols is required, but students do not typically find this topic difficult. Because of the relatively simple operation of new, high-field NMR spectrometers, coupled with the ease of interpretation of the spectra, students are able to perform DEPT (distortionless enhancement by polarization transfer) experiments, as well as traditional 13C NMR spectroscopy. Students perform 13C NMR spectroscopy, which shows all carbons, with or without attached H’s, DEPT 90, which shows only 3° carbons, and DEPT 135, which shows all carbons that have a hydrogen attached. DEPT 135 also distinguishes between carbons with an odd number of hydrogens (CH and CH3) and carbons with an even number of hydrogens (CH2) by the phase of the signal. For example, the DEPT 135 for 1-butanol is shown in Figure 1. The 13C NMR spectral properties for a number of alcohols that could be used as the unknowns are summarized in Table 1. The identity of an unknown alcohol can be identified from the 13 C and DEPT spectra because no two unknowns have the same number of peaks and the same number of CH, CH2, or CH3 signals. For example, there are four unknown compounds that give four signals in the 13C NMR spectra. However, each
he use of spectroscopy for chemical structure determination has been well established in the organic chemistry laboratory.1−7 Spectroscopy is generally introduced in the second semester based on the placement of this topic in the majority of organic chemistry textbooks.8,9 However, with the recent acquisition of a high-field spectrometer, NMR theory and interpretation has been introduced early in the first semester at this institution. In particular, the speed of processing of 13C NMR samples with an FT-NMR, coupled with an automatic sample changer, allows the experiment described here to be completed easily in a 3-h time frame. This approach has at least two advantages. First, it aids in teaching concepts, such as electronegativity and molecular structure, including distinguishing between primary, secondary, and tertiary carbons, that are being presented in the first semester lecture. Second, it allows the use of NMR spectroscopy to be used much earlier in the laboratory for characterization of synthesis products. The more typical sequence for introducing NMR spectroscopy begins with coupled 1H NMR spectra. However, it seems that proton-decoupled 13C NMR is a better introduction to NMR for students. 13C NMR spectra, at least with smaller compounds, are easier to interpret than 1H NMR spectra, as students typically struggle with the more complex concepts of integration and spin−spin splitting needed for 1H spectral interpretation. The three major concepts required by students to interpret 13C NMR spectra are determination of the number of distinct carbons in a structure, the effect of electronegativity on the chemical shift, and the concept of hybridization. By the time students perform their first NMR experiment (the sixth week of the semester), they have been introduced to these concepts in lecture. The identification of alcohols is used in this © 2013 American Chemical Society and Division of Chemical Education, Inc.
Published: September 20, 2013 1365
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samples and send the FID data to students for processing outside of class. An NMR spectrometer equipped with an automatic sample changer allows a class of 20 students to set up the experiment in a 3-h laboratory. A detailed experimental description is given in the Supporting Information.
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HAZARDS Students are warned that the unknowns may be toxic and can be harmful if the compound is inhaled, comes into contact with their skin, or is ingested. Several of the alcohols are flammable; thus, the unknowns should be kept away from flames. Deuterated chloroform is a possible carcinogen. Safety glasses and gloves should be worn when handling these compounds.
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DISCUSSION This experiment has been carried out by several classes of firstsemester, second-year undergraduate organic chemistry students. Approximately 80% of these students obtained the correct result for the unknown structure. The primary reason students did not determine the correct answer was due to their unfamiliarity with NMR spectroscopy coupled with the fact that they were still struggling with the basic concepts of electronegativity, molecular structure, and so forth. As a whole, this experiment gave students a positive and often rewarding introduction to NMR spectroscopy in the time frame of a 3-h laboratory session. Students learned how to prepare a sample for NMR analysis and how to run a state-of-the-art instrument. They also learned how to distill a compound for purification and boiling point determination. Although all of the unknowns could be determined using only NMR data, the boiling point helped students to confirm the identity of the unknown. This confirmation was found to be helpful because this was the first introduction to NMR and students were not as confident about their NMR results as they became later in the course. This early introduction to NMR spectroscopy provided significant benefits over the more traditional second-semester introduction. Not only was NMR spectroscopy used earlier and thus more often, but also this introduction reinforced what was being taught in the lecture about molecular structure. Students were able to apply their knowledge of concepts of structure, electronegativity, and functional groups discussed in lecture with a hands-on laboratory experiment. Finally, early in their organic chemistry course, students were introduced to NMR spectroscopy, a very important technique that organic chemists utilize to determine molecular structures.
Figure 1. DEPT 135 Spectrum of 1-butanol.
Table 1. Number of Peaks in 13C NMR Spectra no. of peaks alcohol 2-Propanol 2-Methyl-2-propanol 1-Propanol 3-Pentanol 2,4-Dimethyl-3-pentanol Cyclopentanol 2-Methyl-2-butanol 1-Butanol 3,3-Dimethyl-2-butanol Cyclohexanol 4-Methyl-2-pentanol 1-Pentanol 4-Methylcyclohexanol 2-Hexanol 2-Methyl-1-pentanol 1-Hexanol 2,2,4-Trimethyl-1-pentanol 2-Methylcyclohexanol 3,5,5-Trimethyl-1-hexanol
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C
2 2 3 3 3 3 4 4 4 4 5 5 5 6 6 6 6 7 7
CH
CH3
CH2
1 0 0 1 2 1 0 0 1 1 2 0 2 1 1 0 1 2 1
1 1 1 1 1 0 2 1 2 0 2 1 1 2 2 1 2 1 2
0 0 2 1 0 2 1 3 0 3 1 4 2 3 3 5 2 4 3
compound generates a unique set of DEPT spectra: 2-methyl2-butanol has no CH and thus no peak in DEPT 90, two CH3 peaks, and one CH2 peak in DEPT 135; 1-butanol has no CH, and one CH3, but has three CH2 peaks; 3,3-dimethyl-2-butanol has one CH peak, two CH3 peaks and no CH2 peaks; and cyclohexanol has one CH, no CH3 and three distinct CH2 peaks.
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ASSOCIATED CONTENT
* Supporting Information S
Notes for the instructor and a complete student experimental handout. This material is available via the Internet at http:// pubs.acs.org.
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EXPERIMENT Before the lab, students are provided with detailed instructions, including a brief introduction to NMR concepts, a complete procedure, and a list of potential unknown alcohols. They are instructed on the use of an NMR spectrometer and accompanying software. Students are provided a sample of one of the alcohols and first determine the boiling point by distillation. They use the freshly distilled alcohol to prepare a sample in CDCl3 for NMR analysis. Students collect their data using the NMR spectrometer and process the FID data using instrumental software. This is most conveniently done if the NMR instrument is set up to acquire the NMR data for the
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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 a NSF Course, Curriculum, and Laboratory Improvement Grant 1366
dx.doi.org/10.1021/ed300833s | J. Chem. Educ. 2013, 90, 1365−1367
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(Grant No. DUE-CCLI 0633346). George Fox University provided matching funds. The author would also like to acknowledge Carlisle Chambers, chemistry department chair, for his support with this project.
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REFERENCES
(1) 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, 1382−1384. (2) Chapman, O. L.; Russell, A. A. Structure, Chirality, and FT-NMR in Sophomore Organic Chemistry. A Modern Approach to Teaching. J. Chem. Educ. 1992, 69, 779−782. (3) Reeves, P. C.; Chaney, C. P. A Strategy for Incorporating 13C NMR into the Organic Chemistry Lecture and Laboratory Courses. J. Chem. Educ. 1998, 75, 1006−1007. (4) Zubrick, J. W. The Organic Chem Lab Survival Manual: A Student’s Guide to Techniques, 9th ed.; John Wiley and Sons: Hoboken, NJ 7030; Chapter 33. (5) Davis, D. S.; Moore, D. E. Incorporation of FT-NMR throughout the Chemistry Curriculum. J. Chem. Educ. 1999, 76, 1617−1618. (6) Ball, D. B.; Miller, R. Impact of Incorporation of High Field FTNMR Spectroscopy into the Undergraduate Chemistry Curriculum. J. Chem. Educ. 2002, 79, 665−666. (7) Liotta, L. J.; James-Perderson, M. J. Identification of an Unknown Compound by Combined Use of IR, 1H NMR, 13C NMR, and Mass Spectrometry: A Real-Life Experience in Structure Determination. J. Chem. Educ. 2008, 85, 832−833. (8) McMurray, J. Organic Chemistry, 8th ed.; Brooks/Cole: Belmont, CA, 2012; Chapter 13. (9) Wade, L. G. Organic Chemistry, 8th ed.; Pearson; Upper Saddle River, NJ, 2012; Chapter 13.
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