In the Laboratory
Structure Determination of Benzene-Containing C9H12 Isomers Using Symmetry, Peak Heights, and Chemical Shifts in 13C NMR Nanine A. Van Draanen* and Richard Page Department of Chemistry and Biochemistry, California Polytechnic State University, San Luis Obispo, CA 93407; *
[email protected] With the advent of the Anasazi FT-NMR conversion of standard Varian EM-360 NMR spectrometers in many undergraduate laboratories, hands-on 13C NMR has become available to students. A few laboratory experiments using 13C NMR to reinforce concepts in organic chemistry have been presented in this Journal (1–4). Carbon-13 NMR is an outstanding vehicle to demonstrate isomerism and symmetry, and in this simple experiment, beginning organic chemistry students can explore the effect of symmetry and nuclear Overhauser effect (NOE) on the predicted and actual carbon spectra of a series of isomeric substituted benzenes. The 13C NMR spectrum is affected by several factors (5). Molecules with one or more elements of symmetry can have fewer resonances than their asymmetric isomers, and the number of peaks in the 13C NMR spectrum can be predicted easily by analyzing the symmetry of the molecule. Additionally, the presence or absence of hydrogen atoms on a benzene carbon directly affects the peak height through the NOE; while unreliable in other parts of the spectrum, the NOE in the benzene region is useful. The relative heights of the peaks in the aromatic region can be determined by the presence or absence of hydrogen atoms on the carbon, coupled with the number of identical carbons. Finally, the chemical shifts in 13C NMR spectra can be predicted reliably using a literature method (5). Combining the number of peaks, relative heights of peaks, and calculated chemical shifts can give an excellent prediction of the actual 13C NMR spectrum. In this lab, the students are first required to draw and name the eight isomers of C9H12 that contain a benzene ring. Then, based on symmetry and gross chemical environment, they predict the number of signals in general regions of the broadband proton-decoupled 13C NMR spectrum. Combining symmetry with NOE, the students assign a relative peak height (very short, short, medium, tall) to each carbon in the benzene region, being warned not to use this method anywhere else in the spectrum. Finally, they calculate the predicted chemical shift of each carbon and draw the resulting predicted spectrum1 for each isomer (6–8). Students should find that two of the isomers give predicted 13C NMR spectra that are too similar to unambiguously assign a structure and are queried as to what method they can use to differentiate those isomers (they should have some familiarity of the identification of benzene substitution patterns in the fingerprint region of the IR) (9). In lab, the students are given one of the isomers as an unknown. They run a 13C NMR spectrum on the neat liquid at 15 MHz and compare their experimental NMR spectrum with those they calculated to determine the structure of their unknown. If a student received one of the two indistinguishable isomers, he or she will need to run an IR spectrum to determine which of the two compounds it is.
In conclusion, this laboratory experiment introduces students to the concepts of symmetry, peak height, and substituent effects in 13C NMR spectroscopy and allows them to predict a 13C NMR spectrum and then use their predictions to identify an unknown sample. The calculations can be done in lab while students are awaiting their turn at the NMR machine, or they can be assigned as homework and the NMR acquisition worked in around other laboratory work, as instrument availability warrants. Hazards The aromatic isomers of C9H12 are flammable and can irritate the eyes, skin, and respiratory tract (10). Note 1. Computer programs such as ChemDraw have the ability to calculate these chemical shifts and generate a predicted NMR spectrum. However, we find it more useful to have the students go through the exercise of generating the theoretical spectrum by hand, as it gives the opportunity to evaluate the molecule’s symmetry and have some understanding of the source of the chemical shifts in the computer-generated NMR spectra.
Literature Cited 1. Blunt, J. W.; Happer, D. A. R. J. Chem. Educ. 1979, 56, 56. 2. Rablen, P. R.; Deuber, M. A.; Lim, A. C.; Dickson, R. M.; Wittner, C. E. J. Chem. Educ. 1991, 68, 796–797. 3. Cook, G. A. J. Chem. Educ. 1993, 70, 865–866. 4. Stephens, C. E.; Arafa, R . K. J. Chem. Educ. 2006, 83, 1336–1340. 5. Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy, 2nd ed.; Saunders: Fort Worth, TX, 1996; pp 146–163. 6. Van Arnum, S. D. J. Chem. Educ. 2006, 83, 429–431. 7. Olivieri, A. C.; Kaufman, T. S. J. Chem. Educ. 1989, 66, 53–54. 8. Brown, D. W. J. Chem. Educ. 1985, 62, 209–212. 9. Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy, 2nd ed.; Saunders: Fort Worth, TX, 1996; p 45. 10. SIRI MSDS Index. http://siri.org/msds/ (accessed Mar 2009).
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Jul/abs849.html Abstract and keywords Full text (PDF) Links to cited URL and JCE articles Supplement
Student handouts
Instructor notes
Experimental 13C NMR and IR spectra
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 7 July 2009 • Journal of Chemical Education
849
