In the Laboratory
A Quick and Easy Simplification of Benzocaine’s NMR Spectrum Suzanne R. Carpenter* and Richard H. Wallace Department of Chemistry & Physics, Armstrong Atlantic State University, Savannah, GA 31419; *
[email protected] We wish to report a quick and easy way to simplify the NMR spectrum of benzocaine. Increasing undergraduate student use of instruments in the sophomore organic chemistry sequence is desirable but requires that costs and complexity of this effort be considered. While others have made suggestions to help contain the cost (1), we provide a tip to maximize the clarity and minimize the confusion associated with the interpretation of the NMR of benzocaine. The preparation of benzocaine is a common experiment among sophomore-level organic chemistry laboratory texts (2). We have used this experiment for several years in the early part of the second semester of organic chemistry laboratory primarily as a way of reminding the students of the fundamentals of running and interpreting NMR spectra (a topic they are first exposed to in the first semester of the sequence). The NMR of benzocaine is particularly useful in this context because it has a fairly simple spectrum that can be obtained by the student on a 60 MHz instrument (2); however, here is a complication. The broad peak for the amine hydrogens frequently overlaps the quartet assigned to the methylene protons of the ethyl group in the spectrum run in deuterated chloroform. This is seen with the spectrum in the laboratory text we use (2). The integration curve the student plots on the 60 MHz instrument is then more complicated than we would like. Instead of a simple and quick review of the basics of NMR interpretation, the exercise becomes more complicated. An obvious solution, and one that has been used, is to obtain the spectrum using a high field instrument but student operation of the simpler 60 MHz spectrometer is more straightforward and routinely practiced. Knowing that solvent changes can result in the shift of signal positions (3–8), we found an inexpensive and easy way to simplify the NMR spectrum of benzocaine. The addition of a small quantity of deuterated benzene to the benzocaine sample in deuterated chloroform does the trick!1 The peak for the amine hydrogens shifts upfield meaning the amine singlet and the quartet assigned to the methylene protons of the ethyl group are nicely separated. This allows the spec-
www.JCE.DivCHED.org
•
trum to be cleanly integrated and more easily interpreted by NMR novices. Following the students’ successful interpretation of the simplified spectrum, comparison of benzocaine’s spectrum in CDCl3 with and without C6D6 provides a relevant example of solvent effects on chemical shifts. Acknowledgment The authors would like to acknowledge support from the National Science Foundation’s Course, Curriculum and Laboratory Improvement Program under grant DUE9952343 that allowed for the purchase of a 300 MHz NMR spectrometer. Note 1. The sample for the NMR was prepared as follows: 0.030 g of benzocaine was dissolved in 0.50 mL of CDCl3 with 5 drops of added C6D6 .
Literature Cited 1. Burke, Jeffrey T. J. Chem. Educ. 1996, 73, 550. 2. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques: A Small-Scale Approach, 2nd ed.; Thomson Brooks/Cole: Belmont, CA, 2005; pp 361–364. 3. Friebolin, Horst. Basic One- and Two-Dimensional NMR Spectroscopy; VCH Publishers: New York, 1991; p 45. 4. Macomber, Roger S. A Complete Introduction to Modern NMR Spectroscopy; John Wiley and Sons, Inc.: New York, 1998; p 170. 5. Lambert, J. B.; Mazzola, E. P. Nuclear Magnetic Resonance Spectroscopy; Pearson Prentice Hall: Upper Saddle River, NJ, 2004; pp 76–79. 6. Claridge, T. D. W. High-Resolution NMR Techniques in Organic Chemistry; Pergamon: New York, 1999; pp 75–77. 7. Kalyanam, N. J. Chem. Educ. 1983, 60, 635–636. 8. Cavaleiro, J. A. J. Chem. Educ. 1987, 64, 549–550.
Vol. 83 No. 4 April 2006
•
Journal of Chemical Education
637
