In the Laboratory
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 Caleb A. Arrington, Jameica B. Hill, Ramin Radfar, David M. Whisnant, and Charles G. Bass* Department of Chemistry, Wofford College, Spartanburg, SC 29303-3663; *
[email protected] Collaborative work in the laboratory is valued at Wofford College (1–3), as it is at many other institutions. Earlier collaborations at Wofford have allowed students in our physical chemistry classes to share data and to work with students from other institutions on computational chemistry projects. We now wish to report on preliminary efforts in establishing a program involving laboratory collaboration of another sort—between students enrolled in organic chemistry and those studying general chemistry. Organic chemistry and spectroscopic analysis methods have begun to find their way into the general chemistry laboratory. These experiments range from studies of electronegativity influences on chemical shift (4), to solving unknown structures (5–6), to the preparation of aspirin and spectroscopic confirmation of the product’s structure and purity by FTIR and FT–NMR (7–8). A number of labs have been developed for organic chemistry classes, which are more discovery-based than traditional synthesis labs. Experiments such as these tend to pique students’ interest since they are less like following a “cookbook” (9–16) and don’t require three to four hours to reproduce a known result. Recognizing that we are better students of chemistry now that we are educators in the field, and further recognizing that mentorships can be fun and informative for all parties involved (17–18), we began a program involving the use of organic chemistry students as peer-mentors for general chemistry laboratory students. We felt that it was important that the labs we developed as part of this project should all contain an element of discovery. Overview We introduce organic chemistry students to interpretation of IR and NMR (1H and 13C) spectra during the first semester of organic chemistry. We then require these students to interpret spectra of several unknown compounds as an out-of-lab exercise. Throughout the first term, we reinforce spectroscopic interpretation on lecture tests and as a normal analysis of reaction products. The first lab of the second semester for the organic chemistry students involves a dehydration of cyclohexanol. During this lab we require students to obtain IR and NMR (1H and 13C) spectra of the cyclohexanol and of the cyclohexene formed. This experience allows the organic chemistry students to learn to operate an NMR spectrometer for the first time. During the week in which organic chemistry students are performing the dehydration of cyclohexanol we present an introduction to molecular spectroscopy to the general chemistry students. Although this topic is not typically covered in general chemistry textbooks, we introduce molecular spectroscopy after 288
our textbook chapters on molecular bonding. Providing experimental evidence for molecular structure helps reinforce the abstract subject of molecular bonding. From IR spectroscopy students see evidence of the effect of a double bond on the vibrational spectra, noting that the stronger double bond has a greater vibrational frequency than does a single bond. We present 1H and 13C NMR spectroscopy to focus on electron shielding and the electron distribution in the molecule. The students thus are presented with a reason for understanding electronegativity, bond polarity, and electron distribution. One advantage of the system selected in this study is that proton coupling is not observed. This powerful structural determination feature of NMR spectroscopy can be reserved for their organic chemistry course next year. The experiment requires students to utilize integration to help them arrive at the correct structure. During the second week of the project we require each organic chemistry student to act as a peer mentor for two general chemistry students. The organic chemistry students teach the general chemistry students how to set up a distillation as each team of three students dehydrates 2,3-dimethyl-2,3-butanediol (pinacol). Each team is told only that they would form water and an organic product with the molecular formula C6H12O. Each team is provided with a set of spectra (IR, 13C NMR and 1H NMR) of the pinacol they started with, and the organic chemistry student (with the assistance of a third- or fourth-year teaching assistant) is to teach the general chemistry students how to acquire their own IR and NMR spectra of their unknown product. Working together, each team is to determine the structure of their product. (It should be noted that the organic students had not been introduced to the pinacol rearrangement prior to this lab and great care was made to avoid calling 2,3dimethyl-2,3-butanediol by its common name, pinacol, during the laboratory exercise.) Although the pinacol rearrangement is covered in many laboratory texts (see for example 19–22) and has been the subject of articles in this Journal (23, 24), we believe that this is the first time that it has been used with general chemistry students. The reaction (Scheme I) appeared ideal to us because the dehydration takes place quickly, it gives reasonable yields
OH
OH
H2SO4
%
O
H2O
Scheme I. Reaction of 2,3-dimethyl-2,3-butanediol.
Journal of Chemical Education • Vol. 85 No. 2 February 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
(~70%), and provides a product with relatively simple spectra (no coupling between hydrogens in the 1H NMR). It also provides a product that is somewhat unexpected and thus presents the organic chemistry students with an interesting enigma (23). (Whereas the previous week’s dehydration provided an alkene, this experiment yields a ketone.) Obviously this project will need to be replaced with other peer-mentoring projects on a rotational basis as first-year students who conduct this experiment will certainly find no mystery at all should they conduct the experiment the following year as a second-year students. Thus we are in the midst of developing other peer-mentoring projects to be performed in conjunction with this one.
• Distillation was used to impress upon general chemistry students the importance of intermolecular attractive forces. That topic had just been covered in the lecture and students were required to explain why the mystery product distilled over rather than the starting material.
• Though approximately 75% of our general chemistry students matriculate to organic chemistry the next year we feel that it is important to show students for which general chemistry is a terminal chemistry course (often physics majors) the power of modern spectroscopy for elucidating molecular structure.
General Aspects 1H and 13C NMR spectra were recorded on a Varian EM360 NMR spectrometer outfitted with an Eft NMR conversion package from Anasazi Instruments. FTIR spectra were obtained using a Perkin Elmer Spectrum One. All chemicals used were reagent grade.
Although this effort was a good beginning to the use of peer mentoring in a laboratory setting, we did note that while some groups did indeed use the organic chemistry students as true mentors, the organic chemistry students dominated other groups. In future projects we intend to specify that the organic chemistry students not be allowed to touch the glassware—only to advise the general chemistry students on how to set up the equipment. Projects underway will pay more attention to the design and evaluation of the team learning approach. A number of useful references are available on the topic (25).
Dehydration of 2,3-Dimethyl-2,3-Butanediol
Acknowledgments
Experimental Procedure and Results
Add 2 boiling chips (or a magnetic stir bar), 2,3-dimethyl2,3-butanediol (6 g), 6 M sulfuric acid (30 mL) to a clean, 100-mL, round-bottomed flask. Set up a simple distillation and collect the product—which has the molecular formula C6H12O—along with some water as it forms. The top, organic, layer should be carefully decanted from the aqueous layer and dried over anhydrous calcium chloride for 5 min. The organic material should then be passed through a Pasteur pipette containing a small plug of cotton and anhydrous calcium chloride into a tared product vial. After recording the mass of the product, obtain the spectra (IR, 1H NMR, and 13C NMR) and determine the structure of the product. (See the online material for example spectra, student handouts, and instructions.) Hazards Sulfuric acid is corrosive and can cause severe burns. 2,3dimethyl-2,3-butanediol can be irritating to the eyes, respiratory system, and skin. Calcium chloride is an irritant; avoid inhalation. Gloves should be worn throughout this experiment, protective eye wear used, and all work should be conducted in a fume hood. Conclusions We believe that this project has been a successful attempt at peer mentoring for a number of reasons:
• General chemistry students indicated that this was one of their favorite experiments of the year. Students in one general chemistry section even requested that their next exam include spectra to interpret. Although this is anecdotal information, we believe that the peer mentoring experience may provide motivation for students to take organic chemistry.
• General chemistry students observe effects of bonding and electronegativity spectroscopically for the first time.
We gratefully acknowledge the support of the National Science Foundation Course, Curriculum, and Laboratory Improvement program (CCLI-A&I Grant DUE-0126407), and Wofford College. Literature Cited 1. Whisnant, D. M.; Howe, J. J.; Lever, L. S. J. Chem. Educ. 2000, 77, 199–201. 2. Whisnant, D. M.; Howe, J. J.; Lever, L. S. J. Chem. Educ. 2000, 77, 1648–1649. 3. Towns, M.; Saunder, D.; Whisnant, D.; Zielinski, T. J. J. Chem. Educ. 2001, 78, 414–415. 4. Davis, D. S.; Moore, D. E. J. Chem. Educ. 1999, 76, 1617–1618. 5. Dávila, R. M.; Widener, R. K. J. Chem. Educ. 2002, 79, 997– 999. 6. Baer, C.; Cornely, K. J. Chem. Educ. 1999, 76, 89–90. 7. Parmentier, L. E.; Lisensky, G. C.; Spencer, B. J. Chem. Educ. 1998, 75, 470–471. 8. Byrd, H.; O’Donnell, S. E. J. Chem. Educ. 2003, 80, 174–176. 9. McGowens, S. I.; Silversmith, E. F. J. Chem. Educ. 1998, 75, 1293–1294. 10. McElveen, S. R.; Gavardinas, K.; Stamberger, J. A.; Mohan, R. S. J. Chem. Educ. 1999, 75, 535–536. 11. Shadwick, S. R.; Mohan, R. S. J. Chem. Educ. 1999, 76, 1121– 1122. 12. Sgariglia, E. A.; Schopp, R.; Gavardinas, K.; Mohan, R. S. J. Chem. Educ. 2000, 77, 79–80. 13. Centko, R. S.; Mohan, R. S. J. Chem. Educ. 2001, 78, 77–79. 14. Cabay, M. E.; Ettlie, B. J.; Tuite, A. J.; Welday, K. A.; Mohan, R. S. J. Chem. Educ. 2001, 78, 79–80. 15. Gallego, M. G.; Romano, S.; Sierra, M. A.; Nieto, E. J. Chem. Educ. 2001, 78, 765–769. 16. Wachter-Jurcsak, N.; Reddin, K. J. Chem. Educ. 2001, 78, 1264–1265. 17. Huseth, A. J. Chem. Educ. 1998, 75, 528A–528B.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 2 February 2008 • Journal of Chemical Education
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In the Laboratory 18. Wilson, A.; Myers, C.; Crull, G.; Curtis, M.; Patterson, P. J. Chem. Educ. 1999, 76, 1414–1416. 19. See for example Landgrebe, J. A. Theory and Practice in the Organic Laboratory: With Microscale and Standard Scale Experiments, 4th ed.; Brooks/Cole: Pacific Grove, CA, 1993; pp 474–479. 20. Schoffstall, A. M.; Gaddis, B. A.; Druelinger, M. L. Microscale and Miniscale Organic Chemistry Laboratory Experiments, 2nd ed.; McGraw-Hill: Boston, 2000; pp 399–403. 21. Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques: A Small-Scale Approach, 2nd ed.; Harcourt Brace: Fort Worth, 2005; pp 429–431. 22. Campbell, B. N., Jr.; Ali, M. M. Organic Chemistry Experiments: Microscale and Semi-Microscale; Brooks/Cole Pacific Grove, CA, 1994; pp 345–347. 23. Wojciechowsi, B. J.; Deal, S. T. J. Chem. Educ. 1996, 73, 85. 24. Sands, R. D. J. Chem. Educ. 1992, 69, 667.
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25. (a) Gosser, D. K.; Cracolice, M. S.; Kampmeier, J. A.; Roth, V.; Strozak, V. S.; Varma-Nelson, P. Peer-Led Team Learning: A Guidebook; Prentice Hall: Upper Saddle River, NJ, 2001. (b) Johnson, D. W.; Johnson, R. T.; Holubec, E. J. Cooperative Learning in the Classroom; Association for Supervision and Curriculum Development: Alexandria, VA, 1994.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Feb/abs288.html Abstract and keywords Full text (PDF)
Links to cited JCE articles
Supplement
Handouts for the students
Notes for the instructor including spectra of product
Journal of Chemical Education • Vol. 85 No. 2 February 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
