Using Essential Oils To Teach Advanced-Level Organic Chemistry

Jul 25, 2013 - *E-mail: [email protected]. .... Students were provided with a list of possible structures from which to choose (Figure 1). ... th...
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Laboratory Experiment pubs.acs.org/jchemeduc

Using Essential Oils To Teach Advanced-Level Organic Chemistry Separation Techniques and Spectroscopy Tina M. Bott and Hayley Wan* Department of Chemistry, University of Alberta, Edmonton, Alberta, T6G 2G2, Canada S Supporting Information *

ABSTRACT: Students sometimes have difficulty grasping the importance of when and how basic distillation techniques, column chromatography, TLC, and basic spectroscopy (IR and NMR) can be used to identify unknown compounds within a mixture. This two-part experiment uses mixtures of pleasant-smelling, readily available terpenoid compounds as unknowns to provide upper-division undergraduate students with experience in two major areas common to organic chemistry: (1) separation of mixtures and (2) structure determination. This experiment also encourages students to use deductive reasoning skills in addition to their expanding knowledge of spectroscopy for structure determination purposes.

KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives, Problem Solving/Decision Making, Chromatography, IR Spectroscopy, NMR Spectroscopy, Separation Science, Spectroscopy

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organic chemistry: (1) separation of mixtures using advanced techniques, such as vacuum fractional distillation and silica gel flash column chromatography, that students have yet to encounter, and (2) structure determination using a problemsolving approach appropriate for upper-division undergraduate chemistry students. This experiment involved the separation and purification of a liquid mixture containing two unknown terpenoid compounds. This was followed by elucidation of each unknown structure using physical and spectroscopic data.

ssential oils that have been extracted from plants contain many colorless, pleasant-smelling compounds, such as limonene, carvone, and pinene.1 The majority of these fall into a class of compounds known as terpenes,2 which have been used in the flavor and fragrance industries for many years, and are, thus, inexpensive and readily available in large quantities. The chemical structures and stereochemistry associated with these types of compounds make them suitable for structuredetermination exercises involving IR and 1D-NMR spectroscopy for upper-division undergraduate organic chemistry students.3 In our experience, students do not get enough practice in separation and spectroscopy techniques, which are used daily in research laboratories. Distillation and chromatography are important laboratory techniques that are commonly used to separate mixtures of compounds effectively. Basic distillation techniques (simple and fractional), gravity column chromatography, and TLC, in addition to basic spectroscopy (IR and NMR), are frequently taught as part of a second-year undergraduate organic chemistry laboratory curriculum at many institutions.4−9 However, these techniques are often taught in separate laboratory periods, and students sometimes have difficulty grasping the importance of when and how these techniques can be used with IR and NMR spectroscopy to enable structural determination of unknown compounds within a mixture. This two-part experiment was designed for an upper-division undergraduate organic chemistry laboratory course, with a prerequisite second-year organic chemistry course, to reinforce how to use techniques together for structure determination, which is a necessary skill in organic chemistry. The objective of this experiment was to provide students with experience in two major areas common to © 2013 American Chemical Society and Division of Chemical Education, Inc.

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EXPERIMENT Each student was provided with a two-component mixture containing a lower boiling point and higher boiling point liquid (in a 10:1 ratio, respectively). The lower boiling point liquid (boiling point range of 150−180 °C) was chosen from β(−)-pinene, eucalyptol, and (R)-limonene. The higher boiling point liquid (boiling point range of 205−270 °C) was chosen from (−)-citronellol, (R)-carvone, and geraniol. In total, there were eight suitable unknown combinations that could be prepared that would allow for suitable separation by distillation (see the Supporting Information). Each unknown combination was placed in a vial with a designated letter code, and each student in the class was issued a sample containing a different letter code. This experiment was carried out over two laboratory classes, each lasting four hours. In the first class, a brief tutorial on TLC analysis and vacuum distillation was conducted before students were told to analyze their unknown mixtures using TLC. It was suggested that Published: July 25, 2013 1064

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gel can cause silicosis, flash columns are prepared using the slurry method and dry silica gel is handled only in the fume hood. Carvone, pinene, eucalyptol, limonene, citronellol, and geraniol are eye, respiratory tract, and skin irritants. Pinene is flammable, and limonene is a skin sensitizer. Chloroform-d is toxic and carcinogenic, as well as a skin and eye irritant.

students start with an eluent of hexanes:ethyl acetate (4:1) and then adjust the polarity as necessary to obtain a desired separation and a suitable Rf value (0.3). Following TLC analysis, fractional vacuum distillation (with a Vigreux column and cow distillation adapter) was used to distill off the lower boiling point liquid until a residue remained. A water aspirator or the house vacuum, equipped with a manometer, was used as the vacuum source. Students recorded the distillation pressure and the boiling point of the lower boiling liquid as it distilled and utilized a nomograph to determine the corrected boiling point. After the distillation was complete, students used TLC analysis to determine which distillation fractions were pure and could be combined. Once students had obtained the pure, low boiling point liquid, IR and 1H NMR spectra of their unknown liquid was obtained. Students ran their own IR spectrum and were shown how to prepare a proper NMR sample; however, a technician ran the NMR samples on a spectrometer. The remaining residue, containing the higher boiling point liquid, likely contaminated with a small quantity of the lower boiling point liquid, was then cooled, stoppered, and stored for part two of the experiment. In the second part of the experiment, students performed silica gel flash column chromatography on the remaining residue to purify the higher boiling point liquid. From the first part of the experiment, students should have determined a suitable eluent system for their separation. The higher boiling point liquid was purified by flash chromatography, and IR and 1 H NMR spectra of the pure compound were obtained. Using the spectroscopic data obtained for both liquids, students deduced the structures of both unknown compounds. Students were provided with a list of possible structures from which to choose (Figure 1). In lieu of this structure list, the instructor could choose to provide more spectroscopic data (MS, 13C NMR, COSY) to make the structural elucidation exercise more complex.

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DISCUSSION The unknown components for this experiment were chosen to challenge students to use more in-depth one-dimensional 1H NMR analysis (such as complicated splitting patterns and coupling constants) to determine the structure of their unknown liquids. The choice of lower boiling and higher boiling liquid combinations was based on whether the compounds in the mixture had a large enough difference in Rf values (>0.4). This was to allow for proper separation during silica gel flash chromatography. A list of suitable lower and higher boiling liquid combinations for this experiment, as well as Rf values for each component and the optimal TLC elution system for each combination, is provided in the Supporting Information. Once students had successfully purified the lower boiling liquid, they used the corrected boiling point (obtained using a nomograph) to eliminate some structures from the list provided in Figure 1. Students used IR and 1H NMR spectral analysis to identify the structure for their first unknown liquid. Looking specifically at the olefin region of their 1H NMR spectrum, analyzing the splitting patterns and calculating coupling constants, students were able to deduce the structure of their first unknown. Following purification of the higher boiling point liquid, students used a similar methodology to justify their choice of structure for the second unknown liquid. For the second unknown, the use of IR was of particular importance to determine if either an alcohol or carbonyl group was present. Following this initial analysis, the aromatic and olefinic regions of the 1H NMR spectrum were once again examined to determine the structure of the second unknown. Representative student data are shown in the Supporting Information. In cases where the student did not successfully purify one or both of the unknowns, and did not have time to repeat the purification steps, the instructor supplied backup spectroscopic data. This occurred for approximately 1−2% of students. At the end of this two-part experiment, students gained practical experience in using fractional vacuum distillation and silica gel flash chromatography. Although some students found this experiment difficult because they were learning two new techniques within the same experiment, they did develop a better sense of different purification techniques available to them. In addition, they developed a better understanding of spectroscopy, especially with proton NMR analysis. Students were exposed to the process involved in structure determination and how to use physical and spectroscopic data obtained in an experiment to determine the structure of an unknown compound successfully. This experiment was first implemented in fall 2010 and has been completed by approximately 100 students for three fall semesters.

Figure 1. Structures and boiling points of the possible unknown components of the liquid mixture.

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HAZARDS For this experiment, students are equipped with safety glasses, lab coats, and gloves. The experiment is performed either in a fume hood or on a benchtop equipped with a ventilation trunk. Acetone, hexanes, and ethyl acetate are flammable and must be properly disposed of after use. Hexane is a neurotoxin. As silica

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CONCLUSION The separation techniques and spectroscopy experiment described has been successfully implemented into upperdivision undergraduate organic laboratories as a two-week 1065

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experiment. In these two four-hour lab classes, students developed their skills in miniscale fractional vacuum distillation, silica gel flash column chromatography, and TLC analysis. From a spectral analysis standpoint, this experiment encouraged students to use both their expanding knowledge of spectroscopy, which is a main focus of this class, and deductive reasoning skills to aid in structure determination. This experiment, although intended for an upper-division undergraduate chemistry course, could potentially be used in the second semester of a second-year undergraduate organic chemistry course for chemistry majors. For situations where the cost of purchasing distillation equipment for a whole class is too expensive, it could be possible to include this experiment in a rotation of experiments to make the best use of limited equipment.

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ASSOCIATED CONTENT

S Supporting Information *

Instructor’s notes, student handout, idealized data, representative student data, and CAS registry numbers. This material is available via the Internet at http://pubs.acs.org.

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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 The authors acknowledge the help and support from the Chemistry Storeroom staff (A. Yeung, M. Munroe, and M. Kingston) in preparing the lab rooms and the student samples. The authors also thank T. L. Lowary, the 2010 fall session teaching assistants, Lisa Shulman, and the Chem 361 students for testing the experiment and providing feedback.

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REFERENCES

(1) Pelter, L. S. W.; Amico, A.; Gordon, N.; Martin, C.; Sandifer, D.; Pelter, M. W. Analysis of Peppermint Leaf and Spearmint Leaf Extracts by Thin-Layer Chromatography. J. Chem. Educ. 2008, 85, 133−134. (2) Glidewell, C. Monoterpenes: An easily accessible but neglected class of natural products. J. Chem. Educ. 1991, 68, 267−269. (3) Alty, L. T. Monoterpene Unknowns Identified Using IR, 1HNMR, 13C-NMR, DEPT, COSY, and HETCOR. J. Chem. Educ. 2005, 82, 1387−1389. (4) Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Organic Laboratory Techniques, A Contemporary Approach, 3rd ed.; W. B. Saunders Company: Philadelphia, 1988,; pp 554−575 and 593−610. (5) Horowitz, G. Undergraduate Separations Utilizing Flash Chromatography. J. Chem. Educ. 2000, 77, 263−264. (6) Feist, P. L. The Separation and Identification of Two Unknown Solid Organic Compounds: An Experiment for the Sophomore Organic Chemistry Laboratory. J. Chem. Educ. 2004, 81, 109−110. (7) Goodrich, J.; Parker, C.; Phelps, R. The microscale separation of lycopene and [beta]-carotene from tomato paste. J. Chem. Educ. 1993, 70, A158. (8) Svoronos, P.; Sarlo, E. Separation of methylene blue and fluorescein: A microscale undergraduate experiment in column chromatography. J. Chem. Educ. 1993, 70, A158−A159. (9) Davies, D. R.; Johnson, T. M. Isolation of Three Components from Spearmint Oil: An Exercise in Column and Thin-Layer Chromatography. J. Chem. Educ. 2007, 84, 318−320.

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