In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp
Mass Spectral Fragmentation Patterns of Deuterated Butyl and Ethyl Acetates
SUNY-Cortland Cortland, NY 13045
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An Easy Microscale Isotopic Labeling Experiment Hengameh Zahedkargaran† and Leverett R. Smith* Department of Chemistry, Contra Costa College, San Pablo, CA 94806; *
[email protected] Gas chromatography–mass spectrometry has become part of the routine characterization of compounds in the organic lab. We have developed an experimentally uncomplicated microscale experiment that illustrates the use of isotopic labeling, in this case deuteration, to help confirm and interpret mass spectral fragmentation patterns in butyl acetate and ethyl acetate. In this way, many fragments can be confirmed experimentally, not just by going on a basis of plausibility or by “looking them up”. Given the expense of the deuterated reagents required, as well as the need for a series of chromatographic runs, a microscale procedure and sharing of data are most appropriate. Background Organic mass spectrometry began its strong growth in the 1950s (1), but expense and complexity of instrumentation limited access to undergraduates until relatively recently. Research use of gas chromatography–mass spectrometry (GC–MS) was well developed by the 1970s. GC–MS became an essential part of regulatory-driven chemical analysis in the 1980s, and analytical methods for high-volume, reproducible analyses were established by the U.S. Environmental Protection Agency (EPA) and other organizations (2, 3). Numerous instrumental options are available (4). This Journal has included such topics as GC–MS of halogenated solvents (5), fuel constituents (6, 7), ozonolysis products (8), fragrances (9, 10), identification of unexpected reaction products (11), and GC–MS–MS (12). Even courses including non-science majors can include the topic (13, 14). While the first step to introducing mass spectrometry may predictably tie in with isotopes and molecular weights, use for more detailed structural verifications need not come far behind. Laboratory Exercise Each student prepares one or more of the following esters, to give materials for analysis in the lab section as a whole: CH3CO2CH2CH2CH2CH3 CH3CO2CH2CH3 CD3CO2CH2CH2CH2CH3 CD3CO2CH2CH3 CH3CO2CD2CD2CD2CD3 CH3CO2CD2CD3 CD3CO2CD2CD2CD2CD3 CD3CO2CD2CD3 †
Student participant, Center for Science Excellence, Contra Costa College.
Experimental Procedure To a small reaction vessel add 50 µL of acetic acid, 15 µL of either (as instructed) 1-butanol or ethanol, and 15 µL of concentrated sulfuric acid. Cap the vessel securely. Next prepare a similar reaction mixture using deuterated acetic acid and/ or alcohol instead of undeuterated starting material. Heat at 90–100 °C for one hour in a beaker of water on a hot-plate. Remove the tubes from the bath and allow to cool to room temperature and add 5 mL of pentane; then wash each product solution successively with 1 mL of water, 1 mL of saturated aqueous sodium bicarbonate, and 1 mL of saturated aqueous sodium chloride. Dry the pentane solutions with anhydrous calcium chloride. Analyze by GC–MS, as directed. Consult with others in the lab section to obtain mass spectral data on the other esters. For your report, use the data to develop as complete an assignment of mass spectral peaks as is feasible. One way to approach this task is to make copies of the various spectra to compare, and jot down your ideas until you are satisfied that you have figured out as much as you can. Discuss your results with your colleagues. Finally, write up your formal lab report, including the usual introduction, summary of experimental procedures, and discussion. One way to present peak assignments would be as tabulations of mass numbers, along with suggested structures or compositions of fragments, for each of the compounds discussed. Hazards Concentrated acetic and (particularly) sulfuric acids are corrosive. Acetic acid and butanol vapors can be irritants. Discussion Ester samples can rapidly be analyzed by GC–MS, and the major fragment ions can fairly easily be deduced. Details of fragmentations and rearrangements are not mentioned here because they can be found in various references (15–18). However, as one source puts it, “The mass spectral behavior of aliphatic esters is more complex than that of many other compound classes” (19), and there are some aspects that may be puzzling, such as elimination of water, hydrogen scrambling, and sometimes ion–analyte reactions. For this reason, we have included additional commentaries and literature citations in the supplemental material.W
JChemEd.chem.wisc.edu • Vol. 78 No. 10 October 2001 • Journal of Chemical Education
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In the Laboratory
While experimentally easy, this exercise is conceptually fairly involved in comparison to a standard preparation– isolation lab. For elucidating fragmentation paths, the fully deuterated compounds are less informative than partially deuterated material, but they do increase the variety of compounds prepared and the number of spectra to compare and interpret. Students are given more time than the usual “due next week” for lab reports, to encourage thought and discussion and because reports are longer than usual. The suggested approach is to make assignments for fragmentations of the undeuterated compounds, then to predict the effect of deuteration and see whether the spectra of labeled compounds verified the original hypotheses. After the introduction, the report should include tabulations of masses and assigned fragment compositions. A generic one-page fragmentation summary is handed out to get students underway, and is included in the supplemental material to this article, along with spectra of all the compounds prepared.W Acknowledgments We thank the students of Chemistry 227 for their readiness to try this lab. Our Varian Saturn 2000 ion-trap GC–MS instrument was funded by the Army Research Office (ARO), grant number DAAG55-97-1-0298. Support for HZ’s participation was provided by the Contra Costa College Center for Science Excellence, through ARO grant DAAH0496-1-0004 and the University of California Office of the President, Mathematics Engineering Science Achievement grant 99-MESA-CCCP-5. We also appreciate the constructive suggestions of the reviewers. W
Supplemental Material
The student experimental procedure, notes to the instructor, extended discussion and additional literature citations, chromatograms, mass spectra, and the generic acetate fragmentation scheme handed out to students are available in this issue of JCE Online.
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Journal of Chemical Education • Vol. 78 No. 10 October 2001 • JChemEd.chem.wisc.edu
