In the Classroom
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No, the Molecular Mass of Bromobenzene Is Not 157 amu An Exercise in Mass Spectrometry and Isotopes for Early General Chemistry Steven M. Schildcrout Department of Chemistry, Youngstown State University, Youngstown, OH 44555-3663;
[email protected] Of all the spectrometric techniques, mass spectrometry gives data that can be most directly related to molecular-level properties that can be readily understood by beginning general-chemistry students. Nevertheless, their textbooks typically give it little attention, emphasizing its historical role in the discovery of isotopes (1). Early in our general course, after being introduced (or reintroduced) to the concepts of atomic weight, molecular mass, isotopes, ions, chemical formulas, and chemical equations, our students do a cooperative guided-inquiry exercise involving interpretation of a simple organic mass spectrum. This exercise illustrates, refines, reinforces, and extends these concepts while the students learn some of the chemical principles of electron-ionization mass spectrometry. Although there are many previous reports in this Journal on using mass spectrometry in more advanced courses, the only ones for first-year chemistry include its use near the end of a year-long course for nonmajors (2) and in a majors laboratory course (3). Both presume a background in bonding and molecular structure. This exercise does not address aspects of bonding or structure, which come later in the course. Nor is it intended to teach the instrumentation or analytical applications of mass spectrometry. Because we integrate mass spectrometry throughout our undergraduate curriculum, at this very early stage in the students’ chemical education we stress only the
principles of the technique as an effective tool to elucidate the above-mentioned fundamental chemical concepts. As students move on to their organic, analytical, physical (4), and research courses, they re-encounter mass spectrometry in the classroom and in the laboratory from other viewpoints and with increasing sophistication. This exercise serves as a prelude. It begins with a straightforward question but then leads students into a paradox that prompts them to refine a common misconception of atomic weight. Given the formula C6H5Br for bromobenzene, they have little trouble using average atomic weights to calculate the correct average molecular weight of 157.0 amu. The electron-ionization process, as it occurs in a mass spectrometer, is then described for them. When they realize that the molecular ion is C6H5Br+ and that the lost electron has negligible mass, they will nearly always predict the mass of this ion to be also 157.0 amu. Next they are confronted with a mass spectrum of bromobenzene (Fig. 1) (5) showing intense peaks at m/z (mass-to-charge ratio) 156 and 158, but not at 157. After they are told that bromine has two isotopes, 79Br and 81Br (with nearly equal natural abundances), they are then able to explain these peaks as arising from C6H579Br+ and C6H581Br+, and they have discovered (or been clearly reminded of) the distinction between average atomic or molecular weights and the actual masses of individual atoms or molecules, which account for the mass
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Figure 1. Electron-ionization (70 eV) mass spectrum of bromobenzene from Finnigan GCQ ion-trap mass spectrometer.
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In the Classroom
spectrum. This also points out a common fallacy, which holds that things, as opposed to numbers, can be averaged. There is neither an average person nor an average bromine atom. The students then investigate the mass spectrum further, using a ruler to measure the heights of the two molecular-ion peaks, relating the relative intensities to bromine’s natural isotopic abundances, and using precise isotopic masses to calculate bromine’s (weighted) average atomic weight to three significant figures, 79.9 amu. Bromine is the only element common in organic compounds that has an average atomic weight near (within 0.15 amu) an integer but has no stable isotope at the corresponding mass number (6 ). (Although the concepts of molar mass and 12C as the reference standard for the amu and for Avogadro’s number are not central to this exercise, the instructor may wish to relate these concepts to it.) After ion fragmentation in a mass spectrometer is described for them in general, the students consider the possible composition of the ion giving the strong peak at m/z 77. Can this ion contain bromine? They respond “no” because its mass is too low to include a Br atom, or, if they learned the isotope lesson well, because it does not show the signature isotope pattern for bromine, a pair of peaks two mass units apart with similar intensities. With the reminder of the molecular formula of bromobenzene, they will find that the formula for this fragment must be C6H5+. Next they are asked to find the balanced chemical equation for formation of this fragment from the molecular ion: C6H5Br+ → C6H5+ + Br. Students may ask whether the familiar Br᎑ anion could be produced here. This lets the instructor remind them of how the gas phase is different from aqueous solution and that, because a chemical equation requires balance of charge as well as of atoms, the cation would then need to be the much less stable C6H52+. Mass spectrometric fragmentation typically follows the format A+ → B+ + C, where ions A+ and B+ give corresponding peaks in the spectrum but neutral C must be inferred by atom balance. This part of the exercise shows students a new aspect of balancing chemical equations. Rather than the familiar problem of finding stoichiometric coefficients from known chemical formulas, here they deduce the chemical formula (from known coefficients). This technique is analogous to that used to balance nuclear equations, where, if other species are known, an unknown nuclide or particle is found by considering conservation of mass and charge numbers. Similarly, given the respective precursor ions (4), they can write equations for the production of the fragment ions giving m/z 51 and 50 and deduce formulas for the neutral molecules that must accompany them. The equations are respectively C6H5+ → C4H3+ + C2H2 and C6H5Br+ → C4H2+ + C2H3Br. If further interpretation is desired, the students may be asked whether there is evidence in the spectrum for doubly charged molecular ions. These are not significant in our spectrum, but would appear at m/z 78 and 79 (now z = 2)
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with nearly equal abundances. The peak observed at m/z 78 is from C6H5+ containing a 13C atom, as evidenced by its relative intensity near 7%, equal to the 1.1% natural abundance of 13C multiplied by the six carbon atoms in the ion. A similar explanation accounts for the weak molecular-ion peaks at m/z 157 and 159. Weak signals at m/z 79 and 81 are attributable to Br+. Students could also be asked to predict the principal peaks for chlorobenzene. It should (and does) give the same phenyl-group fragments at m/z 50, 51, and 77 that bromobenzene shows, but its molecular ion, C6H5Cl+, occurs at m/z 112 and 114 in a 3:1 intensity ratio, corresponding to C6H535Cl+ and C6H537Cl+, respectively. Students do this exercise in small discussion groups during one of their weekly recitation sessions. The instructor may give guidance as needed, but the worksheets are intended to be self-explanatory, challenging the students to think through the concepts. We include with this exercise a brief tour of our mass spectrometry lab so students can see the instrument and get an appreciation of how it operates. They enjoy discovering that they can interpret the output of a modern research instrument using the fundamental molecular concepts that they are encountering in the general chemistry course. WSupplemental
Material
Supplemental material for this article (an abstract, student worksheets, and a tabulation of the data corresponding to Fig. 1) is available in this issue of JCE Online. Acknowledgment I thank the National Science Foundation (DUE9551683) for partially funding the purchase of the mass spectrometer. Literature Cited 1. Griffiths, I. W. Rapid Commun. Mass Spectrom. 1997, 11, 2– 16. This includes a historical account of J. J. Thomson’s invention of mass spectrometry and the first observation of stable isotopes (for Ne). 2. Eichstadt, K. E. J. Chem. Educ. 1992, 69, 48–51. 3. Amenta, D. S.; DeVore, T. C.; Gallaher, T. N.; Zook, C. M.; Mosbo, J. A. J. Chem. Educ. 1996, 73, 572–575. 4. Schildcrout, S. M. J. Chem. Educ. 2000, 77, 501–502. 5. For a mass spectrum of bromobenzene see also NIST Chemistry WebBook; Mallard, W. G.; Lindstrom, P. J., Eds.; NIST Standard Reference Database No. 69; National Institute of Standards and Technology: Gaithersburg, MD, Nov 1998; http://webbook.nist.gov (accessed Jun 2000). 6. Holden, N. E. Table of the Isotopes. In Handbook of Chemistry and Physics, 71st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1990; pp 11-33–11-140. The only other elements with this property are Se, Ag, and Eu.
Journal of Chemical Education • Vol. 77 No. 11 November 2000 • JChemEd.chem.wisc.edu
