The Mechanism of Covalent Bonding - Journal of Chemical Education


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Letters Cinnamaldehyde by Steam Distillation of Cinnamon I read with interest the note by D. F. Taber and A. J. Weiss in the May issue of this Journal (J. Chem. Educ. 1998, 75, 633), as we have also observed excessive foaming when attempting to obtain cinnamaldehyde from cinnamon. For 10 g of commercial ground cinnamon in about 150 mL of water, we have found that the addition of 2 mL of 6 N HCl is very effective in controlling the foaming. This seems simpler and faster than the procedure proposed by the authors. José Castrillón Chemistry Department University of Texas-Pan American Edinburg, TX 78539

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The Mechanism of Covalent Bonding Bacskay, Reimers, and Nordholm (1) are to be commended for their highly readable discussion of covalent bonding. Unfortunately, their discussion seems to have stopped a few steps short of giving a much clearer account of covalent bonding. The space given to the example of H 2+ may have given readers the impression that the extent of electron delocalization in that ion is typical of neutral molecules. Results from modern spin-coupled valence-bond theory (SCVB) show that this is not so, and that while electron delocalization along the bond axis occurs, electrons in molecules usually occupy highly localized, deformed atomic orbitals (2). In the SCVB picture of H2, for example, each electron occupies an orbital derived mainly from one atomic orbital,

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which is deformed slightly towards the other atom. In a minimal basis set consisting of 1s orbitals centered on nuclei A and B, the antisymmetrized ground state electronic wave function is Ψ = [φA(1)φB(2) 1 φB(1)φA(2)] {α(1)β(2) { β(1)α(2)} where φA = 1sA + λ1sB and φB = 1sB + λ1sA . Expansion of the spatial part shows that this is exactly the form of the wave function obtained by mixing the doubly excited and the ground MO configurations. Thus, a single SCVB configuration incorporates nondynamical electron correlation. In almost all of the cases examined, bonding electrons are found in highly localized orbitals (2). This includes the π electrons of benzene (3). Cyclobutadiene (D4h) and the allyl radical are two cases where π electrons occupy orbitals delocalized equally over two atoms (4, 5). The equivalence of the limited CI wave function and a generalized valence bond wave function was noted by Bacskay et al. (1), but they described the valence bond wave function as including ionic configurations, a common but rather counterintuitive idea, instead of as deformations of the atomic orbitals within a single configuration. The two descriptions lead to the same numerical results. The description for H2 afforded by SCVB theory has the advantages of requiring only a single configuration, of describing the bonding in strictly covalent terms, and, thus, of being in accord with chemical intuition. Literature Cited 1. Bacskay, G. B.; Reimers, J. R.; Nordholm, S. J. Chem. Educ. 1997, 74, 1494. 2. Gerratt, J.; Cooper, D. L.; Karadakov, P. B.; Raimondi, M. Chem. Soc. Rev. 1997, 26, 87; Cooper, D. L.; Gerratt, J.; Raimondi, M. Chem. Rev. 1991, 91, 929.

Journal of Chemical Education • Vol. 76 No. 3 March 1999 • JChemEd.chem.wisc.edu

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3. Cooper, D. L.; Gerratt, J.; Raimondi, M. Nature 1986, 323, 699. 4. Wright, S. C.; Cooper, D. L.; Gerratt, J.; Raimondi, M. J. Phys. Chem. 1992, 96, 7943. 5. Karadakov, P. B.; Gerratt, J.; Raos, G.; Cooper, D. L.; Raimondi, M. J. Am. Chem. Soc. 1994, 116, 2075. Curtis Hoganson Department of Chemistry Michigan State University East Lansing, MI 48824 [email protected]

The authors reply: The single most important point made in our paper (1) is that a covalent bond is the natural consequence of electron delocalization, and therefore the concept of delocalization is the vital key to the understanding of covalent bonding. The delocalization process is most obvious if the wave function of a molecule is constructed in terms of delocalized molecular orbitals, from H2+ and H2 to larger and more complex molecules. The presence of delocalization is less obvious if one uses a valence bond (VB) approach, where the orbitals are highly localized, since they are atomic orbitals (AO) or polarized AOs in the case of the spin-coupled valence bond method of Gerratt and co-workers (2). VB wave functions are generally constructed in terms of two-electron functions, and the typical spatial component that describes covalent bonding, namely electron sharing, between two atoms A and B can be written as ΨVB = φA (1)φB (2) + φB(1)φA (2)

of Gerratt and co-workers (2) in developing a VB approach that is closely connected with our traditional chemical thinking and appears to rival the accuracy and predictive capabilities of the mainstream MO-based methods, an explicit discussion of it, in our view, would not have added any fresh insights into the mechanism of covalent bonding. However, given that the use of localized many-electron basis functions in the construction of molecular wave functions is a potential source of confusion concerning the origin of covalent bonding, we are grateful for this opportunity to address this issue. Literature Cited 1. Bacskay, G. B.; Reimers, J. R.; Nordholm, S. J. Chem. Educ. 1997, 74, 1494. 2. Gerratt, J.; Cooper, D. L.; Karadakov, P. B.; Raimondi, M. Chem. Soc. Rev. 1997, 26, 87, and references therein. 3. Reimers, J. R.; Bacskay, G. B.; Nordholm, S. J. Chem. Educ. Software 1997, 10B, No. 2. George Bacskay and Jeffrey Reimers School of Chemistry University of Sydney NSW 2006 Sydney, Australia Sture Nordholm Department of Physical Chemistry Göteborg University, CTH S-41296 Göteborg, Sweden

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(1)

where φA and φB are AOs on the atoms that may be either pure or polarized AOs. The two electrons described by ΨVB are now correlated (to some extent) as well as delocalized! Thus, electron 1 has the same probability of being around nucleus A as being around nucleus B, and the same applies of course to electron 2. Being correlated, in this instance, implies that if one electron is near nucleus A, the other electron will be near nucleus B and vice versa. These points are explored in some detail in our software package, The Basics of Covalent Bonding (3), and the exercises associated with it. Just as the delocalized wave function of H2+ is a linear combination of two localized atomic wave functions, the VB wave function for H2 is a linear combination of two localized wave functions, which are the (correlated) configurations, namely Hartree products, in eq 1. As pointed out in our paper (1), an individual, localized configuration cannot account for covalent bonding—it can only describe a van der Waalstype interaction. In summary, electron delocalization is seen as the physical mechanism behind covalent bonding and the key to its understanding. That it is possible to construct accurate wave functions in terms of highly localized orbitals does not imply, as discussed above, that the electrons are also highly localized. In the end, we also need to be aware that it is the total many-electron wave function that describes the electrons of a molecule rather than the (one-electron) orbitals, which, in a many-electron system, are really no more than convenient mathematical functions in the construction of the total wave function. While we recognize the pioneering work

Re-Blue-ing Blue Litmus Paper In a recent issue (Steffel, M. J. J. Chem. Educ. 1998, 75, 183) a technique was given for the “re-blue-ing” of blue litmus paper using 15 M ammonia. We have found that placing a spatula-tip full (approximately 1 g) of powdered ammonium carbonate [(NH4) 2CO3] in the vial with the blue litmus paper re-blues the paper and keeps the paper blue for an extended period. The inconvenience of having the ammonium carbonate powder in the vial is minimal and seems to have no effect upon the results we have obtained. As Steffel states in her communication, “I cannot guarantee that this blue paper corresponds to ACS specifications, but it certainly works satisfactorily in a typical student laboratory setting”. We strongly echo this statement. David R. Myers and Joseph Crane Simon’s Rock College of Bard Division of Natural Sciences & Mathematics 84 Alford Road Great Barrington, MA 01230-1559

The author replies: In our laboratory, an instructor carries out the re-blueing technique in a hood, the capped bottles are handed to the students, and by the time the students return to their desks and are ready to use the litmus paper, there is only a slight odor of ammonia. Powdered ammonium carbonate also re-blues litmus paper. However, its use is not convenient in

JChemEd.chem.wisc.edu • Vol. 76 No. 3 March 1999 • Journal of Chemical Education

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Letters my laboratory, for the ammonium carbonate that we have is “lump” and it must be ground to a powder. Also, as long as there is ammonium carbonate in the litmus paper bottle, there is a noticeable ammonia odor every time the bottle is opened. Margaret J. Steffel Department of Chemistry The Ohio State University, Marion Campus 1465 Mt. Vernon Ave. Marion, OH 43302

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Galvani As part of its January 1998 diamond jubilee, your Journal ran a series of vignettes on other 1998 anniversaries. While most of these were both informative and complimentary of their subjects, the one on Galvani (1) unfortunately was neither. It stated that Galvani’s idea (that his observations of frog muscle twitching were due to “animal” electricity) “was later proved to be in error by Volta”. Too bad that, some two centuries later, at a time when biochemistry is becoming part of the standard chemistry curriculum, Volta’s incorrect views are still being repeated. We now know that the very processes of brain action, such as thinking, involve ionic (“animal”) electricity that runs entirely without metals. Hodgkin and Huxley got the Nobel prize for showing that this involves the mere movement of simple inorganic ions across membranes. Galvani’s work created the beginnings of not one but two fields of scientific inquiry, which is why it indeed galvanized the scientific community of his day. The first field was that of electrochemistry, which largely followed up on Galvani’s observation that metals can cause the frog legs to twitch, on Volta’s subsequent development of a useful device, the battery, and on his ordering of the metals in an electromotive sequence. Electrochemistry has since emphasized the properties of the metal–solution interface. The second field Galvani originated was that of electrophysiology, the study of ionic electricity as it occurs in the nervous system. While Volta ignored this aspect of Galvani’s work, independent contemporary investigators such as Alexander von Humboldt convincingly confirmed that Galvani’s effects could be reproduced in the complete absence of any metals. That was also about 200 years ago; to be precise, Humboldt published his two-volume Versuche über die gereizte Muskel- und Nervenfässer nebst Vermuthungen über den chemischen Process des Lebens in Thier und Pflanzenwelt in 1797, only 6 years after Galvani’s publication of de Viribus Electricitas in Motu Musculari Commentarius, and just one year before Galvani’s untimely death at the age of 60. To quote Brazier (2), “The design of Humboldt’s experiments and the clarity of his reasoning are a pleasure to study in the welter of acrimonious controversy that greeted Galvani’s findings. Without bias toward either protagonist, Humboldt repeated Volta’s and Galvani’s experiments. He examined their interpretations, designed new experiments to test their hypotheses, and concluded that Galvani had uncovered two genuine phenomena: bimetallic electricity and intrinsic animal electricity, and he felt these were not mutually exclusive.” 320

Ostwald, in his monumental 1896 Elektrochemie, ihre Geschichte und Lehre, devoted considerable space to the Galvani–Volta controversy, and to Volta’s interpretation of Galvani’s experiments in terms of metallic contacts. He concluded as follows (3): “If one has to blame somebody it has to be the later “diehard” supporters of Volta’s theory who, at a time when sufficient definitive material against it was available, did not apply themselves to a reexamination of Volta’s theory but were very zealous in defending it.” We now know that electricity is the result of the flow of charged particles. There are two types of such charged particles: electrons and ions. Electrons can move rather freely in metals, and this has given rise to almost all technological applications of electricity we see around us. However, the body uses ionic electricity instead, precisely what Galvani called “animal” electricity, long before electrons or ions had been identified as such. The fact that we can invent electrical gadgets, or even communicate their existence, critically depends on our brainpower. And that brainpower is based on ionic electricity, that is, on the movement of ions such as Na+, K+, and Ca 2+ across membranes, without any need for metals or metal electrons, since no redox processes are involved. It is too bad that an information-rich scientific journal such as yours, two centuries later, still perpetuates Volta’s self-serving misinterpretation of Galvani’s discoveries. Literature Cited 1. Schatz, P. F. Anniversaries: 1998, J. Chem. Educ. 1998, 75, 24. 2. Brazier, M. A. B. A History of Neurophysiology in the 17th and 18th Centuries, From Concept to Experiment; Raven Press: New York 1984, p 214. 3. Ostwald, W. Elektrochemie, ihre Geschichte und Lehre; von Veit: Leipzig 1896, p 66. The quotation given here is from the recent English translation, Electrochemistry, History and Theory, Amerind: New Delhi, 1980; p 65. Robert de Levie Department of Chemistry Georgetown University Washington DC 20057-1227

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Advanced Organic Chemistry: Reactions and Mechanisms I greatly appreciate the many nice things Daniel Berger said about my book Advanced Organic Chemistry: Reactions and Mechanisms in his review (J. Chem. Educ. 1998, 75, 1558–1559). However, I want to assure Prof. Berger, who was concerned that “The problems are good, but neither answers nor leading references are provided for most of them”, that detailed solutions for all 160 problems are provided in the Solutions Manual (ISBN 0-13-769332-X), which can be obtained along with the text if the instructor wishes. Bernard Miller Chemistry Department University of Massachusetts–Amherst Amherst, MA 01003-4510 [email protected]

Journal of Chemical Education • Vol. 76 No. 3 March 1999 • JChemEd.chem.wisc.edu