Letter pubs.acs.org/jchemeduc
Pericyclic or Pseudopericyclic? The Case of an Allylic Transposition in the Synthesis of a Saccharin Derivative Stephanie R. Hare and Dean J. Tantillo* Department of Chemistry, University of California−Davis, Davis, California 95616, United States S Supporting Information *
ABSTRACT: When new concepts, models, or theories are introduced in a course, their presentation should be accurate, even if depth is not the goal. In a recent publication in this Journal, the Woodward−Hoffmann rules were invoked in the context of a new laboratory experiment, but the associated description was inaccurate. Here we aim to clarify the theoretical background relevant to the described laboratory experiment, and we describe a new computational experiment that could be used to further illuminate the relevant theoretical concepts.
KEYWORDS: Upper-Division Undergraduate, Graduate Education/Research, Organic Chemistry, Misconceptions/Discrepant Events, Computational Chemistry, Theoretical Chemistry, MO Theory
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the Woodward−Hoffmann rules11,12 are incorporated into the experiment is problematic. Here we attempt to clarify this aspect of the experiment and suggest a computational experiment that would bring the relevant theoretical concepts to light.
INTRODUCTION Are [1,3] sigmatropic shifts pericyclic or pseudopericyclic? This is not a question that is introduced in most undergraduate organic chemistry courses. In fact, pericyclic reactions tend to occupy a relatively small fraction of the undergraduate organic chemistry curriculum, and a thorough description of the theory behind the Woodward−Hoffmann orbital symmetry rules is rarely discussed until chemistry graduate school (though the rules are described to some degree in a few undergraduate-level texts1−45). Even in graduate-level courses, the concept of a “pseudopericyclic” reaction is not commonly discussed.6−9 Consequently, we were surprised to see a pseudopericyclic reaction incorporated in Fonseca’s recently published undergraduate organic synthesis laboratory experiment.10 That article describes a two-step synthesis of O-cinnamylsaccharin from saccharin that seems to us to be wholly appropriate for an undergraduate laboratory. In a third reaction, O-cinnamylsaccharin is heated, inducing an allylic transposition to give the final product (Figure 1). This final reaction was described as follows:10 This reaction involves breaking the (−O−CH2−) and migration of a σ bond (migration of the cinnamyl group connected by a σ bond) over the π electron system and formation of a new σ bond (−N−CH2−) with concomitant reorganization of the π system. This is a rare process, forbidden according to Woodward−Hoffmann rules; however, it still occurs with benzisothiazolyl derivatives such as O-cinnamylsaccharin. While the experimental portion of this laboratory activity seems well-suited to a novice in chemistry, the manner in which © XXXX American Chemical Society and Division of Chemical Education, Inc.
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THEORETICAL CONSIDERATIONS
The title of ref 10 refers to the reaction in Figure 1 as a “[1,3] thermal sigmatropic rearrangement”. We agree that this reaction can be termed a sigmatropic rearrangement. If the reaction is actually pericyclic, i.e., if it is not merely a formal sigmatropic rearrangement, it must occur in a suprafacial− antarafacial manner.11 Not all thermal [1,3] shifts are forbidden by the Woodward−Hoffmann rules, as implied in ref 10; only those with suprafacial−suprafacial or antarafacial−antarafacial stereochemistry are forbidden. That, however, does not mean that a thermal [1,3] shift that is allowed on the basis of orbital symmetry considerations will proceed preferentially through a pericyclic pathway. In fact, most do notthe result of strain associated with adopting a suprafacial−antarafacial geometry.13 Instead, one of two alternative, nonpericyclic, pathways is usually followed: the reaction proceeds via two chemical steps, usually fragmentation into a diradical pair followed by recombination, or the reaction is concerted but its transition state structure corresponds to a pseudopericyclic array of orbitals. To be pericyclic, a reaction must be concerted and involve a structure Received: October 27, 2016 Revised: April 20, 2017
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Figure 1. Rearrangement reaction of interest, as depicted in ref 10.
Figure 2. Electron-pushing mechanism that would be drawn for a pericyclic mechanism compared with that for a pseudopericyclic mechanism.
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A COMPUTATIONAL EXPERIMENT How might one acquire evidence that the reaction in question is pseudopericylic? One approach would be to use quantumchemical calculations to predict the geometry of the transition state structure for concerted rearrangement.24−30 We located the relevant transition state structure at two levels of theory (both density functional theory [DFT] methods31−33): B3LYP/6-31G(d) and B3LYP/3-21G.21−23 The former method has been used on many occasions to characterize pericyclic reactions,24−30 and the latter is extremely fastfast enough to be applied in most any classroom setting.34−37 Optimized structures were confirmed as minima or transition state structures by a frequency calculation, where minima exhibited no imaginary frequencies and transition state structures had exactly one imaginary frequency. It is also important when carrying out calculations to ensure that the reacting species are in their lowest-energy conformations. In this case, the saccharyl group has no rotatable bonds and the styrenyl moiety is in its fully extended form, so the structures shown are expected to be the lowest-energy conformations (see the Supporting Information for additional details). While the calculations described here were carried out using commercial software,38 they could also be carried out using freely available software.39−44 Indeed, it was found with either level of theory that the migrating CH2 group “points” not at the N−C−O π system during the rearrangement but roughly at the locations of the nitrogen lone pair of the reactant and the oxygen lone pair of the product (Figure 3). These geometric featureshallmarks of a pseudopericyclic reactionalso can be followed along the reaction coordinate if an intrinsic reaction coordinate (IRC) calculation is performed (Figure 4).45−47 While running an IRC calculation may be beyond what is possible in some chemistry courses, running a B3LYP/3-21G transition state structure calculation should not be. That being said, the subtleties
along its reaction coordinate (not always the transition state structure) in which the rearranging electrons reside in a continuous cyclic array of orbitals.14 To be pseudopericyclic, a reaction must be concerted and involve a structure along its reaction coordinate (not always the transition state structure) in which the rearranging electrons reside in a discontinuous cyclic array of orbitals.15−17 Fragmentation could involve a pair of radicals or a cation− anion pair. Radical pathways for related reactions have been discussed at length in the literature.11,18−20 The transition state structure drawn in ref 10 (see Figure 1) depicts an ion pair, although it is not clear what is meant by this representation. Our computations (see the Supporting Information) predict,21−23 however, that a concerted pathway is preferred over either fragmentation pathway, so the nature of the concerted process is the subject of what follows. A concerted, pseudopericyclic pathway is often observed when an atom participating in a π bond in the reactant or product, which is broken or formed during the reaction in question, also bears a lone pair. That is the situation for the reaction in Figure 1both the nitrogen atom of the CN bond of the reactant and the oxygen atom of the CO bond of the product bear lone pairs. If these lone pairs, in addition to the π electrons, are involved in the rearrangement, then the relevant orbital array is discontinuous (each lone-pair/π-bond combination involves orthogonal orbitals) and the reaction is not subject to the Woodward−Hof f mann rules.15−17 If the rules do not apply, then the reaction cannot be forbidden by them. The distinction can be depicted by way of the curved-arrow drawings shown in Figure 2the left-hand depiction corresponds to the pericyclic (here sigmatropic) process, while the right-hand depiction corresponds to the pseudopericyclic process. B
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Figure 3. Top row: optimized structures of the reactant, transition state, and product of this reaction and their electronic energies (in kcal/mol) relative to the reactant structure, calculated at the B3LYP/6-31G(d) level of theory. Bottom row: The left and middle images show two orientations of the transition state structure to emphasize that the geometry of the migrating group remains “pointing” at the lone pairs of oxygen and nitrogen. The right picture depicts a natural bond orbital calculation result showing overlap between the formally empty p orbital on the migrating methylene and one of the lone pairs on oxygen. All of the orbitals shown have an isovalue of 0.06. See the Supporting Information for details on this type of calculation.
transition state structure indicate that it corresponds to a reaction that is pseudopericyclic rather than pericyclic?
associated with this reaction are likely not appropriate subject matter for all undergraduate organic chemistry curricula. Nonetheless, if the experimental reaction is to be run, we believe that an appropriate theoretical description should be provided to students. If having the students run the calculations described is not practical, then at least the results included here could be shared and described. Atomic coordinates for the structures shown in Figure 3 are included in the Supporting Information so that the structures in question can be viewed directly using standard graphical user interfaces. While it is not expected that students will obtain a deep understanding of the Woodward−Hoffmann rules from this single experiment, the goal is to provide students with tools necessary to characterize pericyclic and pseudopericyclic reactions, information that may be useful in more advanced courses in which the theory of pericyclic reactions is discussed in detail. Assessment of student comprehension could involve responses to the following questions (in addition to those about more technical aspects of computations): (1) How do pericyclic and pseudopericyclic reactions differ in terms of electron flow? (2) What geometric features of an optimized
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CONCLUSION Research supports the value of teaching students why they see the phenomena they see in the lab using first principles.48−51 The Woodward−Hoffmann rules have the virtue of being fundamentalit was Woodward and Hoffmann themselves who said it best in their book The Conservation of Orbital Symmetry in a section entitled “Violations”, where they famously stated, “There are none! Nor can violations be expected of so fundamental a principle of maximum bonding.”11 To our knowledge, no violations of these rules have been described; apparent violations all involve nonpericyclic processes and therefore do not meet the criteria for application of the rules.14 Woodward and Hoffmann then go on to state, “All the more is it then important to give consideration to some reactions which might appear on casual inspection to contravene orbital symmetry conservation.”11 We agree that such apparent violations should be studied and discussed openly, and we have attempted to provide an appropriate theoretical treatment of one such reaction here. We believe that C
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Figure 4. Energy changes along the intrinsic reaction coordinate (IRC) associated with the pseudopericyclic transition state structure (B3LYP/321G). Pictures of structures at the points marked in red on the chart are shown with their electronic energies (in kcal/mol) relative to the reactant structure.
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the delineation between rules based on first principles, the types of rules that are essentially “unbreakable”, and models that are approximated on the basis of physical data, is critical for the development of chemical intuition.52 Thus, we hope that the Woodward−Hoffmann rules are rigorously described when introduced and are applied only when appropriate.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Dean J. Tantillo: 0000-0002-2992-8844 Notes
ASSOCIATED CONTENT
The authors declare no competing financial interest.
S Supporting Information *
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The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00825. A brief introduction to running various DFT calculations and Cartesian coordinates, energies, and three-dimensional CYLview images of all optimized structures (PDF, DOCX)
ACKNOWLEDGMENTS
We gratefully acknowledge support from the National Science Foundation (XSEDE Program via CHE-030089) and the Department of Education’s Graduate Assistance in Areas of National Need (GAANN) Fellowship. D
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energetics, and transition state geometries. J. Phys. Chem. A 2003, 107, 11445−11459. (26) Kirmse, W.; Rondan, N. G.; Houk, K. N. Stereoselective Substituent Effects on Conrotatory Electrocyclic Reactions of Cyclobutenes. J. Am. Chem. Soc. 1984, 106 (25), 7989−7991. (27) Rondan, N. G.; Houk, K. N. Theory of Stereoselection in Conrotatory Electrocyclic Reactions of Substituted Cyclobutenes. J. Am. Chem. Soc. 1985, 107 (7), 2099−2111. (28) Wiest, O.; Houk, K. N. Density functional theory calculations of pericyclic reaction transition structures. Top. Curr. Chem. 1996, 183, 1−24. (29) Houk, K. N.; Gonzalez, J.; Li, Y. Pericyclic reaction transition states: Passions and punctilios, 1935−1995. Acc. Chem. Res. 1995, 28, 81−90. (30) Houk, K. N.; Li, Y.; Evanseck, J. D. Transition Structures of Hydrocarbon Pericyclic Reactions. Angew. Chem., Int. Ed. Engl. 1992, 31 (6), 682−708. (31) Cramer, C. J. Essentials of Computational Chemistry: Theories and Models, 2nd ed.; Wiley: West Sussex, England, 2004. (32) Bachrach, S. M. Computational Organic Chemistry, 2nd ed.; Wiley: Hoboken, NJ, 2014; p xxi. (33) Young, D. Computational Chemistry: A Practical Guide for Applying Techniques to Real World Problems; John Wiley & Sons: New York, 2001. (34) Palazzo, T. A.; Truong, T. T.; Wong, S. M. T.; Mack, E. T.; Lodewyk, M. W.; Harrison, J. G.; Gamage, R. A.; Siegel, J. B.; Kurth, M. J.; Tantillo, D. J. Reassigning the Structures of Natural Products Using NMR Chemical Shifts Computed with Quantum Mechanics: A Laboratory Exercise. J. Chem. Educ. 2015, 92 (3), 561−566. (35) Esselman, B. J.; Hill, N. J. Integration of Computational Chemistry into the Undergraduate Organic Chemistry Laboratory Curriculum. J. Chem. Educ. 2016, 93 (5), 932−936. (36) Marzzacco, C. J.; Baum, J. C. Computational Chemistry Studies on the Carbene Hydroxymethylene. J. Chem. Educ. 2011, 88 (12), 1667−1671. (37) Pearson, J. K. Introducing the Practical Aspects of Computational Chemistry to Undergraduate Chemistry Students. J. Chem. Educ. 2007, 84 (8), 1323−1325. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (39) Turney, J. M.; Simmonett, A. C.; Parrish, R. M.; Hohenstein, E. G.; Evangelista, F. A.; Fermann, J. T.; Mintz, B. J.; Burns, L. A.; Wilke, J. J.; Abrams, M. L.; Russ, N. J.; Leininger, M. L.; Janssen, C. L.; Seidl, E. T.; Allen, W. D.; Schaefer, H. F.; King, R. A.; Valeev, E. F.; Sherrill, C. D.; Crawford, T. D. Psi4: An Open-source Ab Initio Electronic Structure Program. WIREs Comput. Mol. Sci. 2012, 2 (4), 556−565. (40) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14 (11), 1347−1363. (41) Fortenberry, R. C.; McDonald, A. R.; Shepherd, T. D.; Kennedy, M.; Sherrill, C. D. PSI4Education: Computational Chemistry Labs Using Free Software. ACS Symp. Ser. 2015, 1193, 85−98.
REFERENCES
(1) Klein, D. R. Organic Chemistry, 2nd ed.; Wiley: Hoboken, NJ, 2015; pp 797−814. (2) Clayden, J.; Greeves, N.; Warren, S. G. Organic Chemistry, 2nd ed.; Oxford University Press: Oxford, U.K., 2012; pp 877−931. (3) Loudon, G. M.; Parise, J. Organic Chemistry, 6th ed.; Roberts and Company Publishers: Greenwood Village, CO, 2016; pp 1449−1477. (4) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry; W. H. Freeman: New York, 1994; p 614. (5) Solomons, T. W. G.; Fryhle, C. B.; Snyder, S. A. Organic Chemistry, 12th ed.; John Wiley & Sons: Hoboken, NJ, 2016; pp 572− 616. (6) Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry; University Science Books: Sausalito, CA, 2006; pp 877− 928. (7) Isaacs, N. Physical Organic Chemistry, 2nd ed.; Addison Wesley Longman: Harlow, England, 1995; Chapter 14. (8) Smith, M. B.; March, J. March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 6th ed.; John Wiley & Sons: Hoboken, NJ, 2007. (9) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper Collins Publishers: New York, 1987; Chapter 10. (10) Fonseca, C. S. C. Saccharin Derivative Synthesis via [1,3] Thermal Sigmatropic Rearrangement: A Multistep Organic Chemistry Experiment for Undergraduate Students. J. Chem. Educ. 2016, 93 (10), 1781−1784. (11) Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry, 1st ed.; Verlag Chemie: New York, 1970. (12) Fleming, I. Molecular Orbitals and Organic Chemical Reactions; John Wiley and Sons: New York, 2009. (13) Tantillo, D. J.; Lee, J. K. Reaction mechanisms: Part (ii) Pericyclic reactions. Annu. Rep. Prog. Chem., Sect. B: Org. Chem. 2007, 103, 272−293. (14) Nouri, D. H.; Tantillo, D. J. Sigmatropic shifts and cycloadditions on neutral, cationic, and anionic pentadienyl+butadiene potential energy surfaces. Tetrahedron 2008, 64 (24), 5672−5679. (15) Birney, D. M. Theory, experiment and unusual features of potential energy surfaces of pericyclic and pseudopericyclic reactions with sequential transition structures. Curr. Org. Chem. 2010, 14 (15), 1658−1668. (16) Lemal, D. M. Chemistry of hexafluorobicyclo[1.1.0]butane: a computational study. J. Org. Chem. 2010, 75 (19), 6411−6415. (17) Ross, J. A.; Seiders, R. P.; Lemal, D. M. An Extraordinary Facile Sulfoxide Automerization. J. Am. Chem. Soc. 1976, 98 (14), 4325− 4327. (18) Leber, P. A.; Baldwin, J. E. Thermal [1,3] carbon sigmatropic rearrangements of vinylcyclobutanes. Acc. Chem. Res. 2002, 35, 279− 287. (19) Baldwin, J. E. Thermal rearrangements of vinylcyclopropanes to cyclopentenes. Chem. Rev. 2003, 103 (4), 1197−1212. (20) Fleming, I. Molecular Orbitals and Organic Chemical Reactions; John Wiley and Sons: New York, 2009; pp 190−221. (21) Becke, A. D. Density-functional exchange-energy approximation with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38 (6), 3098−3100. (22) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98 (7), 5648−5652. (23) Becke, A. D. A new mixing of Hartree−Fock and local densityfunctional theories. J. Chem. Phys. 1993, 98 (2), 1372−1377. (24) Dolbier, W. R., Jr.; Koroniak, H.; Houk, K. N.; Sheu, C. Electronic Control of Stereoselectivities of Electrocyclic Reactions of Cyclobutenes: A Triumph of Theory in the Prediction of Organic Reactions. Acc. Chem. Res. 1996, 29 (10), 471−477. (25) Guner, V.; Khuong, K. S.; Leach, A. G.; Lee, P. S.; Bartberger, M. D.; Houk, K. N. A standard set of pericyclic reactions of hydrocarbons for the benchmarking of computational methods: The performanceof ab initio, density functional, CASSCF, CASPT2, and CBS-QB3 methods for the prediction of activation barriers, reaction E
DOI: 10.1021/acs.jchemed.6b00825 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Letter
(42) Gordon, M. S.; Schmidt, M. W. Advances in Electronic Structure Theory: GAMESS a Decade Later. In Theory and Applications of Computational Chemistry: The First Forty Years; Dykstra, C. E., Frenking, G., Kim, K. S., Scuseria, G. E., Eds.; Elsevier: Amsterdam, 2005. (43) Valiev, M.; Bylaska, E. J.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Van Dam, H. J. J.; Wang, D.; Nieplocha, J.; Apra, E.; Windus, T. L.; de Jong, W. A. NWChem: A comprehensive and scalable opensource solution for large scale molecular simulations. Comput. Phys. Commun. 2010, 181 (9), 1477−1489. (44) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter 2009, 21 (39), 395502. (45) Gonzalez, C.; Schlegel, H. B. Reaction path following in massweighted internal coordinates. J. Phys. Chem. 1990, 94 (14), 5523− 5527. (46) Fukui, K. The path of chemical reactions - The IRC approach. Acc. Chem. Res. 1981, 14, 363−368. (47) Maeda, S.; Harabuchi, Y.; Ono, Y.; Taketsugu, T.; Morokuma, K. Intrinsic reaction coordinate: Calculation, bifurcation, and automated search. Int. J. Quantum Chem. 2015, 115 (5), 258−269. (48) Schultz, E. Fully exploiting the potential of the periodic table through pattern recognition. J. Chem. Educ. 2005, 82 (11), 1649− 1657. (49) Cooper, M. M. Why Ask Why? J. Chem. Educ. 2015, 92 (8), 1273−1279. (50) Schultz, E. Reflections catalyzed by an assault on a favorite principle. J. Chem. Educ. 2010, 87 (5), 472−473. (51) Jalil, P. A. A procedural problem in laboratory teaching: Experiment and explain, or vice-versa? J. Chem. Educ. 2006, 83 (1), 159−163. (52) Scudder, P. H. Database vs. expert system teaching paradigms: Using organic reaction mechanisms to teach chemical intuition. J. Chem. Educ. 1997, 74 (7), 777−781.
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