Technology Report pubs.acs.org/jchemeduc
3D Printers Can Provide an Added Dimension for Teaching Structure−Energy Relationships David N. Blauch* and Felix A. Carroll* Department of Chemistry, Davidson College, Davidson, North Carolina 28035, United States S Supporting Information *
ABSTRACT: A 3D printer is used to prepare a variety of models representing potential energy as a function of two geometric coordinates. These models facilitate the teaching of structure−energy relationships in molecular conformations and in chemical reactions.
KEYWORDS: First-Year Undergraduate, Second-Year Undergraduate, Upper-Division Undergraduate, Hands-On Learning/Manipulatives, Conformational Analysis, Mechanisms of Reactions, Nucleophilic Substitution, Reactions, Reactive Intermediates, Organic Chemistry
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produce HCl and a hydrogen atom.3 Beginning with a 2D contour map of potential energy as a function of the H−H and H−Cl internuclear distances, MacDonald traced the contour line corresponding to each energy increment onto a separate sheet of Lucite. Each plastic sheet was cut along the contour line with a jigsaw, and then the set of plastic sheets was stacked in proper order to create a stepped 3D surface. Power sanding reduced the steps to a smoother surface, and a final buffing finished the model. Similarly, Dye reported the creation of a physical model for the reaction of H2 with a bromine atom from steel strips, plaster of Paris, and modeling clay.4 The construction of a physical model of a potential energy surface that was carried out with so much effort by MacDonald can now be accomplished relatively easily with a 3D printer. With a MakerBot Replicator 2 printer, models were made of even complicated 3D potential energy surfaces in a few hours of mostly unattended operation (the time depending primarily on the size of the model). The computer model of the potential energy surface may be obtained from computational chemistry software (e.g., Gaussian 09W, Spartan ‘10) or may be an abstract representation based upon intersecting parabolic energy wells. A virtual 3D object is assembled by adding sides and a lower surface to the theoretical potential energy surface with numerical analysis software such as MathCAD 15. Data points from the surface of the virtual object are organized and saved as a stereolithography (STL) file to capture the description of the 3D object. The STL file can be refined using
olecular models are fundamental tools for learning chemical structure. Students who hold physical models in their hands gain a better understanding of molecular geometry, constitutional isomerism, stereoisomerism, and conformational analysis than they could achieve solely from viewing images on a printed page.1 Complementary to physical models are computer models, which can show theoretical aspects of chemical structure (such as electron density and electrostatic potential energy surfaces) in addition to geometric structure. Although physical models have been used extensively to depict molecular geometry, they are almost never used to teach relationships between chemical structure and potential energy. Students usually study conformational energies with graphs of potential energy against a coordinate that represents a particular dihedral angle, and they learn about activation energies with plots of potential energy versus one reaction coordinate. Computer images can represent potential energy as a function of two coordinates, but they rely solely on visual perception. This can be a problem for students with normal vision who have difficulty recognizing the implicit 3D character of surfaces displayed on a monitor, and it is a particularly serious limitation for those who are blind or visually impaired.2 Because a physical model allows students to employ both sight and touch, there have been some attempts to produce 3D potential energy surfaces for chemical education. Such models have not been widely adopted, however, due to the difficulty of making them. For example, in 1948 MacDonald reported the construction of a relatively simple 3D potential energy surface for the reaction of molecular hydrogen with a chlorine atom to © 2014 American Chemical Society and Division of Chemical Education, Inc.
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dx.doi.org/10.1021/ed4007259 | J. Chem. Educ. 2014, 91, 1254−1256
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Technology Report
Figure 1. A 3D model of the conformational energy of butane associated with rotation about both the C1−C2 and C2−C3 bonds. θC1−C2 represents an H−C1−C2−C3 dihedral, while θC2−C3 represents the C1−C2−C3−C4 dihedral. A printed sheet of paper gives a label and scale for each conformational axis.
software such as Blender or Netfabb. Conceptually, the remainder of the process is the same as that used by MacDonald. Instead of stacking sheets of Lucite to build up a solid object, a “slicer” program converts the computer model of the 3D object into thin sections. Then, layers of polymer corresponding to each slice are deposited sequentially by a 3D printer to generate a solid object. With this approach, a variety of 3D potential energy surface models have been produced for teaching important concepts in general chemistry, introductory organic chemistry, advanced organic chemistry, and biochemistry. Students in introductory organic chemistry first encounter 3D potential energy models when studying the conformational energies of propane and butane.5 In conjunction with their molecular model sets, students can use 3D models to explore the potential energy changes associated with simultaneous rotation about two carbon−carbon bonds, first with propane (graphical abstract) and then with the C1−C2 and C2−C3 bonds of butane (Figure 1). The 3D models make it clearer to students that a textbook figure shows just one slice of a conformational energy landscape and that potential energy depends on the conformations of all the bonds. Three-dimensional potential energy surface models also can give what is literally an added dimension to studying structure− energy relationships in chemical reactions. To this end, 3D models were prepared for nucleophilic substitution in which the two geometric coordinates represent the axes of a More O’Ferrall−Jencks diagram.6 One geometric axis is the distance between the leaving group and the α carbon, while the other geometric axis measures the approach of the nucleophile toward the α carbon. By comparing the activation energy along the 3D potential energy surface for an SN2 reaction (Figure 2) with the activation energy for the pathway representing an SN1 reaction on a similarly constructed 3D model (Figure 3), students can better understand why one set of reagents follows an SN1 path, but a different system undergoes an SN2 reaction. The potential energy surfaces in Figures 2 and 3 were not computed but were assembled from parabolic energy wells to illustrate the concept of relative activation energies for different
Figure 2. Three-dimensional potential energy surface for the More O’Ferrall−Jencks analysis of an SN2 reaction of R−L with Nu:−. The 3D model is supported by a stand for this illustration. The labels and reaction path were added to the photo in order to make the structure− energy relationships more evident.
reaction pathways. Analogous models were also created illustrating the Hammond postulate7 and the potential energy surface for the chlorination of methane, two important topics in many introductory organic chemistry courses. For advanced organic chemistry, 3D surfaces were developed for teaching topics such as kinetic and thermodynamic control of product distribution and the Curtin−Hammett principle.8,9 Potential energy surface models prepared with 3D printers are likely to play a significant role in chemical education in future years. It is important for instructors to emphasize that any model is not the chemical system itself but is only a means to understand the chemical system. Thus, a 3D model of a potential energy surface should be presented as a representa1255
dx.doi.org/10.1021/ed4007259 | J. Chem. Educ. 2014, 91, 1254−1256
Journal of Chemical Education
Technology Report
(6) (a) More O’Ferrall, R. A. Relationships between E2 and E1cB Mechanisms of ß-Elimination. J. Chem. Soc. B 1970, 274−277. (b) Jencks, W. P. A Primer for the Bema Hapothle. An Empirical Approach to the Characterization of Changing Transition-State Structures. Chem. Rev. 1985, 85, 511−527. (7) Hammond, G. S. A Correlation of Reaction Rates. J. Am. Chem. Soc. 1955, 77, 334−338. (8) Youssef, A. K.; Ogliaruso, M. A. An Organic Experiment to Illustrate Thermodynamic versus Kinetic Control. J. Chem. Educ. 1975, 52, 473−474. (9) (a) Curtin, D. Y. Stereochemical Control of Organic Reactions. Differences in Behavior of Diastereoisomers. I. Ethane Derivatives. The cis Effect. Rec. Chem. Prog. 1954, 15, 111−128. (b) Hammett, L. P. Physical Organic Chemistry, 2nd ed.; McGraw-Hill: New York, 1970; pp 119−120. (10) Carroll, F. A. Perspectives on Structure and Mechanism in Organic Chemistry, 2nd ed.; John Wiley & Sons: Hoboken, NJ, 2010; pp xv− xvii, 48. (11) For a discussion of the proper uses of models in chemical education, see: Bent, H. A. Uses (and Abuses) of Models in Teaching Chemistry. J. Chem. Educ. 1984, 61, 774−777. Figure 3. Three-dimensional potential energy surface (supported by a stand) for the More O’Ferrall−Jencks analysis of an SN1 reaction of R−L with Nu:−. The labels and reaction path were added to the photo.
tion of structure−energy relationships that complements textbook drawings and computer images. None of these models is complete by itself, but considering the models together can provide students with a more complete understanding of structure−energy relationships.10,11
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ASSOCIATED CONTENT
S Supporting Information *
Details of STL file preparation as well as the STL files for the propane, butane, SN2, and SN1 models discussed here. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Davidson College for supporting the purchase of the 3D printer used to make the models and to Bill Giduz for photographing them.
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REFERENCES
(1) Dori, Y. J.; Barak, M. J. Virtual and Physical Molecular Modeling: Fostering Model Perception and Spatial Understanding. Educ. Technol. Soc. 2001, 4, 61−74. (2) Wedler, H. B.; Cohen, S. R.; Davis, R. L.; Harrison, J. G.; Siebert, M. R.; Willenbring, D.; Hamann, C. S.; Shaw, J. T.; Tantillo, D. J. Applied Computational Chemistry for the Blind and Visually Impaired. J. Chem. Educ. 2012, 89, 1400−1404. (3) MacDonald, N. S. A Potential Energy Model for Displacement Reactions. J. Chem. Educ. 1948, 25, 338−340. (4) Dye, J. L. Model of a Potential Energy Surface. J. Chem. Educ. 1957, 34, 215−217. (5) For example, see: Wade, L. G. Organic Chemistry, 8th ed.; Pearson: Upper Saddle River, NJ, 2013, Chapter 3. 1256
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