Article pubs.acs.org/jchemeduc
Decorating with Arrows: Toward the Development of Representational Competence in Organic Chemistry Nathaniel P. Grove,*,† Melanie M. Cooper,‡ and Kelli M. Rush§ †
Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, North Carolina 28403, United States ‡ Department of Chemistry, Clemson University, Clemson, South Carolina 29634, United States § Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056, United States S Supporting Information *
ABSTRACT: Much effort has been expended in developing improved methods for presenting mechanistic thinking and the curved-arrow notation to organic chemistry students; however, most of these techniques are not research-based. The little research that has been conducted has mainly focused on understanding the meaning that students associate with the curved-arrows during a single moment in time. The current research uses OrganicPad, an innovative, tablet person computer-based structure drawing program, to document our efforts to understand how second-yearlevel organic chemistry students’ mechanism use changes over their study of the subject. Our results reveal a dramatic evolution of mechanistic strategies during the academic year, including a large proportion of students who elect not to use the mechanistic convention in their work. KEYWORDS: Second-Year Undergraduate, Chemical Education Research, Curriculum, Organic Chemistry, Mechanisms of Reactions FEATURE: Chemical Education Research
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In recent times it has become more and more evident that the graphical formulae employed by organic chemists to represent the constitutions of chemical individuals are expressions which inadequately symbolise the properties of substances, and from time to time efforts have been made to introduce additional systems of notation corresponding with more or less definite theoretical ideas.
REPRESENTATIONS AND THE DEVELOPMENT OF REPRESENTATION COMPETENCE It has been argued3 that visualization is central to learning in the sciences, and while the term visualization is commonly used to refer to a range of different skills, we use it in a fashion analogous to Tufte, who defines it as the systematic and focused visual display of information in the form of tables, graphs, and diagrams.4 For the purposes of chemistry, we would add representations of chemical structure to that list. To truly grasp many scientific ideas, students must, of necessity, learn to translate and navigate between different modes of representation and learn to represent different models in meaningful ways. This ability is often referred to as representational competence and has been extensively described and characterized by Kozma and Russell,5 who defines five levels of representational competence that range from level 1, representation as depiction of physical features, to level 5, a reflective, rhetorical use of representations in which they are used to explain the relationship between physical properties and underlying entities and processes. The concept of representational competence is of particular importance in chemistry, and much has been written on the difficulties students have in translating between a symbolic, molecular, and macroscopic understanding of chemistry.6,7 A great deal of effort and time has been spent on the improvement of student visualization skills, including incorpo-
[Kermack and Robinson; see ref 1]
T
he essence of learning chemistry is the ability to merge the physical with the ephemeral, the seen with the unseen, and in so doing, connect the properties and behaviors of substances in the world around us with the atoms and molecules that comprise them. The use of symbolically based “systems of notation” is critical to making these connections possible. Of the innumerable advances made in organic chemistry since the above quote was published in the Journal of the Chemical Society in 1922,1 few have had as great an impact on the teaching and learning of organic chemistry as the use of the curved-arrow notation to convey electron flow during mechanistic processes. For the practicing organic chemist, the curved arrow is an invaluable and indispensible tool for not only mechanisms, but also in predicting the products of novel reactions, drawing resonance structures, and in simplifying target compounds during retrosynthetic analysis.2 For many organic chemistry students, however, its use is plagued by frequent complications. © 2012 American Chemical Society and Division of Chemical Education, Inc.
Published: April 11, 2012 844
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therefore, requires research tools and methodologies that can accurately capture process in addition to product. In the past, this has necessitated the use of qualitative techniques; however, it was our desire to study as large a sample as possible, ultimately making such an approach prohibitive for this investigation. Instead, the current research used OrganicPad, an innovative structure-drawing program, as the primary data collection tool. OrganicPad exploits the unique user interface afforded by tablet PCs to present students with an experience that approaches their work with paper and pencil as closely as possible. Participants draw atoms, bonds, electron pairs, charges, and curved arrows on the screen using the tablet PC stylus. The system is open-ended, enabling students to add the various elements to their mechanisms in the order they deem appropriate. An example of this process is depicted in Figure 1
rating multiple representations into texts and the development of sophisticated virtual modeling systems, although the efficacy of this approach is debatable and not well documented. As Johnstone8 himself has pointed out, while experts can translate seamlessly between the different levels and representations, beginners must first learn to operate along the edges of his famous triangle. It may be that presenting students with such a profligate set of representations only serves to overload their working memory. What is clear from our research9 is that students have great difficulty not only in translating between the levels of representation, but in constructing and using them.
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PRIOR RESEARCH ON MECHANISM USE Mechanisms and the use of the curved-arrow notation are integral components of most organic chemistry courses; however, despite nearly four decades of publications in this Journal and others (for example, see refs 10−22) presenting alternative strategies for teaching these topics to students, little actual research has been conducted exploring the difficulties organic chemistry students encounter in using the curved-arrow notation or how they develop competency over time in regards to this task. Notable exceptions include the work of Bhattacharyya and Bodner,2 and Ferguson and Bodner.23 Although these studies focused on students who differed widely in their experiences proposing mechanisms and in using the curved-arrow notationchemistry graduate students enrolled in a graduate-level organic chemistry course versus undergraduate chemistry majors enrolled in a second-year-level organic chemistry coursethe findings of both were remarkably consistent. Students frequently viewed the curvedarrow notation devoid of any actual meaning. In other words, the arrows did not convey the flow of electrons during the mechanistic process but simply were a means of connecting the disparate pieces of the molecules together. In adopting what the authors described as a “connect the dots” strategy,2,23 the students frequently invoked meaningless and “mystical” operations to transition from reactants to products in a manner that suggested little connection between symbol use and preexisting chemical knowledge. In short, Bhattacharyya and Bodner2 concluded that a disconnect exists between how instructors expect their students to use the curved-arrow notation and how students actually use them: “Rather than solving chemical problems, they [the students] were essentially playing with puzzles.”
Figure 1. A screenshot from OrganicPad showing a user-created mechanism for the SN2 reaction of methyl bromide and hydroxide.
in which the user has drawn a mechanism for the SN2 reaction that occurs between methyl bromide and the hydroxide anion. Beyond merely providing students a medium with which to create their structures and mechanisms, OrganicPad offers a number of tools for researchers, including aggregation of data using Markov modeling;24 however, the most important for our current inquiry is the ability to record and store all user-made actions in an online database. These records can be replayed afterward for the purpose of subsequent analysis. More detailed reports describing OrganicPad’s development, its technical underpinnings, and its features have been previously reported.9,24 During the 2009−2010 academic year, students enrolled in a two-semester organic chemistry course at a research-intensive university located in the southeastern United States were asked to complete a series of mechanisms using OrganicPad. The participants were drawn from three different lecture sections with three different lecture professors. All three sections used the same textbook and followed the same lecture and laboratory schedule. At the conclusion of the spring semester, all students completed a comprehensive, standardized final exam published by the American Chemical Society’s Examinations Institute. Typical mechanistic instruction in the three sections consisted of a short primer presented approximately four weeks into the fall semester, followed by subsequent reinforcement of mechanistic concepts throughout the remainder of the fall and spring semesters. So as to ensure that students were familiar with the program’s use, all participants were required to complete a comprehensive tutorial before completing any research activities. In an effort to track changes in mechanism use over time, students were asked to complete the exercises at four different times throughout the academic year:
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METHODOLOGY Previous research has documented the difficulties that secondyear- and graduate-level organic chemistry students face in employing the curved-arrow notation to convey the movement of electrons during mechanisms.2,23 Using well-designed, qualitative methodologies, these studies have effectively captured students’ experiences at a singular moment in time; our understanding, however, of how mechanistic skills develop over a more extended period is lacking. It was the purpose of this research to investigate how the mechanistic approach of students enrolled in a two-semester, second-year-level organic chemistry course evolved during their study of the subject. OrganicPad as a Mechanism Capture Tool
Mechanisms are process-oriented tasks, requiring students to “envision a continuous flow [of electrons] along the mechanistic pathway that transforms the reactants into the products of the reaction”.23 Any study of mechanisms, 845
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Data Analysis
1. Midfall, approximately two weeks after organic chemistry 1 students were introduced to mechanisms in their lecture course 2. At the conclusion of organic chemistry 1 3. At the beginning of organic chemistry 2 4. At the conclusion of organic chemistry 2 The reactions, which are summarized in Figure 2, were selected to reflect material being taught in lecture at the time and were
Data analysis began by reviewing the 2200 replays that were collected from students as they created their mechanisms using OrganicPad. Two members of the research team (NPG and KMR) initially reviewed the replays to obtain a broad understanding of the mechanistic actions students used to obtain the final products for each reaction. We subsequently viewed the replays a second time, creating a step-by-step record of how each student constructed each of his or her mechanisms. This fine-grained analysis allowed us to construct a series of reaction-specific “mechanism maps” that related the various reactants, products, and intermediates students created with the mechanistic approaches they relied upon to transition among those reactants, products, and intermediates. To ensure that the coding scheme used to create the mechanism maps was sufficiently robust to be used among multiple researchers, we randomly selected a sample of 50 analyzed replays for comparison, resulting in an inter-rater reliability of 0.84. After discussing areas of disagreement, an additional 25 replays were selected, coded, and compared. This time, the inter-rater reliability was 0.93, allowing us to conclude that the scheme as designed (and presented in both the mechanism map included in Figure 3 and those included in the online Supporting Information) was reliable. An example mechanism map for the acid-catalyzed addition of water to 1-butene is shown in Figure 3. In addition to reporting the states students used while constructing their mechanisms, the probability of proceeding from one state to the next is also included on each arrow. For example, all students began with a blank OrganicPad palette and either rewrote the information provided to them in the promptthat is, the reactants or reagents (46 students, 0.69 probability)or immediately drew the final product (20 students, 0.31 probability). Probabilities for subsequent steps are calculated using the total number of students who made it through to that step, not using the original number of students who started the exercise. For purposes of clarity, all transitions with a
Figure 2. Summary of the mechanisms and activities provided to research participants.
provided to the research participants devoid of an answer: that is, participants were instructed to use the mechanism to predict the product of the reaction. In all, more than 2200 mechanisms were collected from approximately 300 different students, in multiple lecture sections with three different instructors. All research participants were provided with detailed information about their rights as human subjects and informed consent was obtained from all participants.
Figure 3. Mechanism map illustrating the pathways used by students who created a mechanism for the acid-catalyzed addition of water to 1-butene, midfall administration. 846
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guiding the course of their mechanistic actions, but instead approached all mechanisms in a consistent fashion regardless of the functional groups involved. Although this may seem surprising, our prior research investigating how students construct Lewis structures documented a similar phenomenon among not only organic chemistry students but also more advanced undergraduate and graduate students.9 Figure 4 summarizes the firstand in many cases, the onlymechanistic step used by students (the mechanism maps
probability of 0.05 or less were omitted from the graphic. The most appropriate pathway that students created is denoted using striped arrows; however, it is important to note that the most appropriate pathway, as is true of the mechanism map depicted in Figure 3, did not always correspond to the correct mechanistic route. In this case, none of the 66 students who participated in the first administration proceeded through the protonated alcohol intermediate. Instead, they depicted hydroxide attacking the carbocation, water attacking the carbocation without showing the final mechanistic step that removes the extra proton, or provided no mechanism for the transformation; that is, they wrote down the intermediates or products without indicating the sequence of events that led to their formation. Finally, it is important to note that although new students entered the sample with each administration, we carefully evaluated their results separately and found no discernible differences in their mechanistic strategy usage. As such, the two groups were combined and mechanism maps for the unified whole are presented throughout this report.
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RESULTS AND DISCUSSION In reviewing the replays and in analyzing the mechanisms maps, we were immediately impressed by the sheer number of students who merely predicted the product without first providing a mechanism for the process as requested. As shown by the data in Table 1, depending on the reaction and the time
Figure 4. The evolution of mechanistic pathways from mid-fall to latespring administration.
Table 1. Comparison of Students’ Mechanism Use for these reactions are included in Figure 3 and in the online Supporting Information). Initially, the mechanism maps, and consequently the number of mechanistic pathways used by the students, were quite simplistic. Indeed, as shown in the mechanism map in Figure 3, only 9 of 66 students (14%) depicted any sort of intermediate during the course of their mechanism, in this case either the primary or secondary carbocation; in general, students either depicted an electronrich species attacking another electron-rich species (41%), or an electron-rich attacking an electron-deficient species (52%). The simplicity of the initial mechanism maps supports the assertion made by Ferguson and Bodner that organic chemistry requires a paradigm change surrounding students’ views of the nature of chemical reactions: from less of a product-oriented view as expounded upon in general chemistry to more of a process-oriented view in organic chemistry.23 Although the overall percentage of students who engaged in mechanism use remained fairly constant during the course of our study, the mechanisms maps from late-fall and early-spring did show shifts that would suggest that some students were beginning to make that transition. In both cases, a significantly greater number of students drew the carbocation intermediate during the course of their mechanism, and further, the most chemically appropriate pathwayincluding the depiction of the protonated alcohol intermediatewas provided by a number of students. Interestingly, these improvements were also accompanied by an increase in the number of mechanistic pathways used by the students, many of which were dubious and inappropriate in nature. During the mid-fall sampling, the majority of the students relied on two mechanistic pathways as described above and presented in Figure 4. However, by the late-fall sampling, this increased to three major pathways, with the addition of the 23% of students who depicted electron-deficient species attacking electron-rich species: in other words, drawing arrows that
Reactions, % Timing of Administration
A
B
C
D
E
Average
Mid-Fall Late-Fall Early-Spring Late-Spring
56 49 57 70
62 62 59
57 66 59
64
40
56 56 62 56
period sampled, anywhere between 30 and 60% of students did not engage in the activity, and on average, this percentage remained fairly constant despite students’ growing familiarity with mechanisms over the course of the academic year. Further, upon watching the replays, it became clear that an additional 15−20% of the students drew their curved arrows only af ter first predicting a product for the reaction. In other words, as many as three-quarters of the students did not engage in the activity or, in all likelihood, only engaged in the activity because we specifically requested they do so. It seems likely, therefore, that many of the students in the population we sampled did not find the mechanism or the use of the curved-arrow notation particularly useful or necessary in helping to determine the products of the reactions we presented to them. The mechanism maps afforded us a unique and convenient means of tracking the actions of the students who did offer mechanisms in response to the reactions they were provided. For the purposes of this report, we focus exclusively on the results from reaction A, the acid-catalyzed addition of water to 1-butene. This reaction specifically provided us with an opportunity to study the development of early mechanistic abilities from their earliest inception to (for most students) their culmination at the end of the two-semester sequence; that was not possible with the other reactions. Further, our analyses showed a remarkable level of congruence among the results for the five reactions, suggesting that many students did not use their growing familiarity with functional groups as a means of 847
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showed the proton attacking the π bond of the alkene. The emergence of this new pathway unfortunately came at the expense of a more appropriate one. It may be that these students truly believed that electron density flowed from the proton to the alkene during the course of the reaction, or, more likely, it may be indicative of a misunderstanding about the symbolic meaning of the curved arrows. There is precedence for the latter idea in the research conducted by Bhattacharyya and Bodner2 who reported graduate students using the curved arrows as a means of connecting pieces of the molecules together instead of a means of representing electron flow during the process. The proliferation of mechanistic pathways continued into the early-spring administration when 24% of students began to use what we referred to as a cyclic approach in proposing their mechanismsnamely, all of the reactants and reagents reacted at once to form the final productin addition to the three major pathways used during the late-fall administration. A screenshot of a cyclic process is included in Figure 5 in which the student has simultaneously depicted the electrons in
in the mid-fall sampling, the late-spring maps were more simplistic in nature and reflected the concentration of mechanistic actions described above. Unfortunately, the percentage of students who depicted an intermediate during their mechanism was also similar (14% mid-fall vs 19% latespring administration) and represented a smaller number than those who had created intermediates either in late fall or early spring. This may be related to the greater numbers who elected to depict a cyclic pathway given that the process-oriented nature of the mechanism was circumvented by this approach and any intermediates were not explicitly drawn by the participants.
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CONCLUSION Mechanisms and the curved-arrow notation can serve as powerful tools for assisting students and professionals alike in visualizing the sequence of steps that transforms reactants into products. This research has documented the evolution of mechanistic actions by second-year-level organic chemistry students during the course of their study of the subject and suggests areas of concern for those instructors who rely heavily upon the use of mechanisms and the curved-arrow notation in their organic chemistry courses. One of the more surprising findings from our research was the sizable proportion of students who failed to provide mechanisms for the reactions that we presented to them. Despite students’ growing familiarity with mechanisms and the more process-oriented view that they represent, this proportion remained remarkably consistent throughout the course of the study. Furthermore, an additional 15−20% of students provided the mechanism and curved arrows only after having predicted the product, adorning and decorating their submissions with arrows in all likelihood because we asked them to do so, not because of any inherent benefit derived from the use of such representations. We are particularly interested in exploring this phenomenon in more detail. While many students did not engage in mechanism use as part of this study, were those that did more successful at predicting the products? Is it the case, as many assume, that students who engage in mechanism use are better equipped to solve organic chemistry problems, or are our requirements to use the curved-arrow notation simply another burden to the students, another obstacle they must successfully navigate around in order to successfully complete the course? In a broader sense, what benefits, if any, do students actually derive from mechanism use? These are all important questions that we are currently working to answer. The mechanism maps provided us with a convenient means of depicting how students’ mechanism use evolved over time. Initial maps were simplistic in nature and showed students using only a handful of mechanistic pathways. Over time, however, the number of improper pathways increased, suggesting that many students were still struggling with how to actually implement proper mechanistic conventions. One such approach to emerge over the course of the study was the use of a cyclic pathway, which was frequently used by those students who provided the mechanism after predicting the product. Additionally, this proliferation in pathways could suggest that students were finally becoming more comfortable with mechanistic convention and were beginning to “play” with them. Unfortunately, without substantive, personalized feedback, the students continued to use many of these pathways whether appropriate or not, but it may be that at this stage, where students are finally becoming familiar enough to
Figure 5. Student-created mechanism using a cyclic pathway.
the π bond attacking the proton, the water attacking the alkene, and one of the hydrogen atoms on the water leaving to form what she provided as the final product. As was the case with the majority of the students who pursued such a mechanistic approach, this specific student first predicted the product before going back and drawing the reactants and curved arrows. Furthermore, there often was little correlation between the traditionally accepted sequence of mechanistic steps taught to students and the actual order in which they drew the arrows. In this instance, the student first drew the arrow to indicate the cleavage of the oxygen−hydrogen bond, showed the π electrons of the alkene attacking the proton, and concluded by having the water attack the alkene. Again, we surmise that many of these students would not have provided a mechanism, cyclic or otherwise, had we not prompted; clearly, they did not find it useful in helping to predict the product as that step preceded the placement of the curved arrows. Another possible explanation for this behavior, which increases toward the end of the second semester, is that students believe this to be suitable “shorthand” for presenting the mechanism. However, because the sequence typically did not correspond to the appropriate pathway, we believe this type of representation should not be encouraged, and may be representative of rote memorization rather than true conceptual understanding. Following the increase in mechanistic pathways observed during the late-fall and early-spring sampling, we expected to see a continuation of this trend toward the end of the academic year; however, that was not the case. The data included in Figure 4 for this time period showed a significant improvement in the number of students who depicted an appropriate pathway as their initial mechanistic step. This was accompanied by significant decreases in the percentage of students using two of the three more inappropriate pathways, the exception being the sizable percentage of students who continued to rely upon the cyclic approach. Similar to the mechanism maps generated 848
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(21) Friesen, J. B. J. Chem. Educ. 2008, 85, 1515−1518. (22) Ault, A. J. Chem. Educ. 2010, 87, 937−941. (23) Ferguson, R.; Bodner, G. M. Chem. Educ. Res. Pract. 2008, 9, 102−113. (24) Cooper, M. M.; Grove, N. P.; Pargas, R.; Bryfczynski, S. P.; Gatlin, T. Chem. Educ. Res. Pract. 2009, 10, 206−210.
experiment, that this is where such feedback may be more effective. In either case, we believe that these results speak clearly to the need for more extensive mechanism instruction and training in most organic chemistry courses. Although most textbooks and instructors provide students with a basic primer on mechanisms and the use of the curved-arrow notation, this clearly was not sufficient for either the students in our study or those in others. The use of the curved-arrow notation is a vital tool for practicing organic chemists and instruction must reflect that. Students must continually be provided with opportunities to practice the development of these skills, and most importantly, be provided substantive feedback as they do so. Often this is difficult to accomplish in the context of large enrollment courses; however, we believe that technology provides the means to do this and we are currently working on the development of such approaches.
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ASSOCIATED CONTENT
S Supporting Information *
Mechanism maps. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ACKNOWLEDGMENTS This work is funded by NSF grant DRL-REESE #0735655 and NSF-CCLI # 0816692. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. The authors would also like to thank Sam Bryfczynski and Sonia Miller Underwood for their assistance.
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REFERENCES
(1) Kermack, W. O.; Robinson, R. J. Chem. Soc. 1922, 121, 427−440. It bears mentioning that it is this manuscript that many credit with the introduction of what would become the curved-arrow notation. (2) Bhattacharyya, G.; Bodner, G. M. J. Chem. Educ. 2005, 82, 1402− 1407. (3) Gilbert, J. K. Visualization in Science Education; Springer: Dordrecht, The Netherlands, 2005. (4) Tufte, E. R. The Visual Display of Quantitative Information, 2nd ed.; Graphics Press: Cheshire, CT, 2001. (5) Kozma, R.; Russell, J. Modelling Students Becoming Chemists: Developing Representational Competence. In Visualization in Science Education; Gilbert, J. K., Ed.; Springer: Dordrecht, The Netherlands, 2005. (6) Gabel, D. J. Chem. Educ. 1999, 76, 548. (7) Johnstone, A. H. J. Comp. Assist. Learn. 1991, 7, 75−81. (8) Johnstone, A. H. J. Chem. Educ. 2010, 87, 22−29. (9) Cooper, M. M.; Grove, N. P.; Underwood, S. M.; Klymkowsky, M. W. J. Chem. Educ. 2010, 87, 869−874. (10) Williams, D. H. Educ. Chem. 1971, 8, 214−216. (11) Caserio, M. C. J. Chem. Educ. 1971, 48, 782−790. (12) Hanson, R. W. Educ. Chem. 1976, 13, 78−79. (13) Norman, R.; Waddington, D. J. Educ. Chem. 1979, 16, 120−122. (14) Buncel, E.; Wilson, H. J. Chem. Educ. 1987, 64, 475−480. (15) Simpson, P. School Sci. Rev. 1988, 70, 77−82. (16) Simpson, P. School Sci. Rev. 1989, 70, 31−38. (17) Simpson, P. School Sci. Rev. 1989, 70, 63−69. (18) Ryles, A. P. School Sci. Rev. 1990, 72, 71−74. (19) Wentland, S. H. J. Chem. Educ. 1994, 71, 3−8. (20) Erdik, E. J. Chem. Educ. 2005, 82, 1325. 849
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