Does Mechanistic Thinking Improve Student Success in Organic

Article pubs.acs.org/jchemeduc

Does Mechanistic Thinking Improve Student Success in Organic Chemistry? Nathaniel P. Grove,*,† Melanie M. Cooper,‡ and Elizabeth L. Cox† †

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 S Supporting Information *

ABSTRACT: The use of the curved-arrow notation to depict electron flow during mechanistic processes is one of the most important representational conventions in the organic chemistry curriculum. Our previous research documented a disturbing trend: when asked to predict the products of a series of reactions, many students do not spontaneously engage in mechanism use even when explicitly prompted to do so. Building upon those results, this study revealed that students who engaged in mechanism use were better equipped to solve organic chemistry problems but only those that involve transfer of knowledge. KEYWORDS: Second-Year Undergraduate, Chemical Education Research, Curriculum, Organic Chemistry, Mechanisms of Reactions FEATURE: Chemical Education Research

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organic chemists derive great value from engaging in mechanistic thought and in using curved arrows, what benefits do students actually gain from using curved arrows? The prior research is silent in this regard. Our previous work focused on how students’ use of mechanisms changed over the course of their study of organic chemistry18 and documented a disturbing trend: when asked to generate mechanisms in the course of predicting the products of a series of reactions, the majority of students either simply predicted the product without providing a mechanism, or created the mechanism and “decorated” it with the curved arrows only af ter predicting the product. In some cases, these two actions accounted for nearly 80% of the students who participated in our research.18 Upon the basis of these results, we surmised it was likely that many students found the use of the curved-arrow notation unnecessary in helping them to predict the products or that they were using the formalism simply because we explicitly requested that they do so, not because they derived any inherent value from it. For a discipline such as organic chemistry that places a tremendous emphasis on the use of the curved-arrow notation, these results were quite surprising and extremely disappointing. Clearly, the majority of students did not find it necessary, or were unable to engage in mechanistic thought or to use the curved-arrow notation. Yet, an important question remained: Are those students who use mechanistic reasoning better equipped to solve the problems presented to them during the course of our study? In other words, did the students who use curved arrows and mechanism predict the correct product a greater percentage of the time? Were they able to solve more difficult problems than the majority of their peers who did not use mechanisms? The current research was conducted as an attempt to answer these questions: to ascertain the benefits, if

rganic chemistry has a fearsome reputation for many students, who often see it as a gatekeeper course: difficult, complex, and overstuffed with material that students may perceive as irrelevant. While organic chemistry may be feared for any number of reasons, what is unfortunately true is that many students after a whole year’s study are unable to articulate and make sense of the underlying principles that connect the structure of a compound to its properties.1 Previous research has shown that even chemistry graduate students have trouble predicting the outcome of organic reactions in a systematic manner.1 To the expert organic chemist, the subject is self-consistent and logical; however, many studentsoverwhelmed by the sheer volume of material, and lacking the skills to decode information contained in the representations and structuresare perforce compelled to memorize and solve problems by analogy or surface-level features.1 Of the innumerable representational conventions used in the undergraduate and graduate organic chemistry curriculum, few are as ubiquitous as the curved-arrow notation to convey electron flow during mechanistic processes. First introduced by Kermack and Robinson in 1922,2 the use of the curved arrow has transitioned beyond depicting mechanisms to encompass the planning of retrosynthetic analyses, the prediction of products of unfamiliar reactions, and the production of resonance structures. Curved-arrow notation occupies an almost revered position at the heart of many organic chemistry curricula. Although a large number of papers have appeared in the literature during the last four decades presenting alternative methods for teaching the use of curved arrows and mechanisms to students,3−15 the vast majority are not research-based. The few research studies that have been conducted16,17 have focused mainly on the meaning that students ascribe to the curved arrows and how they are used during the creation of mechanisms. While there can be little doubt that practicing © 2012 American Chemical Society and Division of Chemical Education, Inc.

Published: May 2, 2012 850

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writing the answers on paper. In addition to recording their final submissions, all user-made actions leading to that submission were also recorded and stored for later replay in a secure database. In all, approximately 2200 submissions were collected. A complete listing of acceptable answers, in addition to the correct mechanisms for reactions A−F, are included in the online Supporting Information. After data collection was completed, the first author reviewed each of the 2200 submissions to ascertain whether the answer provided was correct and to determine whether the student engaged in mechanism use as requested. Three main criteria, which were developed jointly by the first and second authors, were considered in making the latter determination: 1. Did the student submit an answer to the problem? 2. Was a mechanism attempted, and were curved arrows employed during the creation of that mechanism? 3. Was the use of the mechanism and curved arrows meaningful to the creation of the product? That is, did the use of curved arrows support the prediction of the product, or were they added as an afterthought or subsequent to the creation of the product? It is important to note that the correctness of the proposed mechanism was not considered as a factor for determining mechanism use. To illustrate the difference between requirement 2 and 3 above, consider the two mechanistic sequences depicted in Figure 2. In both instances, students showed the electrons in

any, that students gained from the use of mechanisms and the curved-arrow notation.

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METHODOLOGY The data used in the current research were gathered as part of a larger study aimed at better understanding students’ use of the curved-arrow notation to propose mechanisms for a series of organic reactions and is described in detail elsewhere.18 For the purposes of this report, however, those methods as they pertain to the current inquiry are briefly described below. This study was conducted with students (N = 399) at a research-intensive university located in the southeastern United States during the 2009−2010 academic year. The students were enrolled in three separate sections of a second-semester, second-year-level organic chemistry course. Each section was taught by a different instructor who was ultimately responsible for determining the material covered, the presentation style of that coverage, and how to assess students’ learning of the material. Approximately 70% of the students were preprofessional majors intending to attend medical, dental, veterinary, or pharmacy schools after graduation. The remaining students were mostly chemical engineering majors, chemistry majors, and biology majors. Most of the students were between 19 and 22, and about 60% female. Approximately three weeks before the end of the spring semester, students were asked to provide mechanisms for a series of organic reactions and to predict the product of those reactions as they did so. The reactions, shown in Figure 1, were

Figure 2. Example mechanism sequences created by students in response to the reaction A task.

the π bond attacking the proton; however, the student who supplied the top example would be considered a mechanism user according to our scheme, while the student who provided the bottom example would not. Although the bottom answer is correct, it is not clear how the student used the curved arrows to transform the reactant to product. Conversely, the top answer is incorrect; however, the use of the curved arrows supports the student’s belief that the alkene is converted into the corresponding alkane.

Figure 1. Six tasks presented to organic chemistry students. Tasks E and F (shaded) are more difficult.

designed to present students with a series of tasks from the familiar to those that required transfer of knowledge to solve a more difficult task.19 That is, the reactions labeled A, B, C, and D represented tasks that were similar to those presented to students in class and in their textbooks, while those labeled E and F required students to apply their knowledge of organic reactivity to a novel situation that they may not have seen before. To gather responses and to collect data about the students’ mechanism use, OrganicPad, a tablet PC structure-drawing program, was used as the primary data collection tool.1,20 Responses were collected from students during their weekly laboratory sessions; in most cases, while students were waiting for the completion of refluxes. Informed consent was collected from all participants and all students were required to complete a short OrganicPad tutorial prior to completing any research tasks. Students were presented with the six tasks on-screen, one at a time, and asked to use the tablet PC stylus to construct their mechanisms and final products. In this activity, the tablet PC is used like a pen and pencil. In other words, there was no software to learn and the activity takes about the same time as

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RESULTS Overall, students were consistent in their mechanism usage (according to our criteria) with nearly 51% not using mechanisms during the six tasks or only using them for one of the six tasks while over 38% used them during five or all six tasks. Only 11% were more selective in their application of mechanism use and there did not appear to be any pattern to suggest which tasks these students provided mechanisms for and which they did not. The results summarized in Figures 3 and 4 present the success rates for each of the six problems provided in Figure 1. Figure 3 presents results for reactions A− D, while Figure 4 focuses on reactions E and F, the more difficult tasks. In both cases, the success rates for each reaction were calculated separately for students using mechanisms and those who did not. Given the categorical nature of the data 851

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Table 1. Reaction A−D Success Rates Comparisons for Mechanism Users versus Nonmechanism Users Reactions

p Values

A B C D

0.30 0.13 0.34 0.57

substantial effect sizes (reaction E, ϕ = 0.505; reaction F, ϕ = 0.578).

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Figure 3. Success rates reported as percentages for reactions A−D, collected from second-semester organic chemistry students (N = 399).

DISCUSSION The results presented above confirm the assumption that many organic chemists have made all along: students can directly benefit from engaging in mechanism use. However, limits to those benefits are apparent as they pertain to the measures we focused on for the purpose of this research. Although we observed significant differences between mechanism users and nonmechanism users for the two more complex tasks, no significant differences were found when it came to the four simpler tasks. Although disappointing, to a certain extent, the results from reactions A−D are not surprising. In most cases, these tasks were fairly straightforward, and because of this, we believe many of the students simply predicted the product outright because they knew (or thought they knew) the answer. Under these circumstances, any benefits derived from engaging in mechanism use were likely negated. Alternatively, it may be that for some students, the cognitive demands associated with both engaging in mechanism use and accessing knowledge from their long-term memories was too overwhelming, which again may have negated any benefits gained from mechanism use. There are both advantages and disadvantages to using the curved-arrow notation, as is the case with any representational formalism; we posit that it is the interplay of these factors that may explain the trends in success rates for these tasks. Organic chemistry is a process-oriented field and it is the transition from the product-oriented view used most commonly in general chemistry to the more process-oriented view of organic chemistry that some believe contributes, at least partially, to the difficulties that many students face in learning organic chemistry.17 The use of mechanisms and the curved-arrow notation provides students with a systematic and organized approach to guide this transition. At the same time, however, the cognitive demand associated with these activities, especially for novice students such as the ones we were working with, is likely quite high and in some instances may outweigh any benefits gained from their use. Although these activities are second nature to experts, consider just a few of the questions that students must successfully navigate in order to provide a correct mechanism: • At what position in the reactants does the arrow start? • At what position does the arrow end? • What does the arrow actually mean? • Do I use a single-barbed arrow or a double-barbed arrow? • Is the process concerted or does it happen over the course of several steps? • If the latter is the case, what do the intermediates look like? • How do the intermediates themselves react to subsequently form the product?

Figure 4. Success rates reported as percentages for reactions E and F, collected from second-semester organic chemistry students (N = 399).

(correct vs incorrect), differences between the two groups were explored for statistical significance using the χ2 test.21 Effect sizes were calculated for all statistically significant comparisons and are reported as ϕ values22 in the discussion below. It is interesting to note that despite the fact that these results were collected from students who were mere weeks away from finishing their second (and for most, their last) semester of organic chemistry, the success rates for all six tasks were quite low: in all cases save one, below 30%. Furthermore, there did not seem to be any discernable effect of time from instruction to testing. In other words, reactions that had been recently covered in class fared no better than older material. For example, the alkene process depicted by reaction A was included in material taught midway through organic chemistry 1, whereas reaction D was presented only two weeks before the research was conducted during the last few weeks of this second semester of organic chemistry. Despite these temporal differences, the success rates for these two problems are not significantly different. It should also be noted that even though these students attended three different lecture sections taught by three different instructors who used different tests and teaching methods, there were no differences in success rates or mechanism use as defined by our criteria. For reactions A−D, no significant differences were found in success rates between mechanism users and nonmechanism users. The p-values for the comparisons are included in Table 1. The same cannot be said of the differences in success rate for the two more difficult tasks (reactions E and F) as shown in Figure 4. In both cases, the mechanism users scored significantly higher than their nonmechanism counterparts (reaction E, p = 0.011; reaction F, p = 0.007) coupled with 852

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that is foreign to them, with a grammar that is unnatural and counterintuitive.

It is no wonder, therefore, that so many students elect not to use mechanisms spontaneously. It is clear that the issues related to the cognitive demand associated with mechanism use require additional research. As the tasks presented in reactions A−D became more difficult and less straightforwardin essence, more like the tasks students encountered in reactions E and Fsuccess rates began to shift to favor those students who engaged in mechanism use. For reactions A−D, we surmise that the cognitive demands associated with mechanism use negated any of the positive, organizational benefits, and as such, those students who decided to engage in mechanism use were no more or less successful than their peers who did not. However, in instances in which students could not answer the problem through simple recall of facts (for example, reactions E and F), those students who engaged in mechanism use were much more successful. Nevertheless, even for those students, the success rate was quite low, and is indicative of the difficulty of the task.

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ASSOCIATED CONTENT

S Supporting Information *

Complete listing of acceptable answers for the reaction mechanisms. 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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CONCLUSIONS Mechanisms serve as a unifying concept for much of organic chemistry and have come to occupy an important position in most modern curricula. Our research has documented an increase in success rates for students who engage in mechanism use, but only in situations in which students are presented with problems with solutions that actually require the students to engage in mechanistic thinking. These results when coupled with our prior work18 documenting the large numbers of organic chemistry students who do not spontaneously engage in mechanism use, strongly highlight the need to better understand the barriers that students face in trying to use mechanisms and the curved-arrow notation during the course of their study of organic chemistry. We hypothesize that a dynamic equilibrium exists between the positive, organizational benefits of mechanism use and the more negative, cognitive load issues that its use engenders and we are currently engaged in research to better understand this equilibrium and the factors that influence it. We must gain a better sense of how issues like cognitive demand affect the utility of using mechanisms and find ways of helping students overcome these problems. For example, can we develop “chunking” strategies that might diminish the cognitive demand associated with using mechanisms? Finally, this research points to the critical need for research-based instructional strategies that promote the development of more naturalistic and meaningful mechanism use. We are currently exploring the efficacy of such instructional strategies. One of the purposes of the work described in this manuscript is to alert instructors of organic chemistry to the idea that many of our students are moving through (and passing) organic chemistry courses without learning how to use one of the most important tools taught in the course. Many students believe that organic chemistry is all memorization, and for many students this is true; however, our studies show that students who do use mechanisms are more likely to succeed in more difficult tasks. We therefore recommend that instructors focus on developing the tools that lead to mechanistic thinking, and reinforce them during the course. If a student does not learn what the curved arrows mean fairly early in the course, it is unlikely that he or she will be able to pick it up later. We believe that extra time spent on basic principles at the beginning of the course will help students as they learn to navigate a language

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