Exploring Student Thinking About Addition Reactions Solaire A. Finkenstaedt-Quinn*, Field M. Watts, Michael N. Petterson, Sabrina R. Archer, Emma P. Snyder-White, and Ginger V. Shultz Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States 5 ABSTRACT Organic chemistry is a required course sequence for many STEM students, however research indicates that organic chemistry reaction mechanisms are especially challenging for students due to a mixture of underlying conceptual difficulties, the process-oriented thinking inherent to the discipline, and the representations commonly used to depict mechanisms. While student reasoning about many of the 10 reaction types covered in the organic chemistry curriculum have been studied previously, there is minimal research focused specifically on how students think about the mechanisms of addition reactions. This study addresses that gap by probing first semester organic chemistry students’ thinking using think-aloud interviews as they worked through two different addition reactions. To elicit a range of thinking, students worked through the mechanisms using either paper and pencil or 15 an app that dynamically represents the molecules. Overall, students were able to identify the steps of the two addition reactions but did not always successfully apply chemical thinking during the mechanistic steps. Specifically, both groups of students struggled with the concepts related to carbocation stability, frequently misapplying stabilization via substitution and demonstrating difficulty in identifying the potential for resonance stabilization. Our results suggest that instructors should 20 emphasize the conceptual grounding directing mechanistic steps, in particular when determining carbocation stability. GRAPHICAL ABSTRACT KEYWORDS 25 Second-year undergraduate, chemical education research, organic chemistry, computer-based learning, problem solving/decision making, addition reactions INTRODUCTION The organic chemistry sequence is a two-semester prerequisite for numerous fields of study with the aim of teaching students the fundamentals of organic chemistry reactions, developing students’ 30 ability to reason through reaction mechanisms, and strengthening students’ understanding of structure-property relationships.1 Unlike in general chemistry, where reactions are presented simply as reactants and products separated by an arrow, organic chemistry emphasizes reaction mechanisms, which require students to envision electron flow, denoted by the curved-arrow formalism, from reactant through intermediate molecules to the desired product.2, 3 However, reaction 35 mechanisms have been identified as one of the most difficult concepts covered in organic chemistry,1 as students must engage in multivariate thinking, weighing considerations of chemical reactivity and spatial constraints while also interpreting numerous representations.4 Thus, it is important to characterize how students reason through reaction mechanisms in order to better support their learning. 40 Prior research focused on how students approach and think about organic chemistry reaction mechanisms indicates that students use mechanistic steps primarily to move from the reactant to the product.2, 5-8 Additionally, students have a tendency to focus on the explicit features of the molecules involved in the reaction.7-10 However, research focused specifically on student reasoning about addition reactions is minimal.5, 9 Addition reactions are often introduced early in the organic chemistry 45 curriculum and require students to begin applying new content knowledge as they weigh multiple reaction pathways, utilizing reasoning processes that they will engage in throughout the course. As such, investigating student thinking on this topic has the potential to inform our understanding of how students may approach more difficult reactions that similarly require consideration of multiple reaction pathways. This study explores how students reason about addition reactions during think- 50 aloud interviews where students work through reaction mechanisms on paper or through an app. Mechanistic Reasoning An understanding of reaction mechanisms is a key component of the organic chemistry curriculum and requires students to integrate several sources of knowledge including knowledge of chemical reactivity and mechanistic representations.3-4 With its importance and complexity, a number of studies 55 have been conducted that investigate the processes students use when approaching mechanisms.2, 5-15 These studies indicate that students are not successfully applying the conceptual knowledge they have developed throughout general and organic chemistry and focus on explicit structural features, neglecting implicit, electronic features when making mechanistic decisions.2, 7-10, 15 For example, in two studies using card sort tasks, students tended to sort organic reactions based on two main criteria: 60 similarities in the type of reaction, without any discussion of the underlying mechanistic patterns, and by surface features, such as the presence of similar functional groups.9, 15 Additionally, students have been found to rely on memorized mechanisms that lead them to the desired products.2, 5-8, 14-15 These difficulties may be due to underdeveloped knowledge or a poor ability to apply the knowledge of electronics and electron flow during organic chemistry reactions,2, 6, 10 which is indispensable when 65 solving unfamiliar mechanisms. To facilitate thinking about the electronic properties and relationships between atoms and molecules, the ability to draw and manipulate Lewis structures is considered key, yet students are known to have difficulties with this skill.16 Additionally, students are not adept in their use of the curved-arrow formalism,2, 5-6 where they do not always use the formalism when it would be useful to predict reaction products,5 and may follow known reaction steps rather than 70 thinking chemically about the process.6 Additionally, mechanistic arrows have little meaning for students and are not recognized as representing electrons.2, 6 This may be indicative of a profound learning gap—that students do not fully understand the usefulness of mechanistic arrows or what these arrows depict.2, 5-6 Lacking an appreciation for curved arrows may promote reliance on memorization and is especially detrimental as students who use the curved-arrow formalism have 75 been shown to perform better on more difficult problems than those who do not.17 Addition Reactions Pi bond addition reactions are introduced early in the organic chemistry curriculum and start to extend students’ reasoning to incorporate content beyond what they have learned in general chemistry, e.g. decisions about acidity and basicity, yet there is little research focused on how 80 students approach and reason through this type of reaction specificially.5, 9 Grove et al. provided a detailed analysis of students’ drawn responses to an acid-catalyzed addition reaction.5 The percentage of students who provided a mechanism for the reaction, instead of just drawing the product, increased from 56 to 70% over the course of two semesters in organic chemistry. When students did show the mechanism, the diversity of mechanistic pathways depicted, both correct and incorrect, increased over 85 time and only a small percent drew a carbocation intermediate.5 Graulich and Bhattacharyya used variations of a card-sorting task targeting electrophilic addition reactions and found that students focused primarily on surface characteristics.9 When students were directed to consider mechanisms, they created categories based on characteristics of the reactions—e.g. reaction intermediates or steps—but did not always make correct assignments. Throughout the tasks, the most commonly 90 identified mechanistic step was carbocation formation.9 Beyond these two studies, additional research focused on the concepts students use as they reason through addition reactions is merited. Analyzing student reasoning about addition reactions is especially important as the reaction mechanisms begin to be more complex than what students may have seen previously. When reasoning through addition reactions, students must consider multiple reaction pathways—e.g., which carbon of 95 the double bond to protonate—and begin weighing multiple chemical considerations—e.g. substitution versus resonance stabilization. Students are known to struggle with these,6, 8 yet considering multiple factors is especially important in more complex reactions, such as substitution and elimination. This study sought to develop descriptions of students’ thinking about addition reactions by identifying what chemical features and concepts students focused on as they worked through two addition reactions. 100 Student reasoning was elicited through think-aloud interviews that utilized two different modalities, conventional paper-pencil and a touch-screen app. It is important to use multiple modalities to probe how students reason, as there are an increasing number of interfaces with which students can interact with mechanistic representations. Additionally, previous research indicates that multiple modalities can elicit a greater range of reasoning from students.18 This research was guided by the 105 following research questions: 1. How do undergraduate students in organic chemistry think about addition reactions when working through mechanisms on paper or using a touch-screen application? 2. When considering addition reaction mechanisms,