Organic Chemistry Students' Understandings of What Makes a Good

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Article Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Organic Chemistry Students’ Understandings of What Makes a Good Leaving Group Maia Popova and Stacey Lowery Bretz* Department of Chemistry & Biochemistry, Miami University, Oxford, Ohio 45056, United States ABSTRACT: Thirty-six students enrolled in Organic Chemistry II participated in individual, semistructured, think-aloud interviews about the factors that contribute to the stability and reactivity of organic species in the context of unimolecular and bimolecular nucleophilic substitution and elimination reactions. The students were provided with the mechanistic steps for these reactions. Most students correctly identified the leaving groups in these reactions and referred to them as “good leaving groups”. However, less than half of the students could explain the electronic and structural factors that justify characterizing a species as a “good” leaving group. Nearly one-third of the students who were interviewed were unable to provide any explanation of what factors result in a chemical species being a “good” leaving group. These findings are discussed through the lenses of both Perry’s scheme of intellectual development and Ausubel and Novak’s theory of meaningful learning. KEYWORDS: Second-Year Undergraduate, Chemical Education Research, Organic Chemistry, Misconceptions/Discrepant Events, Elimination Reactions, Molecular Properties/Structure, Nucleophilic Substitution FEATURE: Chemical Education Research

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INTRODUCTION The concept of reactivity is essential to learning organic chemistry. To communicate about reactivity and the associated thermodynamic and kinetic factors that influence it, organic chemists commonly use mechanisms to depict the transformations of organic compounds into the target products. Teaching the various mechanisms through which different reactions proceed is a central focus of organic chemistry, and therefore, multiple studies have investigated students’ understandings of reaction mechanisms and their approaches to solving mechanistic problems. It has been reported that organic chemistry students encounter numerous difficulties when learning about reaction mechanisms, including the improper use of curvedarrow notation,1−6 difficulties understanding the dynamic nature of the processes that are portrayed by the static reaction equations,6−8 and challenges with distinguishing between different reaction mechanisms.4,9−12 To address these difficulties, Flynn implemented changes to the standard organic curriculum by beginning the course emphasizing electron-pushing formalisms with students and then introducing students to the reactions themselves, starting with those that proceed through simple mechanisms and moving toward mechanistically more complex reactions.4 For example, unimolecular and bimolecular nucleophilic substitution and elimination reactions are traditionally one of the first reactions that students are taught in the first semester of organic chemistry, but in Flynn’s novel curriculum, these are now taught in the second semester because these reactions are considered to be mechanistically complex and require understanding of multiple concepts such as nucleophile/base strength, solvent effects, and leaving group ability.4 Several published © XXXX American Chemical Society and Division of Chemical Education, Inc.

studies have characterized student thinking regarding these important concepts, including the structure and function of nucleophiles/electrophiles and acids/bases. Anzovino and Bretz reported that students have fragmented understandings of the concepts of nucleophiles/electrophiles and rely primarily on charges to identify these species in mechanisms.13,14 Students use a variety of short-cut reasoning strategies (i.e., heuristics) to evaluate the relative acidity of substances, rather than reasoning with scientifically sound constructs such as polarizability.15,16 Organic chemistry students also rely heavily on the Brønsted− Lowry model of acids and bases, even though the Lewis theory of acids/bases is more broadly applicable in the context of organic chemistry.17 Less is known, however, about students’ thinking with regard to the concept of leaving groups. One recent study reported that organic chemistry students are able to successfully employ backward-oriented reasoning when analyzing multistep reaction mechanisms because they ignore the chronological order of the steps and use information from a subsequent step to explain why the prior step happened.8 Students can readily identify leaving groups in mechanisms and then suggest that the prior reaction step occurred in order to increase subsequent leaving group activity. For example, students explain that the protonation of a hydroxyl group happens in one step to produce a better leaving group (water) in the next step.8 No studies, however, have explored students’ understanding of why some entities are characterized as better leaving groups Received: March 15, 2018 Revised: May 24, 2018

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DOI: 10.1021/acs.jchemed.8b00198 J. Chem. Educ. XXXX, XXX, XXX−XXX

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than others. As part of a larger study investigating how students reason about reactions and reaction coordinate diagrams, students frequently commented upon the leaving groups they identified in the mechanisms. This paper reports findings from an analysis of the students’ comments about leaving groups. Specifically, the research question addressed in this paper is what characteristics do organic chemistry students identif y as responsible for considering a chemical species to be a good leaving group?

Article

METHODS

Sample

This study took place at a liberal arts public university in the midwestern United States. Prior to the beginning of the study, an IRB application was submitted and approved to protect the rights of student participants. Pseudonyms were created for all students who were interviewed in order to protect their identities. Students were recruited from two second-semester Organic Chemistry II lecture courses, one for chemistry/biochemistry majors and one for nonchemistry/biochemistry majors who were majoring in other STEM disciplines. Students were sent an email that contained a link to a survey that gathered demographic information such as undergraduate year, major, gender, race, and the grade earned in Organic Chemistry I. Thirty-six participants were purposefully selected31 to ensure that the sample included students who had earned a range of grades in Organic Chemistry I. Thus, the sample included 14 students who earned a letter grade of “A”, 14 who earned a “B”, and 8 who earned a “C”. Fifteen of the study participants selfidentified as male and 21 as female. The sample included 6 majors and 30 nonmajors, with 8 students enrolled in the major’s course and 28 in the nonmajors’ course (nonmajor students had the option to enroll in the major’s course due to scheduling conflicts).

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THEORETICAL FRAMEWORKS The data collection and analysis in this research were shaped by two theories of how human beings learn, namely, Ausubel and Novak’s Human Constructivism18,19 and Perry’s Scheme of Intellectual Development.20,21 Ausubel and Novak’s Theory of Meaningful Learning

Internal representations are the mental models that are built through the processing of information by the brain.22 Mental models, therefore, shape the learning of chemistry as they organize and transform abstract scientific concepts to be more readily accessible for human perception and cognition.23 Because mental models are formed by an individual, they constitute personal and unique constructs. Ausubel and Novak refer to the processes by which learners actively think about how new information should be interpreted and incorporated into their existing knowledge framework and stored in their long-term memory as meaningful learning.24,25 Students who do not organize and integrate new information, but rather memorize facts, engage in what Ausubel and Novak refer to as rote learning, which results in fragmented mental models. The creation of mental models is not dichotomous in that it can be characterized as either meaningful learning or rote learning, but rather these would be descriptors along a continuum that has been previously used to characterize the learning of organic chemistry.25,26 Previous research has reported that organic chemistry students often rely on rote memorization of the material and, therefore, are characterized as having fragmented knowledge structures.26−28

Instructional Context

The textbook used for the majors’ course was Organic Chemistry by Jones and Fleming,32 and the textbook in the nonmajors’ class was Organic Chemistry by Klein.33 In week 3 of the Organic Chemistry I nonmajors’ course, lectures provided an overview of both the quantitative and qualitative characteristics of acid/base strength. During weeks 7−10, both the majors’ course and the nonmajors’ course included lectures regarding leaving group reactions, namely, both substitution and eliminations reactions. Students were taught during the semester that the loss of a leaving group occurs through a heterolytic bond cleavage during which an atom or group takes the electron pair. To identify the leaving groups in substitution and elimination reactions, students were taught that leaving groups are the weak conjugate bases of strong acids. Students were shown pKa values of several substances (i.e., hydrogen iodide, hydronium ion, hydrogen fluoride, water, ethanol, ammonia, etc.) followed by a lecture regarding the relative stability of the conjugate bases of these substances and their respective leaving group ability. Students were told that several qualitative characteristics could be used to determine relative leaving group ability, i.e., the ARIO priority rule: the size of the atom that carries the negative charge in the conjugate base, its electronegativity, the impact of resonance and induction, and the type of orbital on the atom that carries the negative charge in the conjugate base. An exception to these qualitative priority rules was noted, namely, that pKa values provide a more accurate reference of the leaving group ability. Assessment of students’ knowledge about learning groups involved midterm exams. The exams included multiple choice questions regarding the concept of heterolytic cleavage and selecting the best leaving group among several substances without being provided the pKa values for those substances. The exams also included open-ended questions that asked students to propose mechanisms for substitution and elimination reactions. All students who participated in this study did so after having been taught and tested on these topics regarding the concept of a leaving group.

Perry’s Scheme of Intellectual Development

Perry’s scheme of intellectual development describes a series of stages in the intellectual and epistemological development of college students: dualism, multiplism, and relativism.29 In Perry’s scheme, a dualistic learner would think about chemistry as a science that requires absolute judgments (i.e., true/false, good/bad) and does not leave space for uncertainty or ambiguity.20,30 For example, when learning about the multiple models of acids and bases (Arrhenius, Brønsted−Lowry, and Lewis), students who employ dualistic reasoning would be prone to think that only one of these theories is correct, whereas the rest of the models ought to be deemed not useful and subsequently ignored in future learning. By contrast, students whose reasoning is best described as multiplistic would be inclined to consider all three of these acid/base models as equally accurate and useful, but might lean toward using the model that they think their instructor prefers.20,30 Finally, relativistic students would recognize that knowledge in chemistry is contextual and relative. Therefore, when reasoning about acid/base chemistry, relativistic students, who recognize that models have both strengths and limitations, would invoke an acid/base model as dictated by a given situation.20,30 Both general chemistry students20,21 and organic chemistry students30 have been reported to engage in dualistic thinking.

Data Collection and Analysis

Data was collected in Spring 2016 through individual semistructured think-aloud interviews,34,35 while study participants B

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Figure 1. Example elimination reactions that included the loss of a leaving group printed on Livescribe dot paper to elicit students’ understandings.

not depart as a separate entity, but rather remained within the same chemical species upon ring opening. Three reactions included a bromide ion as the leaving group, and two included a chloride ion; one included an iodide ion, and one reaction included a water molecule as a leaving group. Therefore, the majority of the students in the sample (n = 31) mentioned the loss of a leaving group in their responses when analyzing the assigned reaction mechanisms. None of the students mentioned the term “leaving group” when discussing the epoxide ringopening reaction. Upon listening to students’ comments, the interviewer noticed multiple inaccuracies in their responses pertaining to the concept of leaving groups and, therefore, was careful to ask follow-up questions to better ascertain their reasoning. All seven reactions that included the loss of a leaving group during one step of the mechanism were taught in both the majors’ and the nonmajors’ courses during Organic Chemistry I. Therefore, all students were interviewed after having been taught and tested on these reactions by their instructors.

were enrolled in Organic Chemistry II. Semistructured interviews were chosen to allow for follow-up/probing questions to be asked in order to ensure that the interviewer gained a clear understanding of each student’s mental models.31,34 On average, an interview lasted 53 min, and each participant was offered a $20 gift card as compensation for their time. The interview data was captured through the use of a Livescribe Smartpen to capture students’ writing and drawings,36 an audio recorder to capture students’ utterances, and a video camera to capture students’ gestures. The triangulation of these data sources ensured the validity of interview transcripts and clarification of students’ uses of “this” or “that” while pointing at particular items on the interview prompts. Interviews were transcribed verbatim. The data was inductively coded using the NVivo 11 software.37−39 Codes were generated by identifying and combining common ideas in students’ responses. The generated code-book was adjusted and modified to accommodate any extreme cases or ideas. Constant comparative analysis was used to make sense of the codes and to identify patterns in students’ thinking.40,41 To ensure the trustworthiness of the analyses, the researchers conducted weekly meetings during which the constructed codes were discussed and revised.42 The confirmability and credibility of the findings were established through periodic debriefing sessions with other chemistry education researchers uninvolved with the project.42,43

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RESULTS As the students discussed both bond cleavage and bond formation in their assigned reactions, most of them (n = 31) identified the leaving groups and spoke of them as “good leaving groups”. When asked to explain what makes a specific chemical species a good leaving group, students typically began reciting examples of good and bad leaving groups that were presented during lecture courses rather than discussing underlying chemical constructs. Upon further prompting by the interviewer to explain what specifically makes a chemical species a “good leaving group” rather than a “bad leaving group”, several characteristics of good leaving groups were identified. These characteristics are summarized in Box 1. When asked to provide an explanation as

Description of Interview Prompts

The interview protocol consisted of four phases: (I) prior knowledge about students’ understandings of bonding, relative stability, and reactivity of organic species; (II) students’ ideas about bonding, stability, and reactivity in the context of substitution and elimination reactions; (III) students’ understandings of the meanings of reaction coordinate diagrams’ surface features; and (IV) students’ thinking about how to make connections between the reactions in phase II and the reaction coordinate diagrams in phase III. The full interview protocol and findings regarding phase IV of the interview have been reported elsewhere.7,9 The findings reported herein are from the second phase of the interview. The study participants (n = 36) were randomly divided into four groups of 9 students. Each group was interviewed regarding a different pair of reactions from a set of 8 reactions (4 substitution reactions and 4 elimination reactions). Two reactions that students were asked to discuss are presented in Figure 1. The full set of reactions used in the study has been previously reported.9 Students were probed to discuss the processes underlying the bond making and the bond breaking in one substitution reaction and one elimination reaction that were printed on the Livescribe dot paper. Students were asked to comment upon the relative stability and reactivity of the chemical species in different steps of these reactions. Seven reactions (three substitution reactions and four elimination reactions) included the loss of a leaving group in one step of the mechanism. The final substitution reaction involved an SN2 epoxide opening in which the leaving group did

Box 1. Characteristics of Good Leaving Groups As Identified by Students Halide leaving group Weak base Charge-to-size ratio Very electronegative Octet acquisition It is a halide Water leaving group Absence of charge Octet acquisition

to why these characteristics were important, only two students were able to suggest an appropriate characteristic for a specific instance, although their responses were incomplete. Nineteen students offered reasons that ranged from incomplete or inappropriate for a specific instance to incorrect. Ten students were unable to offer an explanation whatsoever. C

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Halide Leaving Group

could not determine the most important factor when making inferences about why halides function as good leaving groups. Thus, when explaining why halides are good leaving groups, students provided incomplete responses by focusing on the ideas that halides are highly electronegative and that halides gain an octet when they leave, and simply “because they are halides”. Electronegativity is one factor to consider when deciding whether something is a good leaving group, because if the negative charge of the conjugate base is on the more electronegative atom, the charge will be better stabilized. However, this consideration is appropriate to consider only when thinking about atoms of similar size situated in the same period of the periodic table. It is not appropriate to use electronegativity as the sole criteria to justify the leaving group ability of halides because they are very different in size and, therefore, are described by different electron affinities. In fact, the most electronegative halide, fluoride, generally functions as the worst leaving group when compared to the other halides because the substantial ionic character of the C−F bond promotes electrostatic stabilization that is sufficiently strong to limit the formation of a free fluoride ion.44 Students, however, argued that electronegativity was the primary consideration to explain why halides are good leaving groups (see Figure 3 for drawings related to this student remark): “So chlorine is super electronegative. And so, that makes it a good leaving group because it pulls the electrons that are bringing it to carbon towards itself. And so, once it takes both of those [electrons], um, carbon is just there with the positive charge. Since chlorine is very electronegative and is inherently a good leaving group, then it’s almost like the nature of this molecule to dissociate from itself.” (Efim, second-year chemistry major) Some students who invoked only electronegativity to explain leaving group ability could not explain the concept of electronegativity itself. For example, Inna (third-year microbiology major) commented that bromide ion is considered a good leaving group because of its high electronegativity. The interviewer followed up and asked Inna to explain what electronegativity is, to which she responded: “Um, it’s um, I can tell you which atoms are [electronegative], and which aren’t, but I can’t actually say what exactly electronegativity is.” Other students discussed the acquisition of an octet as the key factor that determines whether a specific halide is a good leaving group: “Bromine and chlorine, I believe, are like two of the halides that are really good at taking, um, like breaking their bond... And a lot of times the bond would go to the halide to complete the octet. Because halides have like the seven valence electrons and with the bond, the two shared [electrons] going towards it is going to complete the octet.” (Lev, second-year biochemistry major) Other students further explained that they considered the acquisition of an octet by the halide to be a stabilizing factor, as they could recall that good leaving groups are often described as

When discussing halide leaving groups, only two students were able to provide responses that can be considered scientifically sound, although incomplete. Egor (second-year chemistry major) discussed the impact of the size of a halogen leaving group on its leaving group ability: “Um, a good leaving group is generally something with, um, with a good charge-to-size ratio. Charge over size equals some number n (Figure 2). Um, I would write the halogens in the order of size. I would say that iodine is a better leaving group than fluorine, chlorine, and bromine. It’s a really big atom and the charge, it has really big size, so your charge-tosize ratio would be very low and it’s a weak bond. And fluorine would probably not be a good leaving group because the charge-to-size ratio is much higher.”

Figure 2. Egor’s annotations on the Livescribe dot paper to explain the role of both size and charge in leaving group ability.

The size of the atom that carries the negative charge in the conjugate base is one of the most important qualitative factors to consider when predicting whether a conjugate base will be a good leaving group, especially when reasoning about halides as leaving groups. Egor correctly reported that the iodide ion is the best leaving group among the halides because its large size impacts the internuclear distance between the iodine and the atom to which it is bonded, and therefore, this bonding interaction is the weakest due to an inefficient overlap of orbitals. Inna (third-year microbiology major) also provided a scientifically sound explanation, even though she was not very confident about her response: “It [carbon−bromine bond] is breaking because, um, to me, I want to say it’s breaking because, like, I don’t know if this is right, but it has to do with acidity and basicity. It’s like the conjugate base... no... I think it’s breaking because it [pointed at bromine] is also stable by itself because it’s a weak conju... it’s a weak base by itself.” Inna was able to recall the correct definition of a good leaving group, but she did not discuss any other leaving group characteristics to explain why bromide is a good leaving group. When discussing the leaving group ability of halides, nineteen students provided explanations indicating that they lack a sound understanding of the key electronic and structural factors that determine how good of a leaving group a specific chemical species is. When asked to justify their responses, students could recall one or two characteristics of halide leaving groups, but they

Figure 3. Efim’s annotations of Reaction 7 (green ink) showing that chloride is a leaving group. (Note: The gray lines reflect student annotations later in the interview in response to other questions.) D

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“I know that hydroxide is not a good leaving group, so when you add, when it’s protonated it becomes, it’s easier to leave... If it leaves as this [hydroxide ion], it’s not going to be favorable for it to leave, it would not have enough electrons to fulfill an octet.”

chemical species that are able to stabilize their negative charge (i.e., form stable anions). Consider this dialogue with Yana (second-year biology major): Interviewer: “You said that bromine is a good leaving group. What makes it a good leaving group?” Yana: “Something that is able to handle or stabilize a negative charge better.” Interviewer: “Where does this stabilization come from?” Yana: “Um, so if bromine has like, its first valence shell has two electrons and then second can hold eight. If it gets one more electron to fill the second shell, it becomes more stable than if it had seven in that second valence shell when it was neutral.” These students thought that stabilization is achieved by acquiring an octet of electrons. This thinking is particularly problematic because, of course, the bromine already had an octet prior to the carbon−bromine bond breaking. Students like Yana, however, thought that a covalently bonded bromine atom was not as stable because it “shared” electrons, rather than “having all of the electrons to itself once it has left”. Another explanation commonly offered by students when explaining why halides are good leaving groups was the statement “because they are halides”: “[Bromine leaves because] um, I just learned it is one of the better leaving groups, because it’s a halide. I just know that halides are very good leaving groups.” (Alla, second-year biology major) Even though most of the halides are weak bases and are, therefore, good leaving groups, other nonhalide chemical species can also function as good leaving groups, such as water and sulfonates. These students were unable to articulate any additional reason for why halides are considered to be good leaving groups.

Correct and/or Incorrect Assertions without Any Explanation

Ten students made assertions about leaving groups but were unable to provide any explanation, even after being prompted to explain. Some of these assertions were correct, and some were not. Correct assertions included the ideas that good leaving groups are represented by water and chloride, bromide, and iodine ions, whereas bad leaving groups include hydroxide ion and a proton. For example, Arina (third-year kinesiology major) reported that she considered water to be a good leaving group but not the hydroxide ion. When asked to explain why, she replied: “I know some things are better at being leaving groups than others. Um, I am not sure what it takes to be a good leaving group. I know we learned something... like OH isn’t a good leaving group, but if you add a hydrogen then it becomes a good leaving group. It can leave as water. But I am not sure why that is.” Likewise, Greta (third-year biology major) was unable to explain why she considered a chloride to be a good leaving group: “Chloride is a good leaving group... It’s just these certain things that you are taught and you don’t really remember why they are the way they are.” The incorrect assertions provided by students in this category included the ideas that any halide is an extremely good leaving group, iodine ion is a poor leaving group, or bromide and chloride ions are the best leaving groups: “Bromine and chlorine, I believe, are like two of the halides that are really good at like breaking their bond... And something about the Br and Cl, the good leaving groups, that fluorine and iodide or iodine don’t have. They do not make good leaving groups. And I can’t remember why.” (Lev, second-year biochemistry major) Not only was Lev unable to provide an explanation for his assertions, but his assertions were also inaccurate.

Water Leaving Group

Similar to halide leaving groups, students’ descriptions of water leaving group were incomplete and lacked conceptual sophistication. When explaining why water is a good leaving group, students focused on two ideas. First, students reported that, once water leaves, it is an uncharged chemical species and, therefore, a stable compound in comparison to the original oxonium ion in the previous step (Figure 4):

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DISCUSSION AND CONCLUSIONS

Participants in this sample of Organic Chemistry II students demonstrated very limited understanding regarding the concept of leaving groups, with only 2 of 31 students providing scientifically sound explanations of what factors suggest a good leaving group. An analysis of students’ responses showed that more than half of study participants provided incomplete responses and failed to reason with the most appropriate explanations regarding the factors that are associated with leaving group capacity. Although these students were able to recall some characteristics of good leaving group ability, they failed to provide substantive explanations. Students invoked their understanding of the “octet rule” as an indicator of stability to explain leaving groups, consistent with previous research that students associate acquisition of an octet with stability.45 Many of these students used the concept of electronegativity as the primary, or only, factor to explain why halides are good leaving groups. Song and Carheden had previously reported that students used the term “electronegativity” to explain another chemistry concept (in this case, the concept of “polar”) but, likewise, could not explain the meaning of electronegativity.46

Figure 4. Oleg’s annotation of Reaction 4 (green ink) to explain how water becomes a leaving group. (Note: The gray lines reflect student annotations later in the interview in response to other questions.)

“...that [isopropanol] grabs onto that [H+] and then it forms water, which can be a pretty good leaving group, because it’s pretty stable itself because it’s an uncharged species.” (Oleg, second-year chemical engineering major) Other students focused on the idea that as water leaves it “gains an octet” which makes it a more stable chemical species than the oxonium ion in the previous step. For example, Daria (second-year biology major) suggested that water is a good leaving group because it gains an octet when it leaves; she further went on to explain that the hydroxyl group is not a good leaving group because it would not have an octet when it leaves: E

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about this concept. Future research is warranted to explore and describe the specific heuristics that students might be using to make inferences and assertions about leaving groups.

While electronegativity can be used to draw inferences about some leaving groups, it is not the most appropriate criterion when discussing leaving group ability of leaving groups of very different sizes, as in the case with halides. These students did not demonstrate the ability to identify nor reason with multiple constructs (charge, size) and their relative importance, suggesting they may not be capable of multiplistic thinking, let alone relativistic thinking in the Perry scheme. To the contrary, the responses to the interviews conducted in this research suggest that students who invoked the octet “rule” as connected to leaving group ability have fragmented knowledge structures rather than meaningful learning of these chemistry concepts. More than 30% of the students in this sample were limited to simply making assertions (correct or incorrect) about leaving groups, but they were unable to offer any explanation when asked to justify their responses beyond claiming the statement was what they had been taught. This suggests that these students had not made meaningful connections between what they knew and what they were expected to learn, but rather that they engaged in a rote memorization approach to learning.24,25 Furthermore, these students not only memorized these assertions as facts, but also talked about these ideas in absolute terms (i.e., “OH is a bad leaving group”, “water is a good leaving group”, “Br and Cl [are] the good leaving groups”). They did not discuss the relative nature of leaving group ability, e.g., the continuum from extremely poor leaving groups (very strong bases) to very good leaving groups (very weak bases). This dichotomous “good and bad” mindset, namely, that leaving groups are either “good” or “bad”, is a characteristic hallmark of Perry’s dualistic thinking. These findings are consistent with previous research that suggests that students enrolled in organic chemistry are in the dualistic stage of their intellectual development.30

Implications for Teaching

Understanding the construct of a leaving group, let alone understanding the nature of comparisons among stronger and weaker leaving groups, requires meaningful learning of multiple concepts and reasoning with multiple concepts at once. Because good leaving groups are the weak conjugate bases of strong acids, an understanding of leaving group capacity requires students to first build a coherent conceptual framework of the concepts of acids/bases, pKa values as a measure of acid strength, and the skills to qualitatively inspect the structural composition of different compounds to predict relative acid/base strength. This qualitative inspection in and of itself requires an understanding of multiple subconcepts, namely, atomic size, electronegativity, resonance, induction, orbital hybridization, and others. The research literature contains multiple reports that students find it very challenging to qualitatively predict the relative acid strength of chemical compounds,16,17 and use heuristic reasoning rather than reasoning grounded in scientifically acceptable arguments.15 Therefore, when teaching these concepts, faculty must address students’ alternative conceptions and guide them toward reasoning skills that focus on relevant cues and weighting principles. That is to say, faculty must be explicit with students that there are multiple factors that might be considered and teach students how to examine the relative importance of these factors. Interestingly, despite instruction, when asked to explain what makes something a good/bad leaving group, only one student in this study spoke of leaving groups as weak conjugate bases of strong acids, suggesting a lack of meaningful connections between acid/base concepts and the construct of a leaving group in students’ mental models. Asking students to create concept maps could be a useful activity to determine what connections students perceive between concepts and for the students themselves to realize the fragmented nature of their understanding regarding the connections between these concepts.13,19 Instructors need to challenge students’ tendencies to engage in dualistic thinking (good vs bad leaving group, obeys the octet rule or does not) and provide experiences to help them to shift toward multiplistic (and eventually relativistic) perspectives. It has been suggested that progression in learners’ intellectual development occurs only when they are challenged to consider ideas and principles that are beyond the dualistic level at which they typically operate.21,30 Instruction on qualitative inspection ought to extend beyond the mere memorization of an acronym (ARIO) for the factors that predict leaving group ability and ought to include the conceptual elements of understanding the relationships between these factors and leaving group ability. For example, rather than just memorizing that the size of the atom that carries the negative charge in the conjugate base is an important factor, students need to understand the relationships between concepts of leaving group, atomic size, respective electron affinity, and the efficiency of the overlap of orbitals between the substrate and the leaving group. Additionally, faculty ought to explain the relationship between leaving group ability and the stabilization of the additional electron density in the transition state of the rate-determining step in substitution and elimination reactions. Learning opportunities ought to incorporate discussions and debates that revolve around contradicting ideas to force students to reason with multiple, relative factors and recognize the inadequacy of dualistic thinking.21 Assessments

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LIMITATIONS As students’ understandings of the concept of leaving groups were not the primary focus of this study, the interview protocol was not designed to investigate this in depth. For example, as was reported earlier, the reactions chosen for this study featured only four different leaving groups. The reactions overrepresented some leaving groups such as halides (e.g., six out of seven included a halide as the leaving group) and underrepresented others (e.g., only one reaction included water as the leaving group). All the findings reported herein are from the follow-up questions carefully asked by the interviewer in response to students’ incomplete descriptions of the leaving groups. An intentional in-depth study should be conducted to further ascertain students’ reasoning when discussing leaving groups and the processes associated with the loss of a leaving group. Additionally, a broader range of leaving groups should be used in interview prompts to ensure the elicitation of a wider range of students’ ideas.

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IMPLICATIONS

Implications for Research

Previous research on decision-making about acid strength identified that students employ short-cut reasoning procedures rather than analytical reasoning.15 Because leaving groups and acid/base concepts are related, it is possible that students also employ similar short-cut strategies when thinking about leaving group ability. Thus, the development of effective instructional strategies and interventions that foster meaningful learning about the concept of leaving groups should be based on research that would characterize students’ reasoning strategies when thinking F

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ought to require students to propose explanations to support their claims. Students who are assessed solely by multiple choice tests may have their dualistic thinking reinforced by the impression that there is a single right answer to any question in organic chemistry. Assessments need to require students to answer openended questions and to provide written explanations to justify their answers.21

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stacey Lowery Bretz: 0000-0001-5503-8987 Notes

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 declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Volwiler Family Endowment to the Miami University Department of Chemistry and Biochemistry and by Grant 1432466 from the National Science Foundation. The authors thank the students and instructors of the Organic Chemistry II courses. The authors also thank R. H. McKinney for assistance with the graphical abstract.

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DOI: 10.1021/acs.jchemed.8b00198 J. Chem. Educ. XXXX, XXX, XXX−XXX