Mechanistic Reasoning Using the Electron-Pushing Formalism

Article pubs.acs.org/jchemeduc

From Source to Sink: Mechanistic Reasoning Using the Electron-Pushing Formalism Gautam Bhattacharyya* Department of Chemistry, Clemson University, Clemson, South Carolina 29634 United States S Supporting Information *

ABSTRACT: Since the introduction of Morrison and Boyd’s textbook in organic chemistry over 50 years ago, reaction mechanisms and mechanistic reasoning using the electron-pushing formalism (EPF) have become a mainstay of organic chemistry courses. In recent years there have even been several papers in this Journal and others detailing research on how students attend to various aspects of this formalism. However, there are no explicit articulations of a definition or framework in the chemical or science education research literature on mechanistic reasoning using EPF. Although practicing chemists intuitively know what constitutes mechanistic reasoning, this paper presents results of a nationwide study of organic chemistry faculty regarding their understanding and use of this technique. Although a consensus definition did not emerge from this research, there were several common features to them. These features suggest an activity that has a back-of-the-envelope quality meant to generate possible pathways based on established patterns of reactivity. Consistent with this view, the experts’ focus for skills required to develop dexterity in this type of mechanistic reasoning was on applied ones rather than those that are more theoretical in nature. Finally, the principal uses of mechanistic reasoning using EPF, according to the respondents, are to explain and predict outcomes of chemical processes. KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Graduate Education/Research, Organic Chemistry, Chemical Education Research, Mechanisms of Reactions FEATURE: Chemical Education Research

S

diagram. It is this nonempirical approach to proposing reaction mechanisms and their subsequent use that is the focus of this paper. The skill of proposing mechanisms of reactions using the electron-pushing formalism (EPF)or merely, electron-pushing not only is of value to organic chemists but also is important for students enrolled in organic chemistry courses at all levels. For example, Straumanis and Ruder argue that electron-pushing is the principal means by which students can develop their conceptual understanding.5 As such, electron-pushing may be the single most important technique taught to students in second-year-level organic chemistry courses. In the past decade, we and others have studied many aspects of mechanistic problem-solving in organic chemistry.4,6−11 Over the course of our work,4,6,7 we noticed that neither “mechanistic reasoning” nor “mechanistic thinking” are explicitly defined in the chemical education research literature. A recent search of the chemical education literature yielded a small handful of papers that even contain either of those phrases,10−14 and of those only 310−12 are relevant to the topic of this paper. Additionally, Russ et al. noted that mechanism and, more specifically, mechanistic reasoning have received inadequate attention from the broader education and psychology research literatures.15,16 They further note that the characterizations that do exist in those literatures are too vague to be of practical use to researchers and practitioners. Although practicing chemists intuitively know what constitutes mechanistic reasoning, an explicit articulation is important for

ince the introduction of Morrison and Boyd’s textbook in organic chemistry over 50 years ago, reaction mechanisms have become a mainstay of organic chemistry courses.1−3 In this context, reaction mechanisms are most frequently represented by the electron-pushing, or arrow-pushing, formalism, in which curved arrows are used to show the movement of electrons from source to sink. An example of an electron-pushing diagram of a mechanism is shown in Figure 1.4 (Although the terms

Figure 1. Sample electron-pushing diagram. Note that the curved, double-headed arrows denote the movement of an electron pair from source to sink. Single-headed arrows, in contrast, reflect the movement of single electrons. B represents a generic base.

“electron-pushing” and “arrow-pushing” may have separate connotations for some chemists, they will be used interchangeably in this paper.) Reaction mechanisms are empirically determined from extensive and careful kinetic studies of reactions. As such, the process of establishing a reaction’s mechanism tends to be laborintensive and time-consuming. Seeing that it is not practicable to experimentally establish a mechanism for every single reaction, it is common practice among organic chemistsespecially for those specializing in synthesis and methodologyto propose a reaction’s mechanism in the form of an electron-pushing © 2013 American Chemical Society and Division of Chemical Education, Inc.

Published: September 16, 2013 1282

dx.doi.org/10.1021/ed300765k | J. Chem. Educ. 2013, 90, 1282−1289

Journal of Chemical Education

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multiple educational purposes, especially assessment in which such a framework would be a key component of establishing construct validity. As a first attempt at filling this void, this paper presents results of a nationwide study of organic chemistry faculty regarding their understanding and use of mechanistic reasoning using EPF. This research was developed in three phases, each of which is described in more detail in the following pages.

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chemistry, but can occasionally venture into more exotic hypotheses. 3. Understanding as much as possible about what happens between A and B in the reaction A → B. Understanding the distinctions between thermodynamic and kinetic control in their many manifestations. 4. Mechanistic reasoning in organic chemistry is the predictive/deductive process for predicting/interpreting the results of molecular transformations that occur by electron redistribution during reactions. 5. Thinking about reactions in terms of the atom−ion− molecule collisions that occur step-by-step taking the starting materials to the products of a reaction. 6. The cognitive and psychomotor skills employed that allow one to manipulate representations (constructing these representations is assumed to be a prerequisite) of electrons within and between molecules, while conforming to a set of defined parameters. 7. Using the knowledge of reaction mechanisms to make predictions about reaction outcomes. The three remaining respondents wrote that they were unable to articulate concise definitions that would contain all the important components of mechanistic reasoning. For the most part, these definitions emphasize the hypothetical nature of this type of mechanistic reasoning as well as the explanatory and predictive roles of electron-pushing mechanisms. Another characteristic of the experts’ definitions is that EPF mechanisms are meant to show the transformation in a stepwise fashion, which is in stark contrast to what others have reported that students in introductory organic chemistry courses will do: show the entire transformation in a single step.8,9 For the other two questions, a simple frequency analysis was conducted, in which items that appeared on five or more of the responses were included in the respective lists.17 According to the professors’ responses to the second question, the following tasks may be performed using mechanistic reasoning:

PHASE ONE

Methods

Although there are far more organic chemists working in industrial and corporate settings than in academia, they were not recruited for this study because generally they do not participate in the day-to-day education of undergraduate and graduate students. As such, the participants of this study were limited to faculty. Obtaining a sample reflecting the diversity of organic chemistry faculty would necessitate the use of a survey for nationwide administration. However, due to the dearth of research on mechanistic reasoning in the chemistry and science education literatures, I began with a qualitative study to generate the survey items. As such, 44 organic chemistry professors were contacted by e-mail with the following three questions: 1. What is your definition of “mechanistic reasoning using the electron-, or arrow-pushing, formalism”? 2. What types of tasks, problems, etc. can one solve using this type of mechanistic reasoning? 3. What skills does one require to develop proficiency in proposing and interpreting electron-, or arrow-pushing mechanisms? The questions were purposely broad so that the participants would not be cued to any specific topics or issues. Also, “electron-pushing” is emphasized in each question to remind the respondents that the study is limited to only this type of mechanistic reasoning. The professors contacted represent organic chemists I have met through a variety of professional interactions over the past decade. Of the 10 who replied, 5 were from large, researchoriented (Ph.D.-granting) institutions, 1 was from a comprehensive university (M.Sc.-granting), 2 were from primarily undergraduate institutions, and 2 were from community colleges. Although the 10 respondents represented institutions from all the regions of the continental United States, there was less diversity in experience: the average amount of experience as independently practicing organic chemists was ∼15 years with a range of 2 to 40+ years.

• Explaining the products of a reaction, especially unexpected ones or side-products • Explaining the products of a reaction, especially unexpected ones or side-products • Predicting the products of a reaction given the starting materials • Explaining stereo- and regiochemical outcomes of reactions • Choosing the appropriate experimental conditions including stoichiometry, solvent, and temperaturefor a reaction Again, note that these EPF mechanisms are intended to help explain or predict (potential) outcomes of reactions. Our previous work has shown that even students who are beginning graduate study in organic chemistry may not be aware of these roles of EPF mechanisms.6 At first glance, the experts’ responses to the last question (regarding the skills required to develop proficiency in mechanistic reasoning using EPF) appeared to be, proverbially, “all over the place”. However, after further inspection, the characteristics cited by the experts, while numerous, were either content knowledge or cognitive skillseither reasoning or representationalas shown in Table 1. To help establish reliability of the results,17 a second rater with graduate degrees in organic chemistry and chemical

Phase One Results

To analyze the data, responses were first divided by question. Respondents offered the following definitions for mechanistic reasoning using EPF: 1. Mechanistic reasoning involves explicit consideration of the movement of electrons and atoms, and the formation of intermediate structures, in explaining the outcome of a chemical reaction. 2. A mechanistic “working hypothesis” that allows one to rationalize (or predict) the outcome of a given transformation based on the tradition of “arrow-pushing” (representing the shifting of single electrons or e.l.p. [electron lone pairs]). This is mostly based on our established body of knowledge of mechanistic organic 1283



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Table 1. Experts’ Determination of Skills Needed for Proficiency in Mechanistic Reasoning Using EPR Cognitive Skills Content Knowledge

Reasoning Skills

Bond strengths Molecular orbital theory Valence

Creativity Problem-solving abilities Pattern recognition

Electronegativity

Ability to create and follow a multifaceted logical argument

Bond polarity, especially to determine solvent polarity, and areas of high and low electron density Stereochemistry Sterics Counting and accounting for electrons Reaction kinetics Reaction thermodynamics Brønsted acidity/pKa Identification of functional groups and classification of (parts) of functional groups as electron donors or acceptors Identification of reagents and classification of them as electron donors or acceptors Knowledge of basic reactions and their mechanisms

Ability to create 3D drawings Handle/interpret/understand the visual representation being used Understanding of the meaning and implications of arrows: curved (single- and double-headed), equilibrium, and resonance Being able to draw the structures of resonance hybrids and transition states for unfamiliar structures/reactions Develop an understanding of the visual symbolism with which these external representations are conveyed. Visualize reactions on the molecular level

Finally, as the initial data were taken from a small participant pool, an “Other” option was added to allow participants to share definitions and ideas not adequately expressed in the first five choices. Cooper and colleagues recently demonstrated that providing survey respondents an option to add a free response is one way to ensure content validity of a construct.19 With the exception of the cognitive reasoning skills, all of the items in Table 1 were incorporated into the subsequent pages of the survey as Likert-type questions using a 5-point scale ranging from very important (1) to very unimportant (5). Content knowledge items were separated from those on visualization skills to emphasize the change in focus and to also prevent loss of interest due to the extensive list of skills in Table 1. Items that had more than one skill listed, such as “counting and accounting for electrons”, were separated into multiple survey items to reduce potential ambiguity in interpreting the results of the survey. Valence bond theory and VSEPR were added to this part of the survey to include two of the other models of bonding typically taught at the undergraduate level. Finally, the participants’ responses to the question regarding types of tasks that may be solved using mechanistic reasoning were included as part of a separate question, which also used a 5-point scale that ranged from very important (1) to very unimportant (5). The cognitive reasoning skills were excluded because it would have been very difficult, if at all possible, to establish clear and concise meanings for items such as creativity, which are large constructs in and of themselves.

education reviewed the anonymized faculty responses. The inter-rater reliability for the last two questions was greater than 95%. To establish a measure of validity, member-checking was done by e-mailing the results of the analysis to all of the respondents.17 None of the respondents disagreed with any of the analysis. Rather, several made comments such as, “I had thought of [item name] after sending you my responses” or “I should have included [item name] in my list”.

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Representational Skills

PHASE TWO

Survey Development

The participants’ responses collected in phase one were subsequently used to build the survey on the Web site, Obsurvey.18 The first page of the survey contained the participant informational letter as mandated and approved by the Internal Review Board (IRB) of Clemson University. (The entire survey is available in the Supporting Information as Appendix A.) This letter included the purpose of the study, a solicitation to take the survey, anticipated risks to the participants, anticipated benefits for the participants, and the anticipated use of the outcomes. Those who consented were able to proceed to the survey in which the first question asked the respondents to select their choice for the best definition for mechanistic reasoning using EPF. Definitions 1−4 from Phase One Results were used almost verbatim; the only changes made were grammatical or syntactical in nature. Compared to the others, definitions 5 and 7 were terse and were not as comprehensive. Definition 6 was excluded because it had jargon, such as “cognitive and psychomotor skills”, not usually used by practicing organic chemists. In their place, a fifth definition was added by extracting the essence of the other definitions. Components such as the explanatory role and tentative nature of EPF mechanisms were combined in the following articulation of a new definition 5: The representation of the movement of electrons and atoms to demonstrate the stepwise transformation of a set of reactants into the products of a chemical process. The resulting mechanisms are “working hypotheses” based on established paradigms of chemical reactivity.

Survey Administration

A list of organic chemistry faculty at all U.S. colleges and universities (primarily undergraduate institutions, Mastersgranting, and Ph.D.-granting) was compiled with the help of two colleagues. A total of 2540 professors were identified. Twoyear college faculty were excluded because in a large number of instances neither the chemistry faculty members’ names nor their subdisciplines were explicitly identified. For this phase, about half the group, 1200 professors nationwide, were selected randomly and sent a recruitment letter containing the URL of the survey by e-mail. All of those who responded to the e-mail survey in phase one were excluded from the participant pool. Participants were given four weeks to respond, with reminders 1284



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Table 2. Survey Choices for the Definition of Mechanistic Reasoning Using EPF during Phase Two of the Study Option

Numbera

Definitionb

1

8

2

29

3

0

4

44

5

9

6

13

Explicit consideration of the movement of electrons and atoms, and the formation of intermediate structures, in explaining the outcome of a chemical reaction. A working hypothesis that allows one to rationalize or predict the outcome of a given transformation by representing the shifting of single electrons or electron lone pairs. This is mostly based on our established body of knowledge of mechanistic organic chemistry, but can occasionally venture into more exotic hypotheses. Understanding as much as possible about what happens between A and B in the reaction A → B, including the distinctions between thermodynamic and kinetic control in their many manifestations. The representation of the movement of electrons and atoms to demonstrate the stepwise transformation of a set of reactants into the products of a chemical process. The resulting mechanisms are “working hypotheses” based on established paradigms of chemical reactivity. The predictive/deductive process for predicting/interpreting the results of molecular transformations that occur by electron redistribution during reactions. Other

a

Number indicates the number of individuals choosing that definition out of a total of 103 participants. bOption 4 is the hybrid of several of the responses from phase one. Those who chose “Other” were asked to enter a free-response comment.

sent at the beginning of weeks 2 and 3. A total of 111 professors responded, and 103 of those respondents completed the survey, for a response rate of 8.6%.

pushing is the idea that each step in a mechanism has to be a reasonable step. 6. EPAs [electron-pushing arrows] show the change in disposition of electrons as bonds are formed and broken during a chemical reaction. To the greatest extent possible, EPAs conform to patterns established by known mechanisms and reflect an understanding of partial or formal charges that may exist among the reactants and intermediates. None of the averages for the items in the remaining three categoriescontent knowledge, visualization and representational skills, and uses of EPF mechanismswere higher than 3; that is, at worst, respondents, on average, felt neutral about them. A total of 25 comments were left by those who used the “Other” in the remaining three parts of the survey. Most of these comments were interesting insights from professors’ experiences in teaching mechanisms and organic chemistry. However, in the content knowledge section, multiple respondents noted the absence of Lewis acid−base theory, formal charge, principle of microscopic reversibility, recognition of nucleophiles and electrophiles, and recognition of reaction conditions as acidic−basic, oxidative−reductive, and so on. Several participants also noted the absence of stereochemistry in the visualization and representation section and recommended splitting up explaining regio- and stereochemistry in the section on tasks for which mechanistic reasoning can be used.

Phase Two Results

Table 2 contains the choices participants selected for the definition of mechanistic reasoning using EPF, and Figure 2

Figure 2. Distribution of professors’ preferences during phase two of the study for the definition of mechanistic reasoning using EPF. Note that Option 6 is “Other”.

shows the distribution of the participants’ responses. Options 2 and 4 were the two top choices, while 1, 3, and 5 were largely passed over: a greater number of respondents chose the “Other” option over definitions 1 and 5, and no one opted for 3. Of the 13 individuals who chose the “Other” option, 7 either wrote “all of the above” or “none of the above” along with further commentary. The remaining 6 offered the following alternatives: 1. The fate of electron pairsthose migrating from one bond to another, those coming from a lone pair and becoming a bond, and those coming from a bond and becoming a lone pair. 2. The use of Lewis structures and arrows to show electron motion to rationalize hypothetical, lowest-energy pathways between stationary states in chemical reactions. 3. Electron accounting for converted steps in a reaction mechanism, based on the principles of quantum topology. 4. Electron pushing is a formal way to represent breaking old bonds and making new bonds by recognizing the tendency of nucleophilic centers seek electrophilic centers. 5. Electron pushing defines clearly the changes in bonding that occur in each step in a mechanism, and the practice of electron pushing teaches what constitutes reasonable mechanistic steps. Mechanistic reasoning using electron

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PHASE THREE

Survey Revision

Due to the low frequency with which they were chosen, definitions 1, 3, and 5 were discarded for the revised survey. They were replaced with participants’ definitions 2, 5, and 6 from the Phase Two Results section. The others were not used because of their being either limited in scope or based on constructs not common to organic chemistry instruction in the United States. None of the items in the remainder of the original survey were discarded, as all of the averages were on the positive side of the Likert scale. However, the items suggested by multiple respondents were incorporated into the revised survey. These were • Lewis acid−base theory, formal charge, principle of microscopic reversibility, recognition of nucleophiles and electrophiles, and recognition of reaction conditions as 1285



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Table 3. Survey Choices for the Definition of Mechanistic Reasoning Using EPF during Phase Three of the Study Option

Numbera

Definitionb

1

28

2

47

3

51

4

12

5

32

A working hypothesis that allows one to rationalize or predict the outcome of a given transformation by representing the shifting of single electrons or electron lone pairs. This is mostly based on our established body of knowledge of mechanistic organic chemistry, but can occasionally venture into more exotic hypotheses. The representation of the movement of electrons and atoms to demonstrate the stepwise transformation of a set of reactants into the products of a chemical process. The resulting mechanisms are “working hypotheses” based on established paradigms of chemical reactivity. Electron-pushing arrows (EPAs) show the change in disposition of electrons as bonds are formed and broken during a chemical reaction. To the greatest extent possible, EPAs conform to patterns established by known mechanisms and reflect an understanding of partial or formal charges that may exist among the reactants and intermediates. The use of Lewis structures and arrows to show electron motion to rationalize hypothetical, lowest energy pathways between stationary states in chemical reactions. Electron pushing defines clearly the changes in bonding that occur in each step in a mechanism, and the practice of electron pushing teaches what constitutes reasonable mechanistic steps. Mechanistic reasoning using electron pushing is the idea that each step in a mechanism has to be a reasonable step.

a

Number indicates the number of individuals choosing that definition out of a total of 170 participants. bOnly options 1 and 2 were retained from the original survey.

This choice was also necessary because we did not have any evidence to suggest regular intervals between the response choices of “very important”, “important”, and so forth; that is, the survey yielded ordinal data. For these tests, the null hypothesis was that there is no difference in the item medians of the five definition groups, while the alternative hypothesis was that at least one of the medians is different from the others. Except for “visualize reactions at the molecular level” (H = 13.53, df = 4, p = 0.009), the null hypotheses could not be rejected for any of the other items. (The full results of the Kruskal−Wallis tests are presented in the Supporting Information, Appendix C.) Post hoc pairwise comparisons using Mann−Whitney U tests with the Bonferroni correction for α (p ≤ 0.005) yielded only two pairings whose difference in distribution medians was statistically significant.21,22 (The post hoc Mann−Whitney analysis is presented in the Supporting Information, Appendix D.) Both pairs, however, had effect sizes around 0.3, suggesting that the differences were not very meaningful. Parametric ANOVA analysis of the data yielded similar findings regarding the means. With the exception of two items, “recognition of reaction conditions as acidic/basic/reductive/ oxidative/radical-promoting” [F(4, 165) = 2.47, p = 0.047] and “visualize reactions at a molecular level” [F(4, 163) = 3.86, p = 0.0050], the null hypothesis that all the means are equal could not be rejected for any of the remaining survey items. (The full results of the ANOVA analysis are tabulated in the Supporting Information, Appendix E.) Post hoc pairwise comparisons using Games−Howell tests21,23 showed only three pairings whose difference in means was statistically significant. The small effect sizes of 0.3 in all three cases, however, suggest that the difference, while statistically significant, was not really meaningful. (The results of the post hoc tests may be found in the Supporting Information, Appendix F.)

acidic−basic, oxidative−reductive, etc. in the content knowledge section • Stereochemistry in the section on visualization and representation • Separation of regio- and stereochemistry in the task section Additionally, the “Other” option was removed from all of the sections of the revised survey, which may be seen in its entirety in the Supporting Information, Appendix B. Phase Three Survey Administration

The remaining 1340 professors from the previously generated list were contacted by e-mail with the same recruitment letter containing a new URL of the revised survey. Once again, participants were given four weeks to respond, with reminders sent at the beginning of weeks 2 and 3. A total of 188 professors responded and 170 of those respondents completed the survey in its entirety, for a response rate of 12.7%. Data Analysis

Table 3 and Figure 3 show the definitions and the respondents’ distribution of choices, respectively. None of the options received

Figure 3. Distribution of professors’ preferences during phase three of the study for the definition of mechanistic reasoning using EPF. None of the definitions garnered a consensus among the respondents.

Phase Three Results

Because the choice of definition did not appear to affect a participant’s choices in the remainder of the survey, the data were not subdivided by definition choice for further analysis. The means, standard deviations, and medians for each item in both versions of the survey are presented in Table 4. Note that lower means imply greater importance of the item. Several conclusions can be readily drawn from a scan of the data in Table 4. First, the highest average and median for any item was 3, which suggests that participants valued all of the items on the survey to a lesser or greater extent. Second, the closeness between the means and medians suggests symmetrical distribution about the mean. Third, with the exception of the

more than 50% of the tally. Even though definitions 2 and 3 were the top choices, it is clear that a significant proportion of the sample preferred 1 or 5. Without a consensus definition, the next step in the data analysis was to determine whether choice of definition affected the participants’ responses to any of the other items in the survey. Using definition choice as the independent variable (IV), one-way analysis of variance (ANOVA) was performed at the p < 0.05 level for each of the remaining 37 items on the survey. As none of the dependent variables (DVs) were distributed normally, the nonparametric Kruskal−Wallis test was chosen.20,21 1286



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Table 4. Results of Original and Revised Survey by Item Survey 1 Scoresa Survey Items (Dependent Variable) Content knowledge Valence Electronegativity MO theory Valence bond theory VSEPR Drawing Lewis structures Bond polarity Bond strength Accounting for electrons Counting electrons Formal charge Determining high/low electron density Kinetics Thermodynamics Principle of microscopic reversibility Brønsted−Lowry theory; pKa Lewis acid−base theory Recognition of nucleophiles and electrophiles Functional group identification Classification of functional groups Classification of reagents Knowledge of reactions a

Mean

SD

Median Mean

1.2 1.4 2.6 2.1 2.8 1.4 1.6 2.3 1.1 1.3

0.5 0.5 1.0 0.9 1.0 0.8 0.7 0.9 0.4 0.6

1 1 3 2 3 1 2 2 1 1

1.8

0.9

2

2.9 2.6

1.1 1.0

3 2

1.8

0.8

Survey 2 Scoresb

2

SD

Survey 1 Scoresa Survey Items (Dependent Variable)

Median

1.3 1.4 2.9 2.1 2.4 1.3 1.4 2.4 1.2 1.4 1.4 1.8

0.6 0.6 0.9 0.9 1.0 0.6 0.6 0.8 0.4 0.7 0.6 0.9

1 1 3 2 2 1 1 2 1 1 1 2

3.0 2.7 2.6

1.0 1.0 0.9

3 2 2

1.8

0.8

2

1.6 1.1

0.7 0.3

1 1

1.8

0.9

1

2.0

1.0

2

1.5

0.7

1

1.5

0.6

1

1.4 2.0

0.8 0.8

1 2

1.6 2.2

0.6 0.7

1 2

Knowledge of basic mechanisms Recognition of reaction conditions Representational and visualization skills Create 3D drawings of molecules Manipulate 3D drawings of molecules Interpret 3D drawings of molecules Differentiate types of arrows Draw resonance hybrids Draw transition states Visualize reactions at the molecular level Stereochemistry Uses of EPF Explain products of reactions Predict products of reactions Explain regio- and stereochemistry Explain stereochemical outcomes Explain regiochemical outcomes Determine experimental parameters

Mean

SD

1.5

0.6

Survey 2 Scoresb

Median Mean 1

SD

Median

1.5

0.6

1

1.6

0.7

2

2.1

0.8

2

2.3

0.8

2

2.2

0.8

2

2.4

0.9

2

2.0

0.8

2

2.1

0.8

2

1.4

0.6

1

1.5

0.7

1

1.4 2.1 1.8

0.5 0.9 0.8

1 2 2

1.4 2.2 1.8

0.6 0.8 0.8

1 2 2

1.7

0.7

2

1.4

0.6

1

1.4

0.6

1

1.3

0.5

1

1.4

0.6

1

1.4

0.7

1 1.4

0.6

1

1.4

0.5

1

2.4

0.8

2

2.6

1.0

3

N = 103 for the phase two survey. bN = 170 for the phase three survey.

greater than 2.5: molecular orbital theory, kinetics, thermodynamics, and the principle of microscopic reversibility. Ironically, kinetics received the highest average of all the items on a survey on mechanistic reasoning. Conversely, more utilitarian concepts, such as valence, electronegativity, bond polarity, and so on, had averages lower than 1.5. Thus, all the “hard core” theoretical constructs and the most sophisticated model of bonding, MO theory, were deemed far less important for mechanistic reasoning using EPF than those that were more directly applicable, such as drawing Lewis structures. This distinction is, perhaps, one way in which practicing organic chemists in academia differentiate this form of mechanistic reasoning from its more rigorous counterpart involving the interpretation of experimentally obtained kinetic data. This theme of assigning greater weight to operational constructs is also seen in the latter half of the section on content knowledge. For example, identification of functional groups was rated far higher than classification of functional groups as nucleophiles and electrophiles. Similarly, knowledge of basic reactions received a higher score than knowledge of basic mechanisms. Based on these differences, it would appear that professors are more concerned with students’ abilities to understand and apply their knowledge rather than merely knowing facts. Finally, it is also important to note that the average score of 1.5, which falls between “very important” and “important” on the survey’s scale, for knowledge of basic mechanisms is consistent with the respondents’ preference for definitions of mechanistic reasoning using EPF that emphasized their connection to the body of established mechanisms.

principle of microscopic reversibility, all of the new items in the revised survey had highly favorable scores. Fourth, comparing the data between the original (Survey 1) and revised (Survey 2) surveys shows remarkable consistency across all of the items common to the two instruments. This result, along with using participant feedback from the original survey lends validity to the content of the proposed framework. Strictly speaking, the results should focus on the medians and nonparametric analysis. However, due to the congruence between the nonparametric and parametric data, I will focus on the latter for ease of discussion.24 Although this study failed to establish a consensus definition of mechanistic reasoning using EPF, several factors do seem to be common to the participants’ descriptions. Extracting these ideas from the various renditions suggests that this type of mechanistic reasoning: • Relates the stepwise reorganization/redistribution of electrons during a chemical process • Arises from an established body of knowledge in chemical reactions and reactivity • Results in working hypotheses that can be used to rationalize, explain, and predict the outcomes of chemical processes The recognition of EPF mechanisms as “working hypotheses” is particularly important because it emphasizes their “back-of-the-envelope” nature. This inference is supported by considering the content knowledge items that were less important to the faculty, that is, those whose average was 1287



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Table 5. Key Characteristics of Mechanistic Reasoning by EPF Mechanistic Reasoning by EPF Relates the stepwise reorganization/redistribution of electrons during a chemical process Arises from an established body of knowledge in chemical reactions and reactivity Results in working hypotheses that can be used to rationalize, explain, and predict the outcomes of chemical processes

Content Knowledge Valence

Representational/ Visualization Skills Differentiate between different types of arrows Draw resonance structures and hybrids

Electronegativity Drawing Lewis structures Bond polarity Accounting for electrons Counting electrons Formal charge Lewis acid−base theory Recognition of nucleophiles and electrophiles Classification of functional groups as nucleophiles and electrophiles Classification of reagents as nucleophiles and electrophiles Knowledge of basic reaction mechanisms

Tasks Using EPF Explain products of reactions Predict products of reactions Explain stereochemical outcomes of reactions Explain regiochemical outcomes of reactions

an apparent paradox regarding the teaching of organic chemistry. Straumanis and Ruder5 and Grove, Cooper, and colleagues10,11 note the importance and ubiquity of the EPF and mechanistic reasoning in the teaching and learning of organic chemistry. However, based on the survey results, there are at least 10 items of content knowledgeone of them being knowledge of basic mechanismsin Table 5 that are highly important before one can effectively engage in this type of mechanistic reasoning. As such, how can instructors expect students to engage in mechanistic reasoning before learning any mechanisms? As a corollary, at what point then should mechanistic reasoning using EPF be expected of students enrolled in the yearlong, secondyear-level organic chemistry sequence? Should this topic even be taught in one-semester, introductory-level organic chemistry courses? It is these and many other questions raised by this research that will have to be resolved as instructors continue to pursue ways to better deliver organic chemistry courses.

In the section regarding representation and visualization skills, once again, more operational constructs such as drawing resonance structures and hybrids and differentiating between types of arrows were looked upon more favorably by the respondents, receiving average scores of 1.5 and 1.4, respectively. From the results from the final section of the survey, it is clear that professors value EPF mechanisms as primarily explanatory and predictive tools, and find less importance in their role as determinants of experimental conditions. It is quite important, therefore, that these roles are made clear for students, as previous research has shown that even first-year graduate students can be unaware of the uses of EPF mechanisms.6

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CONCLUSIONS This paper presents results of a nationwide survey of organic chemistry professors regarding their understanding and use of mechanistic reasoning using EPF. Although a consensus definition did not emerge from this study, there appear to be a set of key characteristics that are consistent with the various viewpoints offered by the study’s participants. These features are highlighted in Table 5 along with content knowledge, visualization and representational skills, and uses of EPF mechanisms that were deemed by the respondents to be the most important (i.e., an average score of 1.5 or below, which falls between “very important” and “important” on the survey’s scale, or median of 1). The definitions suggest an activity that is less formal than the mechanistic reasoning that is a product of interpreting kinetic data. Commensurate with this back-of-theenvelope quality of the process is the focus on applied skills rather than those that are more theoretical in nature. Note the emphasis on nucleophilicity and electrophilicity, even more so than pKa, which does not appear on this list. This absence may be surprising to some given the importance ostensibly given to Brønsted−Lowrey theory in regard to all of organic chemistry, especially electron-pushing. Finally, the primary use of this type of mechanistic reasoning according to the respondents is an explanatory one, with prediction as a concomitant objective. In attempting to set a foundation for a framework on mechanistic reasoning using EPF, the intent of this research is descriptive rather than prescriptive. The results, however, pose

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ASSOCIATED CONTENT

S Supporting Information *

Original and revised survey versions; detailed results of statistical analyses. This material is available via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS I would like to express my heartfelt gratitude to all of the faculty members nationwide who took time away from their busy schedules to participate in the different phases of this study. Jacob Schroeder served as a second rater in phase one. He also helped with the compilation of the list of faculty, as did Yu Shen. Thanks to Gauri S. Datta, who helped with the statistical analysis presented in this paper. Finally, I would like to thank the anonymous reviewers of the previous versions of this manuscript for their time and insights. 1288



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Mechanistic Reasoning Using the Electron-Pushing Formalism

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