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, what components of mental models do students use to guide their reasoning?
110 THEORETICAL FRAMEWORK This study is grounded in the models and modeling framework as described by Briggs for use in
chemistry education.19-20 Models are defined as mental representations of processes or systems that
allow for scientific reasoning. The development of models requires experiences with related phenomena
and models are used to make sense of the structure or behavior of a system. This framework aligns
115 with the understanding of mechanisms in organic chemistry, as students are exposed to mechanisms
throughout the organic curriculum and encouraged to use mechanisms as a predictive or explanatory
tool for reaction outcomes. Students gather experience from mechanisms learned in various situations
in their coursework (e.g., lecture, homework, studying, etc.) and have the opportunity to develop
mental models of mechanisms, which they can then apply to make sense of the behavior of unfamiliar
120 reaction mechanisms using fundamental chemical reasoning.
The models and modeling framework theorizes models to be made up of five components: (1)
referents, (2) relationships, (3) rules/syntax, (4) results, and (5) operations.21 The first four of these are
static ways to make sense of system components that build upon one another. Referents are the
symbols used to represent the components of a system, and they have a certain set of relationships to
125 other referents. Rules, or concepts, are the meaningful ways in which referents relate to one another
and syntax is how the rules may be utilized in a specific context. Results are the changed referents
after operations, which are the actions performed by the referents. Essentially, referents have specified
relationships and interact with one another through operations to produce results, all constrained by
rules and syntax.
130 In the context of addition reactions, referents are the atoms and molecules involved—e.g., a
molecule of hydronium and an alkene. These molecules are related to each other in that hydronium is
an acid and the alkene is nucleophilic. The rules and syntax of acid-base chemistry dictate the first
operation of the reaction between the two molecules: that the acid will protonate the alkene double
bond. However, a decision must be made to determine which carbon of the double bond will be
135 protonated. This decision is guided by additional rules for alkene addition reactions: namely, that
carbocation stability as determined by degree of substitution or, when applicable, resonance, will
influence which carbon is protonated. Depending on this decision, the result will be a molecule of
water and the protonated alkene with the most stabilized of the two possible carbocation
intermediates. This result now has a new set of properties—i.e., the carbocation is positively charged
140 and electrophilic—and a new relationship to the nucleophilic water molecule in the system. The rules
of how nucleophiles and electrophiles interact can then be used to make sense of the next step, or
operation, of the reaction to produce the next result.
When working through organic reaction mechanisms, students may use different representations
to model the mechanistic process. Most commonly, this takes the form of drawn Lewis or line-angle
145 structures to represent molecules and curved-arrows to represent the movement of electrons during
operations. Students can extract knowledge about the relationships between or within molecules from
the molecular representations of referents, and can use this knowledge when considering the pertinent
rules and syntax to predict a mechanistic step. Increasingly, there also exist applications which can
also support students working through organic structures and mechanisms.22-24 The app used in this
150 study, “Mechanisms”, may provide students with additional sources of knowledge about the progress
of the reaction. This may take the form of an operation depicted by a task card to students at the
beginning of a puzzle or a hint in the app directing students to consider how the concept of
carbocation stabilization through resonance applies syntactically to inform the operation they perform.
Students’ ability to use and extract information from models during the mechanism may impact their
155 success with mechanistic reasoning.
METHODS Context and Participant Recruitment This study was part of a larger effort that targeted student reasoning about acid-base and addition
reactions.18 Herein we present results pertaining to addition reactions. The study was conducted at a
160 large, research-intensive university in the Midwestern United States. Approval for data collection and
analysis was obtained from the Institutional Review Board (HUM00156602). Students were recruited
across three semesters from the first of a two-class introduction to organic chemistry shortly after
covering addition reactions in the course. The course content covered prior to the interviews included
a review of pertinent general chemistry content, the curved-arrow formalism, resonance, acid-base
165 reaction mechanisms, and strong and weak acid addition. In the first semester of data collection,
purposeful sampling was used to invite students to participate based on their performance on the first
exam.25 In the second and third semester, convenience sampling was used to recruit students via
course announcement to increase participation in the study.25 During recruitment students were told
that following the interview they could ask the interviewer any organic chemistry content questions,
170 and no additional incentives were provided. In total, thirteen students participated across the three
semesters and consented to participate in the study at the beginning of the interviews. Students were
divided into two groups, where six of the students worked through the problems using the
conventional paper-pencil style of problem solving and the other seven solved problems using the
“Mechanisms” app, identified herein as paper-pencil and app students respectively (pseudonyms are
175 given in Table 1).
Table 1. Student participants by think-aloud interview group type Reaction Participants modality groups Paper-pencil Ana, Aurora, Daisy, Francis, Mary, Perdita App Angela, Belle, Flynn, Jasmine, Pepper, Peter, Tiana
Description of app software The “Mechanisms” app contains a wide range of puzzles where students can work through organic
reaction mechanisms on a touch-screen device.23, 26 At the beginning of a puzzle, students are
180 presented with a task card that depicts the initial reactants, intermediate, or a step in the reaction
mechanism. Students can manipulate and move the molecules using the touch-screen, reveal or hide
implicit hydrogens and lone pairs by tapping on atoms, and break and form bonds by moving bonds or
lone pairs from one atom to another. The app prevents students from making incorrect chemical
moves and when students form a bond that then requires breaking a bond the app will only let them
185 perform bond-breaking moves. Additionally, the app provides information about the reaction in the
form of goals students can access that describe the steps needed to complete the puzzle and hints that
appear in certain cases, such as if they have formed a minor product. The goals for the pertinent
reactions are given in Figure S1.
Problem Selection 190 To select problems for the study, the research team chose five reactions from the app reflective of
those presented in first-semester organic chemistry. We then performed cognitive interviews with three
organic chemistry instructors to validate and select two of the reactions. Of the instructors
interviewed, one was the instructor during the first semester of data collection and two had taught the
material previously. They all agreed that the selected problems mirrored the difficulty level expected of
195 students and provided feedback on the presentation of the reactions to the paper-pencil students.
The paper-pencil students were presented with the line-angle representation of the reactant
molecule along with the molecular formula of the product, with reagents depicted above the reaction
arrow. The reactions were presented this way to mirror the information given to the app students at
the beginning of the reaction. The molecular formula of the product was given to provide students with
200 an end-point to parallel the completion goals provided to the app students. The reactions as seen by
both groups of students are presented in Figures 1 and 2.
Think-aloud interviews Interviews were conducted in the think-aloud style where students verbalized their reasoning as
they worked through the reactions.27 Prior to each interview, students were given a practice think-
205 aloud to familiarize them with verbalizing their thinking. For both groups of students, we would ask
probing questions during the interviews to further elicit participants’ reasoning, such as why they
were making certain moves or what they were thinking about between actions. Examples of these
questions are, “Why did you choose to protonate that carbon?” and “What are you thinking about
now?” Both groups of students were provided with the pKa table used in the course. The order of the
210 reactions was varied for each participant.
To match the format of the app, paper-pencil students were asked two general questions at the
completion of each problem: “Are there any resonance structures for your product?” and “Why do you
think you are done?” The think-aloud interviews for the paper-pencil students were performed using a
LivescribeÔ pen and notebook to concurrently capture students’ drawing and verbalizations. The app
215 students went through a modified tutorial in the “Mechanisms” app to minimize any difficulties arising
from working through the reactions in a new modality. The tutorial showed students how to reveal
implicit hydrogens or lone pairs, how to make or break bonds, and how to manipulate the molecules.
Audiovisual recordings were captured for the app-students, so that we could identify the moves they
made as they worked through the reactions in the app as well as their verbalized thought processes.
220 Development and application of coding scheme After the interviews, the coding scheme was developed using open-coding and constant
comparative analysis.28 The research team read the transcripts individually, noting recurring themes.
The team met to discuss findings and began to develop a coding dictionary based on the features
identified by each member of the team. The coding dictionary is given in Table S2. To establish what
225 sections of the transcript should be coded, all transcripts were divided into units of analysis
corresponding to different stages in the mechanism: planning stages where students vocalized ideas
about electron movements and chemical reactivity, and operation stages where the student
manipulated bonds/electrons to advance the mechanism. Two members of the research team met and
agreed on the units before coding. Following this, two researchers independently coded the interviews
230 of both reactions for four participants (30% of the data), met to clarify code definitions and reached
consensus on application of the coding scheme. The same two researchers then coded the remaining
70% (nine participants). The fuzzy kappa for this data was initially 0.78, indicating substantial
agreement between the two coders.29-30 Furthermore, the two researchers discussed discrepancies in
the codes applied to each transcript and reached consensus agreement for the final application of the
235 codes.
RESULTS AND DISCUSSION In this study, students worked through two addition reaction mechanisms using either paper-
pencil or the app interface. As students depicted the mechanisms, they were prompted to describe
what they were thinking and doing in a think-aloud interview. The interviews were qualitatively
240 analyzed through inductive coding and then interpreted through the lens of the models and modeling
framework.19-20 The results are presented below, and common problematic reasoning students
exhibited across the levels of the models and modeling framework is presented in Table 2.
Table 2. Common student difficulties across modalities Problematic student Problem Level(s) in the models and thinking modeling framework Overgeneralization of Resonance-stabilized Rules substitution addition Focusing on resonance Resonance-stabilized Relationship in the reactant rather addition than the potential product Mixture of resonance Resonance-stabilized Relationship structures addition Inverting substitution Resonance-stabilized Syntax stabilization rule addition and acid- catalyzed addition Ignoring octet rule Resonance-stabilized Rules, relationship addition and acid- catalyzed addition Identifying acid- Acid-catalyzed Relationship, rules and syntax catalyst, but not addition knowing how to apply it
Acid-Catalyzed Addition
Figure 1. Reaction schemes for the acid-catalyzed addition reaction as presented during the think-aloud interviews to A) paper-pencil students and B) app students in the task card prior to beginning the reaction. 245
The acid-catalyzed addition reaction was a two-step addition of water to a carbon-carbon double
bond (Figure 1). For this reaction, students needed to recognize that the conjugate acid, hydronium,
was given to them as a pre-formed referent. They additionally needed to identify the most stable site
for carbocation formation by considering the rules and syntax related to carbocation stabilization due
250 to degree of substitution of the two double-bonded carbons. Following these decisions, students could
perform the associated operations to first form a carbocation intermediate and then form the final
hydroxyl product.
The students either explicitly identified the reaction type as acid-catalyzed before proceeding
through the mechanism (four of the app students and five of the paper-pencil students) or at some
255 point while working through the problem (three app students and one paper-pencil student).
Specifically, students recognized the hydronium as a key reactant, or referent, in the reaction. Belle
said:
I know that this reaction probably wants me to use the H3O+ and then with the hydrocarbon. I
see a double bond, so it probably wants me to do something with that, and then regenerate H3O+ 260 at the end, probably.
The ways students referred to the hydronium indicate that they used it as a cue for which steps they
needed to perform during the reaction mechanism, similar to findings by Bhattacharyya and Bodner
focused on how chemistry graduate students approached reaction mechanisms.6
Some students, Peter, Pepper, Jasmine, and Perdita, did not initially vocalize their recognition of
265 the hydronium referent. Of these students, it was unclear whether Peter and Pepper only vocalized
their recognition later or if they did not recognize the reaction type until they had regenerated the
catalyst. Jasmine, an app student, did not recognize the role of hydronium at first, determining that
the hydronium would be deprotonated based on the task card for the reaction, and then remembering
and justifying the move by identifying hydronium as a strong acid. This exemplifies how students’
270 knowledge can be prompted by engaging with the reaction through the app. Perdita, a paper-pencil
student, only identified the reaction as an acid-catalyzed addition after they had drawn out the full
reaction mechanism whereupon, looking over the operations they had performed, Perdita recognized
the steps as an acid-catalyzed addition and used that knowledge to confirm the mechanism they had
drawn. Perdita said:
275 I'm pretty sure for this problem there should be ... I think the first step is a proton transfer. And then there would be a complexation, and then there would be clean-up. And I guess this is what I'm thinking just to check that this is kind of what was supposed to happen. I don't know, this is what I did, but I think that's what's supposed to happen in this, so now I'm just looking to see if that's what did happen.
280 It is promising that Perdita was able to reason their way through the mechanism without recognizing
the reaction type, only comparing what they had done to known steps afterwards. However, we only
observed the technique of retrospectively confirming the proposed reaction mechanism by aligning it
with known reaction steps once during all of the think-aloud interviews. Instructors could potentially
facilitate such metacognitive strategies to help students review their mechanisms for feasibility.
285 Some students struggled to use hydronium in their depicted mechanisms despite recognizing it as
a referent in the reaction. Four of the students who identified hydronium at the beginning of the
reaction did not use it appropriately. Flynn and Aurora both exhibited difficulties with the rules and
syntax of the hydronium, despite identifying it at the start of the reaction. Flynn focused on the water
molecule and tried to use the delocalized electrons in the double bond to form a bond with the oxygen
290 in the water molecule, which the app did not allow, before realizing the hydronium was available to be
deprotonated. Aurora, a paper-pencil student, determined that water was added to the reactant during
the course of the reaction by comparing reactant and product formulas, analogous to mapping
strategies described in the literature.2, 13, 31-32 This led Aurora to deprotonate the water as opposed to
the hydronium, using the delocalized electrons in the double bond. Aurora then formed a bond
295 between the resultant hydroxide and carbocation. Hence, Aurora did not use the hydronium during
the reaction and depicted an unfavorable acid-base reaction that formed hydroxide and a carbocation.
Angela and Daisy, on the other hand, had difficulties with the relationship of the hydronium to the
other reactant species. Neither student recognized that the hydronium was already present in its
catalyzed form and both started by trying to protonate it with the available water molecule. This
300 indicates that while they both recognized the reaction type, neither student recognized that the
conjugate acid had already been formed. Thus, they either lacked or did not apply the chemical
intuition to identify the electrophilicity of the hydronium. This could also be due to the instructional
context in which our study took place, where instructors often show forming hydronium as the first
step in this type of reaction before protonating the alkene. Hence, these students might have felt the
305 need to show a step before protonating the alkene even though the hydronium had already been
formed. In Angela’s case, the app did not allow the chemically inaccurate move so they instead used
the double bond to deprotonate the hydronium. Daisy, a paper-pencil student, drew the result of this
step as an uncharged H4O molecule and continued to show a poor understanding of chemical
reactivity and Lewis structures as they continued the mechanism. While Daisy correctly deprotonated
310 the H4O ‘catalyst’ with the delocalized electrons in the carbon-carbon double bond, they then used the
lone pairs on the oxygen in the reformed hydronium to form a bond with the carbocation intermediate,
despite the positive charge on the oxygen. Following this operation, Daisy drew the product structure
without any charges and determined they had completed the reaction. Angela and Daisy’s attempts to
protonate the already positively-charged acidic oxygen, in addition to the unfeasible chemical
315 structures that Daisy continued to form and use, exemplifies previously characterized difficulties that
students have drawing and applying Lewis structures to the more complex molecules in an organic
chemistry context.8, 16
The last step of this mechanism required students to reform the acid catalyst in the process of
forming the hydroxyl group on the product. While all but two of the students did reform the catalyst as
320 part of their reaction mechanisms, most did not discuss the underlying chemistry. Belle, Pepper,
Flynn, Ana, and Perdita all talked about knowing that the last step of the reaction was to regenerate
the acid, but did not specify why this was the case. Jasmine and Peter were focused on forming the
alcohol product given as the target on the app goal card (Figure S1A) and regenerating the catalyst
seemed to be a byproduct of that step rather than a recognized part of the reaction. Students’ focus on
325 forming the product when drawing mechanisms has been documented frequently in the literature.2, 4, 6-
8 Aurora and Daisy, both paper-pencil students, did not reform the catalyst. For Aurora this may have
been because they did not use the catalyst during the reaction and so it did not need to be
regenerated. Additionally, Aurora’s drawn mechanism did not result in any unstable products. While
Daisy had recognized the catalyst at the start of the reaction and mis-applied a memorized step to
330 form it, they did not consider regenerating the catalyst at the end of the mechanism. For both of these
students, the lack of explicit cues indicating that another mechanistic step was required in the
referents they had drawn may have hindered their chemical thinking about the final step of this
reaction.
Three students verbalized chemical thinking about the last step of their mechanisms. Tiana knew
335 there was a final mechanistic step and remembered that it involved protonation. Tiana first tried
protonating the water they had added to the carbocation and then, when the app did not allow this,
protonated the free water instead. While Tiana initially showed a lack of chemical thinking, they
subsequently used charge to explain why their original move was incorrect. In juxtaposition, Mary and
Francis were the only students who provided chemical reasoning about the last step of the reaction
340 prior to or as they were performing the mechanistic operations. Mary focused on using the free water
to deprotonate and remove the positive charge on the oxygen as the motivation for their last step,
whereas Francis discussed removing the positive charge on the oxygen and regenerating the catalyst.
Overall, it appears that students are able to recognize the reaction as containing an acid catalyst, but
they have some difficulties with the relationships, rules, and syntax affiliated with acid catalysts that
345 led to incorrect operations during their mechanisms. This may be indicative of a surface-level
understanding of the role the catalyst plays in the reaction, as well as the chemistry dictating the role,
as characterized in the literature.33
The other key element of this reaction was for students to identify the correct site for carbocation
formation following protonation at the double bond. While many students (eight out of thirteen)
350 correctly identified that they should use substitution to determine the most stable cation intermediate,
they did not discuss why carbocation stability increases as the number of non-hydrogen substituents
increases. This is exemplified by Belle who states that tertiary carbocations are more stable than
secondary but, following an interviewer question about why this is the case, could not explain the
phenomenon:
355 A tertiary is attached to three carbons and yeah. I don't know. I just know that it is. Despite correctly identifing substitution as the mode of stabilization for the carbocation intermediate,
Aurora and Perdita did not describe substitution correctly. Aurora inverted the syntax associated with
the concept of carbocation stabilization via substitution, which led to their performing an incorrect
operation:
360 ...so then this carbon is attached to three other carbons, and this one is only attached to two. And sp is the most stable, so that means this one [attached to two carbons is] more stable, this one [attached to three carbons is] less stable... These results indicate that students lack an understanding of the chemical principles that relate
degree of substitution to carbocation stability. Students similarly struggled with carbocation stability
365 in the resonance-stabilized addition reaction, as discussed below.
Five students did not initially consider substitution to determine carbocation stability. Jasmine
and Pepper selected the site for protonation based on the app task card (Figure 1B) and Daisy and
Tiana did not verbalize reasoning behind their protonation decision. However, following prompting by
the interviewer, both Tiana and Pepper talked about using substitution to determine carbocation
370 stability. Lastly, Ana used a steric hindrance argument to justify their decision, but focused on the
sterics in the final product rather than during the protonation step. Ana said:
Where would an H fit better? I'm just gonna say the H fits better here because this is a big [five membered ring]. Ana uses similar reasoning in the resonance-stabilized reaction, which is discussed further below.
375 Think-aloud interviews about this reaction indicated that even when students were familiar with
the reaction type and referents, they did not necessarily know how to correctly use the referents or
apply the related steps. While most students recognized the hydronium, they did not always use it
correctly and may have had a rote understanding of the role it played in the reaction. Additionally,
most students correctly used substitution to determine carbocation stability, but did not exhibit an
380 understanding of the underlying chemistry. While there is merit in students’ use of rules, such as
knowing to focusing on substitution when determining the most stable carbocation in this reaction, it
can lead to syntactic difficulties and improper operations when they do not understand the chemical
principles from which they derive.
Resonance-Stabilized Addition
Figure 2. Reaction schemes for the resonance-stabilized addition reaction as presented during the think-aloud interviews to A) paper-pencil students and B) app students in the task card prior to beginning the reaction. 385
The resonance-stabilized addition reaction involved students protonating a carbon-carbon double
bond with a strong acid, HBr, and then forming a bond between the resultant bromine anion and
carbocation to produce the addition product (Figure 2). In this reaction, students needed to assess
carbocation stability by considering the rules and syntax associated with the available resonance
390 stabilization due to the delocalizable electrons on the oxygen adjacent to the double bond. Then they
needed to recognize the electrophilic-nucleophilic relationship between the carbocation intermediate
and bromine anion and perform the associated operation to form the final product.
The first step of this mechanism was determining the most stable carbocation intermediate. Two
students did not consider carbocation stability. Jasmine, an app student, first tried protonating the
395 oxygen. When the app would not allow the move, Jasmine recognized that protonation should occur at
the double bond and chose a carbon at random. Ana, a paper-pencil student, focused on sterics as
they had in the acid-catalyzed addition reaction, applying the rule of steric hindrance to determine
which carbon the bromine would later add to. Ana said:
Oh but that methyl, or that group [the ethyl], that's a big group. So would a Br want to be next to 400 [an ethyl] or an oxygen? They debate the relative merits of having the bromine adjacent to the oxygen versus the ethyl group on
the opposite side of the double bond, in the end deciding that steric hindrance will favor bromine
placement on the carbon adjacent to the oxygen and thus protonating the other double bond carbon.
While this allowed Ana to form the favored product, it indicates limited chemical reasoning as they
405 neglected to consider the stability of the carbocation intermediate.
The remaining students used the rules and syntax of carbocation stabilization to determine the
most stable carbocation, with half considering resonance stabilization and the other half considering
substitution. Four paper-pencil students and one app student correctly identified that resonance
stabilization would dictate where the carbocation would form. Of the four paper-pencil students, Mary
410 incorrectly applied the rules of resonance stabilization, focusing on the potential resonance forms of
the reactant rather than the intermediate when making their decision. This is similar to results in a
parallel study focused on acid-base reaction mechanisms.18 The one app student who focused on
resonance, Peter, accessed the goals directly from the task card prior to starting the puzzle and used
the goals to guide their movements. Peter paraphrased the goal card, saying:
415 So, my goal is to use resonance structures to show how positive charge with the carbocation can be delocalized to form the major alkyl bromide. The goals directed Peter to consider resonance rather than substitution for the rules to determine
carbocation stability, and Peter then correctly applied the syntax affiliated with resonance
stabilization. The remaining students, four app and one paper-pencil, focused on substitution to
420 determine the site of protonation. Daisy, the paper-pencil student, protonated the correct carbon
without stating their reasoning, but upon prompting recognized that both carbons in the double bond
could be protonated and stated that one would be the major product due to substitution of the
carbons. However, Daisy did not reconcile the fact that this principle was not used to determine the
carbon they protonated and proceeded to carry on with the reaction. Tiana and Flynn, both app
425 students, first identified the two double-bonded carbons as secondary and then started thinking about
how the oxygen could stabilize the adjacent carbon through induction. This allowed Flynn to form the
correct carbocation despite lacking some chemical intuition. Tiana’s reasoning, however, focused on
how induction would impact the ultimate placement of the bromine, causing them to incorrectly
identify that the oxygen would donate electron density to the adjacent carbon. With this reasoning,
430 Tiana decided that the carbon further from the oxygen would be more electron deficient and thus
would more readily accept the electrons on the bromine anion during the second step of the
mechanism, which led them to protonate the carbon adjacent to the oxygen. Pepper and Belle
exhibited incorrect knowledge of substitution syntax, as both identified the sp2-hybridized carbon
bonded to the oxygen as primary. Pepper said:
435 So, this is the secondary carbon where this [carbon bonded to an oxygen] is the primary carbon so I would say [the secondary carbon] is the more stable carbocation... Belle hesitated, having noticed resonance on the task card, but continued with their reasoning despite
this. Aurora, a paper-pencil student, demonstrated similar incorrect knowledge in their think-aloud,
despite correctly focusing on resonance. Aurora called the double-bond carbon adjacent to the oxygen
440 sp-hybridized in a misapplication of hybridization rules, discounting the heteroatom. The syntactic
difficulties leading to the mis-assignment of substitution and hybridization may be indicative of poor
understanding of the affiliated concepts.
The app students who did not initially consider resonance to determine carbocation stability did so
following prompting by the app, whereas the two paper-pencil students did not during the post-
445 reaction interview questions. However, Daisy demonstrated in their response to the question about
resonance they did not understand what the term ‘resonance structures’ meant, conflating it with
major and minor products. The five app students who did not consider resonance when initially
determining carbocation stability were prompted to do so by the app in two different ways. As
described above, Pepper and Belle both selected the incorrect double-bond carbon to protonate,
450 forming the minor product. This resulted in a hint from the app that one of the carbons would be
resonance stabilized which led both students to form the resonance structure of the carbocation
intermediate prior to addition of the bromine. Pepper said:
Okay, so, resonance is always more important than based on what's primary and secondary and tertiary. I guess I didn't consider that just because I only saw those double bonds and didn't see 455 these lone pairs but if we think about putting it on this carbon instead then the lone pairs from this oxygen can make a bond between this oxygen and carbon which would stabilize it because then this carbon wouldn't really be a carbocation all the time. While this prompted Pepper to consider resonance-stabilization, it also elucidated a misunderstanding
that resonance forms are present as a mixture of structures in solution, an error also identified in the
460 parallel acid-base study.18 Although Tiana also received the hint after forming the minor product, they
immediately applied the syntax of resonance stabilization to form the major product and did not depict
the resonance contributor until prompted by the goals card in the app (Figure S1B). The two app
students who chose the correct double-bonded carbon to protonate without considering resonance
were prompted by the goals card to form a resonance structure after they had achieved the major
465 product. Following the operation of forming a resonance structure in the app, only one of the two
students, Flynn, verbalized that they should have considered resonance to determine the intermediate
structure.
In the last step of the reaction, students tried adding the bromine anion formed during protonation
of the carbon-carbon double bond to either one of the carbon atoms or the oxygen atom in the reaction
470 intermediate. Three app students and four paper-pencil students formed the major product, while
three app and two paper-pencil students formed the minor product. As mentioned above, the app
prompted students who formed the minor product to consider resonance stabilization to determine the
major product which led those students to eventually form the major product. After forming the
correct carbocation and resonance structure following the hint, Belle then tried to add the bromine
475 anion to the positively charged oxygen in the resonance structure. When the app did not allow this,
Belle tried to reason through what the next step in the mechanism was, focusing on the intermediate
in its resonance contributor form, before turning to the app goals for guidance:
A negatively charged bromide, I have a positively charged oxygen, and it doesn't ... Okay, oxygen is never gonna wanna have four bonds. I can check what the goals are, though. Major alkyl 480 bromide product. Belle was then able to use both their chemical intuition and the scaffolding provided by the app to
correctly form the major product. Jasmine, another app student, also tried to form a bromine-oxygen
bond based on directions from the goal card, exhibiting a poor understanding of Lewis structures and
nomenclature. After trying a few moves in the app, Jasmine undid the carbon-oxygen double bond to
485 form a bond between the bromine and the carbocation. However, they then exhibited poor
understanding of the forces driving major versus minor product and inductive effects by implying that
the bromine would be stabilized by being located closer to the oxygen. Despite making some
chemically inaccurate moves, students generally justified the last step by focusing on forming a bond
to stabilize the charged species.
490 Generally, it appears that students exhibited a series of difficulties with the first step of this
problem. Only four of the thirteen students identified resonance as the source of carbocation
stabilization without additional direction. With the exception of Jasmine and Peter, the other students
reasoned based on substitution or sterics. This could be due to features such as substitution and
sterics being more explicitly represented to students in the referents for the reaction, compared to
495 resonance being a more implicit feature, which aligns with previous literature indicating that students
tend to rely more heavily on explicit chemical features.7-10 As exemplified by Pepper’s statement above,
the hidden oxygen lone pairs in the app may have limited app students’ ability to identify the oxygen's
potential to provide resonance stabilization to the neighboring carbocation, however the app does
require students to consider resonance to complete the puzzle. Additionally, students’ overapplication
500 of substitution to determine reaction progress indicated some difficulties with the syntax of
substitution when carbon was bonded to a heteroatom. Determining degree of substitution when a
carbon is bonded to a heteroatom would be an unfamiliar situation for students as they would not
normally have to consider this situation when resonance should be the leading factor in determining
carbocation stability. We also identified a few students who focused on the placement of the bromine
505 as a driving force for the reaction, which aligns with a product rather than process-oriented approach
to the reaction.7-10
Limitations This study was qualitative in nature with a small pool of participants and as such the results are
descriptive and may not be generalizable. Additionally, the participants were all recruited from the
510 same institution and so we could not characterize potential differences due to curricular differences.
However, our experimental design, recruiting students with a range of ability levels and using two
modalities to probe student thinking, did capture an array of students’ conceptions. Claims about
differences between the two groups, paper-pencil and app students, may also be limited by our sample
size and as there may have been inherent differences in the think-aloud interviews that could not be
515 controlled for which influenced student responses. However, we incorporated several measures in the
interview protocols to mitigate differences, such as aligning the reaction representations and leading
the app students through a tutorial to minimize added cognitive load from using a new interface.
CONCLUSION In this study we elicited student thinking about addition reaction mechanisms using two
520 modalities: paper-pencil and app-based. Qualitative analysis of the resulting think-aloud interview
data provided insight into how students were utilizing the chemical information contained within the
referents of the reaction, as well as what concepts students focused on and how they applied them to
the given reactions. Additionally, comparing the two groups of students elicited a wider range of
student thinking and indicated the potential benefits and drawbacks of each modality.
525 When approaching the acid-catalyzed addition reaction, many students recognized the hydronium
and used it as a cue to inform the initial steps of their mechanisms. However, some students appeared
to have a surface level understanding of the chemistry influencing the role of the acid-catalyst as they
did not know how to use it correctly during the reaction. Additionally, most students performed the
steps of using and reforming the catalyst with a focus on completing potentially memorized reaction
530 steps rather than discussing the chemistry driving the reaction in a specific direction.
For both reactions, students recognized that they needed to identify which of the double-bonded
carbons would form a more stable carbocation to determine the favored reaction pathway, aligning
with previous literature that carbocation formation is the most commonly recognized step in addition
reactions,9 but they had some difficulties identifying and applying the different modes of stabilization
535 between the two reactions. In the acid-catalyzed reaction, most students successfully identified that
the stability of a carbocation would increase as its substitution increased but did not necessarily know
why and exhibited some difficulties with the associated syntax of applying stabilization via
substitution. In juxtaposition, over half of the students failed to identify the potential for resonance
stabilization in the resonance-stabilized addition reaction. Some of those students then struggled to
540 apply substitution or relied on arguments about induction and steric effects to determine the favored
carbocation intermediate. Of the students who correctly identified resonance as the primary source of
carbocation stabilization for this reaction, most did so successfully. Interestingly, while students
appeared to understand the underlying concepts affiliated with resonance better than those associated
with substitution, they appeared to favor substitution when determining carbocation stability. This
545 may be related to students’ tendencies to focus on explicit chemical features compared to implicit
electronics,7-10 as they can readily determine substitution by counting bonds whereas resonance
requires students to visualize or draw resonance contributors. Delocalizable lone electron pairs could
serve as explicit cues for students to consider resonance, and as such it would be important for lone
pairs to be depicted in whatever modality students use when considering a mechanism. However,
550 more research is needed targeting student conceptions of resonance.
The two groups of students were relatively similar in how they approached the mechanisms. Both
paper-pencil and app students considered multiple reaction pathways, unlike in the companion study
where paper-pencil students primarily considered one pathway.18 Students also demonstrated some
reflective thinking during the think-aloud interviews. Specifically, some of the app students provided
555 chemical reasoning after receiving prompting from the app and some paper-pencil students were
reflective upon completing the reactions. While fewer app students identified resonance without
prompting, they were directed to do so by the app in order to complete the puzzle.
Results from our study substantiate previous research indicating that students tend to recognize
explicit, surface features and use those as cues to apply known steps.2, 6 Additionally, our results
560 indicate students have a surface-level understanding of the concepts relevant to addition reactions
that inhibit their ability to apply them in unfamiliar situations. Thus, it is important that, as
instructors, we include discussions of the chemical principles behind the rules presented to students
in class and provide them with opportunities to apply concepts such as carbocation stabilization to
identify gaps in their understanding before moving on to more complex reaction types. Additionally,
565 instructors can include the explicit cues contained in line-angle representations—i.e., lone pairs on
heteroatoms—that experts recognize inherently but may still be difficult for students to identify at the
introductory organic chemistry level. While a few students exhibited some reflection during the course
of the think-aloud interviews, instructors could help students develop these skills by providing
additional scaffolding or modeling reflective behavior.
570 ASSOCIATED CONTENT Supporting Information Coding Scheme and goal cards presented by the app (PDF)
AUTHOR INFORMATION Corresponding Author 575 *E-mail: [email protected]
ACKNOWLEDGMENTS This work has been supported, in part, through a grant to Alchemie from the National Science
Foundation Small Business Innovation Research program, #1659983, and the Michigan Corporate
Relations Network (MCRN), funded by the Michigan Economic Development Corporation (MEDC) and
580 administered by the University of Michigan Business Engagement Center and the U-M Economic
Growth Institute's Small Company Innovation Program (SCIP). This material is based upon work
supported by the National Science Foundation Graduate Research Fellowship Program under Grant
No. DGE 1256260. Additionally, we would like to thank the students who participated in our study
and Alchemie for creating the graphical abstract.
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