Mechanism comics as a task in a written exam in organic chemistry for pre-service chemistry teachers

Introduction

Students starting to learn organic chemistry have to occupy themselves with topics such as formula language, structure-property relationships, substance classes, basic concepts such as acids and bases or electrophilicity and nucleophilicity, and the understanding and designing of reaction mechanisms. Many students simply try to memorize as much organic chemistry content as possible (Grove & Bretz, 2012). This can be seen as the main reason for the problems they encounter when learning organic chemistry. Relying on rote memorization prevents meaningful learning. Learning is meaningful if it is an active process where existing cognitive structures are relevant for the new content, where new ideas interact with existing ideas and where the learning content is reworded (Ausubel, 2000). If we want the students to be capable of this meaningful learning, we must prepare course concepts and learning environments that support and enable meaningful learning in organic chemistry.

To ensure that students can learn meaningfully, several aspects have to be taken into account. First, it has to be clear which knowledge theory forms the basis of the teaching and learning. The idea that knowledge can be transferred from the mind of the teacher to the mind of the learner is outdated (Bodner, 1986). The constructivist view that knowledge has to be constructed in the mind of the learner is much more suitable if we want to initiate learning. If knowledge is constructed in the mind of the learner, this happens on the basis of preexisting cognitive structures or schemes (Ausubel, 1968). If the students relate the new knowledge to relevant concepts they are already familiar with, they learn meaningfully. For this, the students must consciously choose to learn meaningfully by seeking connections and not default to rote memorization. To be successful, they have to see the relevance of the content, self-monitor their learning and reflect upon which learning strategies work and which do not (Grove, Cooper, & Rush, 2012).

To support students and foster their meaningful learning, we developed, used and evaluated mechanism comics. This method was applied in the course to achieve two different didactic goals. The students had to write captions under each step of a given reaction mechanism. This idea builds on the method of writing-to-learn (WTL) as developed, used and evaluated by several groups, as well as on results from research on mechanistic reasoning. Rivard (1994) described WTL strategies as offering an opportunity to enhance knowledge acquisition and cognitive skill development in science disciplines. The writing enables students to understand the relevant content and concepts (Reynolds, Thaiss, Katkin, & Thompson, 2012). For chemistry teaching, several WTL strategies have been developed, used and evaluated (Finkenstaedt-Quinn et al., 2019, 2020; Gupte et al., 2021; Moon et al., 2018, 2019; Schmidt-McCormack et al., 2019; Shultz & Gere, 2015 Finkenstaedt-Quinn et al., 2019, 2020; Gupte et al., 2021; Moon et al., 2018, 2019; Schmidt-McCormack et al., 2019; Shultz & Gere, 2015; Watts et al., 2021; Watts, Zaimi, Kranz, Graulich, & Shultz, 2021). Tasks using WTL strategies are suitable for allowing students to make connections between new concepts and existing knowledge. WTL assignments, therefore, provide opportunities to foster the meaningful learning of chemistry (Gupte et al., 2021).

Students writing captions for the steps of a reaction mechanism engage in mechanistic reasoning. Particularly the understanding of fundamental chemical mechanisms is an important goal in chemistry education (Talanquer, 2018). Instructors should therefore focus on developing tools that students need to develop their mechanistic thinking (Grove, Cooper, & Cox, 2012). Knowledge of reaction mechanisms and the ability to design them are essential for understanding organic chemistry. For both of these, students need the ability to use chemical formulas to represent the processes that occur, as well as applicable knowledge on the use of the representations and concepts connected to them, such as the concepts of nucleophilicity and electrophilicity, for example. If the representations are the tip of an iceberg, their usage and underlying concepts are below the water surface (Graulich, 2015). As we all know, the largest part of an iceberg (and for organic chemistry, in a figurative sense, the important part) is below the surface. Students therefore have to learn how to use the representations to achieve a deeper understanding and, ultimately, develop their mechanistic reasoning. To ensure that students can build up the competences they need, organic chemistry teaching should focus on processes rather than results; as stated by Talanquer and Pollard (2010), we should “teach how we think instead of what we know.”

Chemists use the electron-pushing formalism as a support for the design of reaction mechanisms. For students, drawing electron-pushing arrows is not seen as any kind of support (Ferguson & Bodner, 2008; Grove, Cooper, & Rush, 2012), but as something that just has to be done – sometimes even after their mechanism is complete (Bhattacharyya & Bodner, 2005). As a learning opportunity to prepare students for mechanistic reasoning in organic chemistry, the use of electron-pushing arrows can already be introduced in general chemistry courses (Crandell, Kouyoumdijan, Underwood, & Cooper, 2019). This would possibly prevent the situation where even good students discontinue their studies due to problems in organic chemistry (Anderson & Bodner, 2008). To make sure that students do not simply memorize reaction mechanisms and in preparation for teaching them to design reaction mechanisms, a new organic chemistry curriculum starts with mechanisms before reactions (Flynn & Ogilvie, 2015). In this curriculum, the concept of a mechanism and the electron-pushing formalism are taught first. In our course, we also use this approach. If students learn why they should use the electron-pushing formalism, they will use it correctly and routinely (Webber & Flynn, 2018).

To support students when they are designing reaction mechanisms, several approaches have been published in recent literature. As the mapping of atoms of the starting material onto the target has proven to be a good strategy, teaching it as a supporting strategy can be recommended (Bodé & Flynn, 2016; Galloway, Stoyanovich, & Flynn, 2017). Furthermore, to initiate students’ mechanistic reasoning, case comparisons can be used. Students have to activate prior knowledge to consider which different pathways are possible and which ones are more likely (Graulich & Schween, 2018; Watts et al., 2021). Scaffolds can be used to this end, such as the concrete prompts that have already been developed, used and evaluated (Caspari & Graulich, 2019). The reasoning scaffolds have proven to be suitable tools for supporting students in their reasoning. However, scaffolds should only help students on tasks that they could not accomplish without this support (Graulich & Caspari, 2021).

By writing captions under the steps of a given reaction mechanism, students have to think through every step in detail and apply their knowledge of basic concepts such as those of electrophilicity and nucleophilicity. The writing should support their thinking, as well as their application of conceptual knowledge. Writing captions should give the students a opportunity to more deeply understand the reaction mechanisms and can be seen as a pre-exercise for the second use of mechanism comics in the learning process. Here, the students have to both design the steps of a reaction mechanism by themselves and write the captions as well. For the students to write out the steps for the complete reaction mechanism on their own, they will need to apply their knowledge of formulaic language, (partial) charges and the concepts of electrophilicity and nucleophilicity. Writing the captions is now, as described above, a learning opportunity enabling students to understand the reaction mechanism more deeply, but is also a way to check the design of the mechanism for plausibility and correctness.

Design of the study

The design of the study will be described in detail below. The results of the study will be used to diagnose the students’ reasoning as well as their ability to design mechanisms for reactions that are unknown to them.

The “mechanism comic” task applied to a previously unknown reaction

On the written exam, the task “an unknown reaction” should be solved by the students by designing a mechanism comic for this reaction. The mechanism comic includes all steps of the reaction mechanism and captions that explain those steps. In this case, the reaction was that between acetaldehyde and methanol with protons as a catalyst. Figure 1 shows the task (for this publication, the task has been translated from German into English).

Figure 1:

The task “an unknown reaction” in the written exam.

On this task, 14 points could be achieved in total. For all eight tasks of the written exam, the students could achieve a maximum of 100 points.

Figure 2 shows an example of a student’s reaction mechanism; the captions have been translated from German into English and the students’ drawings were redrawn by the author.

Figure 2:

An example of a student’s reaction mechanism with captions (coding of the captions).

Sample

35 students took the written exam, of whom 22 students (63%) passed the exam (they received at least 50% of the total sum of points in the exam). Solutions for the task (max. 14 points) from 24 students were included in the study (69%). Those students produced mechanism comics for the task, although not all of these were correct. The other solutions have not been taken into account because they were only rudimentary and therefore not suitable for evaluation. The results for the solutions considered here are summarized in Figure 3.

Figure 3:

Rating for the solutions of the mechanism comic.

We contacted all students whose solutions we used in the study and informed them of our plan to examine their solutions. All students gave their consent. Approval from the ethics board was not necessary at our university.

Results and discussion

To assess the quality of the students’ written captions, in the first round of coding the students’ captions were coded as either “descriptive” or “causal” (for examples, see Figure 2). Coded with “causal” were captions that included words such as “as,” “because,” “thus,” “since,” “which is why” or “so that.” If a student’s reaction mechanism included captions that were coded as “descriptive” as well as captions that were coded as “causal,” the complete reaction mechanism was coded as “descriptive-causal.” Table 2 shows the assignment of the codes to the students’ captions and one example for each code.

Of the 24 reaction mechanisms, 7 were coded as completely “descriptive” (29.2%), 15 as “descriptive-causal” (62.5%) and 2 as completely “causal” (8.3%). It cannot be concluded from the data whether the students were aware that their captions were of varying quality. It seems possible that some students rated their descriptive captions as sufficient explanations because experts in the field would understand what they meant. The example “partially negatively charged O of the methanol attacks positively charged C” can be seen as one such explanation, although the caption does not explain why this step occurs in this way. Overall, the majority of the students (70.8%) used causal explanations at least once in their reaction mechanisms. Figure 4 shows the percentage of captions that were coded with “causal” for the students whose reaction mechanisms were coded as “descriptive-causal” or as completely “causal.” The majority (11 students) used causal arguments for 14–50% of their captions. Only 6 students used those arguments for 55–100% of their captions.

Figure 4:

Percentage of captions that were coded as “causal”.

However, it seems unlikely that they consciously shifted between descriptive and causal. It is more likely that the difference between descriptive and causal was not clear: Students mostly describe “what” happens and rarely “why” this happens, as also discussed by Cooper et al. (2016). In the future, the task description should therefore stipulate that the explanation has to be understandable for beginners in the field of organic chemistry and not only for experts.

In the second round of coding, the written captions were assigned to the categories “properties of entities” and “activities” (Machamer et al., 2000; Russ et al., 2008). Entities are, for example, electrons or atoms (Caspari, Weinrich, Sevian, & Graulich, 2018; Russ et al., 2008) and are characterized by their properties (Machamer et al., 2000). Their properties, in turn, include being basic, nucleophile or positively charged (Caspari et al., 2018). Examples of activities include the movement of electrons or the formation and breakage of bonds. In organic reaction mechanisms, those activities cause structural changes (Caspari et al., 2018). Table 3 (properties of entities) and Table 4 (activities) give a summary of the results of this coding. The codes were assigned each time they appeared. Therefore, mostly both codes were assigned for each written caption.

Most written captions (72; 79.1%) were coded to the category “charge.” Almost all students (91.7%) argued by using the charge of the particles that were part of the reaction mechanism. This shows that the students focused more on surface properties then on underlying concepts (Gentner, Loewenstein, & Thompson, 2003); the concept of nucleophiles and electrophiles was used only by 37.5% of the students explicitly. However, they did apply the underlying concept, as the description of particles’ charges or electronegativity shows. As discussed before, it seems that the students wrote their captions for experts; the necessity and usefulness of explaining why oppositely charged particles interact in a reaction were obviously not apparent to the students.

All students described electron movement implicitly; only half of the students also described those movements explicitly. The main focus of the students when describing implicit electron movement was on the breaking and making of bonds without explaining the reasons why it occurred. We expected at least that the students would use the attraction between positively and negatively charged particles as an explanation. As observed before, the analysis of this round of coding confirms that the students focused mainly on “what” was happening and less on “why.”

To discuss the quality of students’ reasoning, in the third round of coding the written captions were assigned to the categories “technically correct,” “partially correct” and “not correct” because, especially for teachers of organic chemistry, it is important to know what learning opportunities students need to ensure that they can apply basic concepts correctly. Table 5 shows one example for each category.

In total, the students wrote 132 captions. Of those, 73 (55.3%) were technically correct, 40 (30.3%) were partially correct and 19 (14.4%) were not correct. To evaluate students’ problems and misconceptions, the captions that were coded with “partially correct” or “not correct” were evaluated in a fourth round of coding. Table 6 gives an overview of the resulting subcategories and one example for each subcategory.

The subcategory “charge” includes students’ writings that show an incorrect understanding of the term “charge.” The students here treat the charge of a molecule or ion as a particle; a charge is something that can be added or removed, as the following citation shows: “the positive charge is produced due to taking the H ⁺ .” The reason for the positive charge, namely the lack of one electron, seems not to be clear to the students who argued in this way. The subcategory “formulaic language” includes students’ careless use of the formulaic language. Especially the proton (H⁺) is repeatedly mixed with hydrogen (H). Whether the students are sloppy or simply do not know the difference remains uncertain. The concept of mesomerism is also not clear to all students. It is not seen as the description of delocalized electrons in more than one Lewis formula, but of some kind of drive for the shift of electrons during reactions, as can be illustrated by the following citation: “because of the mesomerism the double bond shifts upwards and a negatively charged oxygen and a carbenium ion are produced.” Several misconceptions can also be observed in the captions written by the students. Mostly these appear when students describe reactions that are not possible, such as “the OH ⁻ group splits off a hydrogen,” where the acid-base concept of Brønsted was not applied correctly.

To sum up, the research question “What is the quality of the mechanism comics the students designed during their written exam?” can be answered. Most students described the steps of their reaction mechanism correctly. However, only a minority described “why” the steps occurred. The quality of the comics can therefore not be assessed. If the correct explanation as to “why” the steps of a mechanism occur is found to be at the highest level of quality, then the description of “what” happens has to be ranked at a lower level. The evaluation of the students’ captions cannot answer the question of why the students chose a more descriptive approach for their captions. One reason we discussed above may be that the students did not see the need to explain the steps because their target group were experts in the field. Another explanation could be that the students internalized the concepts in a way that they applied those concepts unconsciously and quite naturally.

Limitations

There are some limitations to this study. First, the number of students (N = 24) was not very high, although for a qualitative study this number can be seen as sufficient. Second, not all students solved the task such that their solution was suitable for evaluation in this study; only 69% of the solutions could be analyzed here. It can be assumed that especially those students who did not attend the course (attendance is voluntary in Germany and only 50% of the students who registered for the course attended regularly), and who maybe also did not use all the course materials that were provided via Moodle on their own time, had no experience designing the mechanistic steps and writing captions, and therefore did not solve the task as intended or did not even try to solve the task. Although the quality of students’ written captions can be evaluated, the reasons for the observed quality remain unclear.

Conclusions and outlook

Students write appropriate captions for the steps of a reaction mechanism they design. The captions describe the interaction of the particles, but seldom why this interaction occurs. However, only a minority of the captions (14.4%) are technically incorrect. The majority of the students who took part in this study (91.7%) achieved at least half of the points possible for this task in the written exam. The task with its double focus on designing mechanistic steps and writing explanations can therefore be seen as a suitable task in an exam situation. Because the quality of the students’ solutions could not be evaluated, in the next course the task will be adjusted. The students will be asked explicitly to describe “what” happens for experts in the field and then to explain “why” this happens for beginners. We hope that this division into two different writing tasks will encourage and enable the students to perform mechanistic reasoning on a higher level. Such an approach was already successful for testing students’ mechanistic reasoning pertaining to acid-base reactions, as described by Cooper et al. (2016); if students have to write first what happens and then why it happens, the majority do explain why the reaction occurs as it does. Our approach to describe “what” happens and “why” to different target audiences (experts for the “what” and beginners for the “why”) should, on the one hand, make transparent that chemists often adapt their language to the target group and, on the other hand, give the students a learning opportunity in preparation for their future profession as a chemistry teachers, where their students at school can be seen as “beginners.” Another possibility to promote students’ engagement in mechanistic reasoning is the use of scaffolds. Caspari and Graulich (2019) described scaffolds that asked students the questions they should always answer while explaining mechanistic steps. Students need to be prompted to write down their strategies for solving reaction mechanisms; otherwise, they are less likely to do so (Bodé & Flynn, 2016).

We will develop a different sort of scaffold that provides students with technical terms (e.g. “nucleophile”) and verbs (e.g. “attract”) they can use in their written captions. We want to support the students while giving them sufficient opportunities to use their own language structures. For this reason, we will not give them structured elements, as described by Nagel and Lindsey (2021). Their students used “reasoning elements,” “connecting words” and “conclusions” as building blocks for their reasoning. The development, use and evaluation of our scaffold is planned for the next summer term. For students who need additional learning opportunities to acquire basic knowledge, such as on chemical bonding, several digital learning materials that students can use in their own time will also be developed and evaluated in interview studies.