Analysis of undergraduate chemistry students’ responses to substitution reaction mechanisms: a road to mastery

1.1 Research question

The study was guided by the following research question: How do undergraduate chemistry major students draw and explain substitution reaction mechanisms in organic chemistry?

2.1 Research design

The research design for this study was a descriptive case study (Yin, 2014) that focused on analyzing and describing students’ successes and challenges in solving problems involving organic substitution reaction mechanisms.

2.2 Setting and course

This study, which used a case study design with an interpretive lens, was carried out in the Organic Chemistry IV class (organic reaction mechanisms and stereochemistry) at a Ghanaian teacher training University. Organic Chemistry IV is offered in the first semester of students’ third-year studies. This course is taught in English and usually lasts twelve teaching weeks, one 2-h weekly class, accompanied by a 2-h laboratory experience. Assessment for the course comprises two quizzes, a mid-semester examination, a homework assignment, a final examination, and class participation. This study, however, concentrated on students’ responses to test items during a lecture (a diagnostic test), but not the laboratory experience.

2.3 Sample and sampling procedure

All the 70 third-year Chemistry major students who registered for the Organic Chemistry IV course in the 2022/2023 academic year participated as the sample for this study (census sampling). These students were purposively chosen because they had already taken three organic chemistry courses (including functional group chemistry) and possessed the prerequisite knowledge of the reactions used in this study. Also, this third-year class was used for this study because two of the authors taught the Organic Chemistry IV course.

2.4 Instrumentation

The main instrument used for data collection in this study was a diagnostic test that consisted of four (4) items, which required students to draw the mechanistic arrows in given reactions and explain the mechanisms drawn (how and why the reaction occurs), Appendix 1.

In the diagnostic test, students were given questions on substitution reactions where they were provided the reactants and products and additional instructions to draw mechanistic arrows to show the reaction mechanism (see sample in Table 1). The second part of the questions required students to explain how and why the reactions occur. The questions were adapted from Klein (2017) in line with the learning outcomes of the course. The diagnostic test was administered to the students to identify students’ strengths and weaknesses regarding the drawing and explanation of substitution reaction mechanisms (S_N1 and S_N2) to inform instructors on the teaching strategies to use and areas to emphasize when teaching these reaction mechanisms. The test’s validity was ensured by giving the items to two of the authors, who are experts in organic chemistry, to examine their validity (face and content validity). To ensure the reliability of the test, two of the authors assessed students’ responses to the items independently using a prepared marking scheme. The agreement between the two authors’ assessments was 98 %.

Table 1:

Sample of questions used in this study.

Question number	Question type	Questions	Requirements
1.a	Arrow-type		Using correct arrow-pushing formalism in drawing reaction mechanism.
1.b	Explanation-type	Provide explanations on how and why the reaction occurs.	Explanations based on implicit features and electronic effects of the reaction.

2.5 Data collection and analysis procedure

The test was administered in week 2 of the first semester of the 2022/2023 academic year. It was administered by authors 1 and 2, and students were given a maximum of 45 min to respond to the items on the test. Students’ responses to the questions were graded (using a marking scheme agreed upon by the researchers) by one of the researchers, and the scores were recorded on paper. Another researcher verified that the scores were assigned according to the marking scheme (checking for grading errors and consistency) by comparing the assigned score to the same marking scheme. Students’ responses were further categorized into frequencies and presented in tables for discussion. For the arrow-type questions (questions that required students to draw the electron-pushing arrows of reaction mechanisms), students’ responses were classified into ‘correct mechanism’, ‘incorrect mechanism’, and ‘no response’. A correct mechanism involved the drawing of curved arrows: (1) from an electron-rich species to an electron-deficient species in the given reactions or (2) from a lone pair or ℼ-bond to an appropriate electron-deficient site in an intermediate carbocation. Incorrect mechanisms included electron-pushing formalism (EPF) errors previously identified in Chemical education research (CER) literature such as missing arrows, nonspecific arrows, electron-deficient species attacking electron-rich species (reverse arrows), among others (Wilson & Varma-Nelson, 2018). No response meant that the students did not provide any answer to the question. Students’ responses to ‘explain the mechanism’ questions were categorized into ‘correct explanation’,’ partial explanation’, ‘incorrect explanation’, and ‘no response’ and presented as frequencies. Details of the coding system used for this study are provided in Table 4 (Appendix 2).

3.1 Students’ responses to arrow-type questions

Presented in Table 2 are the frequencies of students’ responses to the test items, which required them to provide the mechanisms for given reactions (questions 1.a-4.a).

The percentage of correct responses from Table 2 varies from 1.4 % to 17.1 %, indicating that a lot of the students could not provide correct electron-pushing arrows for the given reactions. and that students found some of the questions more challenging than others, with question 2.a being the most challenging and question 1.a being the easiest based on the correct response rates. Some common errors that rendered students’ drawing of reaction mechanisms incorrect included reverse arrows, omitting arrows for some reaction steps, and non-specific arrows (for example, arrows drawn from space to an atom or from an atom to space), and arrows drawn from charges (which are accepted by some instructors in the organic chemistry community but was not accepted in this study). All these errors are students’ errors in drawing organic reaction mechanisms that have been reported in Chemical education research literature (Carle et al., 2020; Flynn & Featherstone, 2017; Wilson & Varma-Nelson, 2018), and revealed that students did not attribute meaning to the electron-pushing arrows and did not consider whether the mechanisms they had drawn were chemically reasonable or not (Graulich, 2015; Grove et al., 2012). The data also suggests that students have not mastered the concepts of nucleophiles and electrophiles and therefore, could not successfully apply these concepts when drawing electron-pushing arrows in reaction mechanisms. The generally low correct response rates for the questions suggest the need for targeted instruction on arrow-pushing formalism, and providing students with more opportunities to practice the drawing of substitution reaction mechanisms, with more emphasis on S_N1 reaction mechanisms.

Considering students’ response rates according to the type of reaction mechanism, students’ correct response rates for the S_N2 reaction mechanism questions were higher than that of S_N1 reaction mechanism questions. The higher correct response rates for the S_N2 reaction mechanism questions could be attributed to the fact that an S_N2 reaction mechanism is a simple simultaneous backside attack of the substrate and leaving of the leaving group while an S_N1 reaction mechanism involves multiple steps, that is, the formation of a carbocation intermediate, followed by the nucleophilic attack to form the product. Although the authors sought to simplify the questions on S_N1 reaction mechanisms by providing the reaction intermediates, students still struggled with these questions. One common error that accounted for students’ low correct response rates for S_N1 questions was that students drew S_N2 reaction mechanisms as responses to the questions involving S_N1 mechanism (see Figure 1). Most students ignored the carbocation intermediates provided in the questions for the S_N1 reaction mechanism and drew S_N2 mechanisms for these questions. This observation implies that students were more familiar with S_N2 reaction mechanisms and found them easier to draw than S_N1 reaction mechanisms.

Figure 1:

Akosua’s mechanistic drawing.

Besides, a lot of students only drew the mechanistic arrows for the first step of the S_N1 reactions and omitted the arrows for the other steps, leading to incomplete reaction mechanisms. In this study, incomplete reaction mechanisms/omitted arrows were considered as wrong reaction mechanisms. The data from this study suggests that students were more conversant with the departing of the leaving group to form a carbocation in S_N1 reaction mechanisms. A similar observation was also made in students’ explanations of reaction mechanisms in the second part of the questions where quite a number of students talked about the formation of carbocation intermediates for S_N2 reaction mechanisms.

3.2 Students’ responses to ‘explain the mechanism’ questions

The available CER literature suggests that students’ ability to draw correct reaction mechanisms is not always a good gauge of their understanding of organic reaction mechanisms. That is, students can either draw arrows from rote memorization, based on superficial features or without attaching any meaning to the arrows drawn (Crandell et al., 2020). Just as multiple-choice assessment items have been found to overestimate students’ knowledge, reproducing a reaction mechanism may also overestimate students’ understanding of an organic reaction mechanism. Therefore, if teachers want to make claims about what students know and can do in organic chemistry, it is crucial to gather more comprehensive evidence of their thinking beyond drawing arrow-pushing mechanisms or predicting products. In simple terms, teachers must elicit students’ reasoning about how and why reactions occur (Crandell et al., 2020). This study therefore sought to find out students’ reasoning about organic reaction mechanisms by asking them to explain the mechanisms (the how and why) they had drawn in the first part of the questions.

Students’ responses to ‘explain the mechanism’ questions were categorized into correct explanation, partial explanation, incorrect explanation, and no response and presented as frequencies in Table 3.

Although the data presented in Table 3 suggests that students were able to describe S_N2 reaction mechanisms better than S_N1 reaction mechanisms, our focus in this study was not on comparing students’ performance in explaining the two types of substitution reactions, but rather, on their explanations of the reactions in general. From Table 3, the category with the highest frequencies/percentages was the incorrect explanation category (ranging from 57.1 % to 65.7 %), meaning that most students who attempted to provide explanations for the reaction mechanisms could not do so correctly. There are a lot of factors that could account for these high percentages of incorrect responses, but prevalent among them was the fact that students were mixing up the types of reaction mechanisms that the given reactions proceeded by. For example, Akua’s response to the prompt to explain the reaction mechanism drawn in question 1.b (which was an S_N2 reaction mechanism) was as follows: “This elimination and substitution reaction occurs simultaneously. As the bromine atom is cleaved away, a nucleophile attaches to the carbocation formed at the same time”. The description provided by Akua, which is similar to the descriptions of a lot of students in the incorrect response categories revealed that students misunderstood the concepts of substitution and elimination reactions. They seemed to think that the departure of the leaving group is synonymous with an elimination reaction while a nucleophilic attack is also synonymous with a substitution reaction.

Again, S_N2 reaction mechanisms do not proceed with the formation of a carbocation intermediate, but students were talking about the formation of carbocations and nucleophilic attack of carbocations in their explanations. The presence of descriptions of the formation of carbocation intermediates in students’ explanations of S_N2 reaction mechanisms revealed that students had the ‘chemical vocabulary’ but were not using it in the right context. There were other instances where students mentioned different types of reaction mechanisms for the given reaction mechanisms. For example, after drawing an S_N2 reaction mechanism for question 3.b, Akosua’s explanation was as follows: “The mechanism undergoes E1 substitution. The bromine atom is a better leaving group”. This explanation further revealed students’ good knowledge of ‘chemical vocabulary’ but their inability to apply it in the right context. A study by Crandell et al. (2020) made similar assertions that students’ use of organic chemistry vocabulary, such as electrophile, did not necessarily demonstrate the appropriate causal mechanistic reasoning and an understanding of electrostatic or explicit electron movement. Crandell et al. reported that, although most of the students were able to draw mechanistic arrows that corresponded with the mechanism that they explained, quite a few of them explained an S_N2 mechanistic process but drew S_N1 mechanism.

The frequency of partial explanations was also quite significant (ranging from 10 % to 21.4 %). Explanations where students described only a part of the reaction mechanism were classified as partial explanations. Our data set revealed that a lot of students described only the first part of the given S_N1 reaction mechanisms (the leaving of the leaving group leading to the formation of a carbocation intermediate) and mentioned that the halides, for example, Br, are good leaving groups. Other students just mentioned the mechanism through which the reaction proceeded without explaining further. For example, Kwaku, in explaining the S_N1 reaction mechanism in question 4.b just mentioned that “this is an S _N 1 reaction mechanism” and did not provide any further explanation. Although the aforementioned responses of students are good, their inability to provide further and deeper explanations could be a result of students’ rote memorization of the steps involved in the reaction mechanisms, and their ability to identify good leaving groups and reaction mechanisms based on surface features. A significant number of students did not respond to the questions. Students’ inability to provide any response to the questions asked could be due to a lot of reasons, including the fact that they might have been confused about how to answer the questions, and not having the needed vocabulary to express their reasoning, among many others.

A relatively small number of students were able to provide correct explanations (as expected per the requirements of the course and this study) for the reaction mechanisms. Various CER literature have classified students’ descriptions/explanations of organic reaction mechanisms. For this study, Dood et al.’s (2020) classification of students’ explanations into level 1 (description only), level 2 (surface level explanation), and level 3 (deeper level explanation) was used to classify students’ explanation level sophistication. About 97 % of students’ explanations in this study were at level 1, which involved just a description of what was happening in the reaction without any further explanation of why the reaction occurred. For example, a level one explanation to question 2.b provided by Kofi read: “This reaction occurs by an S _N 1 mechanism which takes place in two steps. Br (the leaving group) departs leading to the formation of a carbocation intermediate. The O in EtOH then attacks the carbocation. The resulting oxonium ion is deprotonated by EtOH to give the product”. Some students went further in their explanations to identify the type of carbocation formed (3° carbocation) but could not link or use it to explain why the reaction occurred. Similar explanations (level 1 explanations) were given by students for S_N2 reaction mechanisms.

Only a few students (3 %) were able to provide level 2 explanations for the given reaction mechanisms. Level 2 responses described the reaction mechanisms at a surface level, including explanations of explicit and implicit features of the reaction without detailed explanations (Dood et al., 2020). Both level 1 and 2 explanations could be an indication of the fact that students had memorized trends of the various reaction mechanisms. None of the explanations provided by the students was at level 3, which is also referred to by CER literature as causal mechanistic explanations (Bodé et al., 2019; Crandell et al., 2020). For an explanation of a reaction to be considered to be at level 3, it should provide reasons why the reaction is occurring at a deeper level; including descriptions of implicit features inferred from explicit features, and an explanation of electronic effects (Dood et al., 2020). The fact that students did not provide level 3 (causal mechanistic) explanations for the reaction mechanisms reveals that they had either memorized descriptions of the types of reaction mechanisms or simply relied on surface features to explain the mechanisms, and these findings have been corroborated in the studies of some CER researchers (Galloway et al., 2019; Graulich, 2015).

3.3 Comparison of students’ drawing of mechanisms to explanation of mechanisms

The percentage of correct mechanisms for questions 1.a-4.a (ranging from 1.4 % to 17.1 %), although relatively low, is notably higher than the percentage of correct explanations for questions 1.b-4.b (ranging from 1.4 % to 7.1 %), which seems to indicate that students had a slightly better understanding of drawing the correct mechanisms than providing accurate explanations. This observation, could also, however, be an indication that students could not assign meanings to the arrows that they had drawn, hence, their inability to provide correct explanations. Notably, the no response percentage for questions 1.a-4.a (ranging from 7.1 % to 25.7 %) is lower than the no response percentage for questions 1.b-4.b (ranging from 7.1 % to 35.7 %). This difference in no response rates suggests that more students attempted to answer the mechanism-based questions (1.a-4.a) than the explanation-based questions (1.b-4b), possibly because they found it easier to draw mechanistic arrows than explaining reaction mechanisms.

4.1 Implications for research and practice

The study revealed disparities in students’ drawing of organic reaction mechanisms and written explanations of the reaction mechanisms. It is therefore important that in teaching organic reaction mechanisms, instructors should not only place emphasis on the use of electron-pushing formalisms but should also include explanations of how and why the reactions occur to encourage mechanistic thinking in their students. Students should also be given more opportunities to practice problem-solving in organic reaction mechanisms, including explaining concepts in class to their peers to enable them to gain mastery (Webber & Flynn, 2018). Added to that, in assessing students’ conceptions of organic reaction mechanisms, instructors are encouraged to not only use test items that require students to draw mechanistic arrows but also, use items that require students to explain reaction mechanisms in order to get a wholistic representation of students’ conceptions. Furthermore, this study focused on only simple nucleophilic substitution reactions, other studies could use other types of reactions.