Students' understanding of alkyl halide reactions in undergraduate organic chemistry

This journal is©The Royal Society of Chemistry 2014 Chem. Educ. Res. Pract.

Cite this:DOI: 10.1039/c3rp00089c

Students’ understanding of alkyl halide reactionsin undergraduate organic chemistry

Daniel Cruz-Ramı́rez de Arellanoa and Marcy H. Towns*b

Organic chemistry is an essential subject for many undergraduate students completing degrees in science,

engineering, and pre-professional programs. However, students often struggle with the concepts and skills

required to successfully solve organic chemistry exercises. Since alkyl halides are traditionally the first functional

group that is studied in undergraduate organic chemistry courses, establishing a robust understanding of the

concepts and reactions related to them can be beneficial in assuring students’ success in organic chemistry

courses. Therefore, the purpose of this study was to elucidate and describe students’ understanding of alkyl

halide reactions in an undergraduate organic chemistry course. Participants were interviewed using a think-

aloud protocol in which they were given a set of questions dealing with reactions and mechanisms of alkyl

halide molecules in order to shed light on the students’ understanding of these reactions and elucidate any

gaps in understanding and incorrect warrants that may be present. These interviews were transcribed and

analyzed using a qualitative inquiry approach and a modified Toulmin scheme. In general, the findings from this

study show that the students exhibited gaps in understanding and incorrect warrants dealing with: (1) classifying

substances as bases and/or nucleophiles, (2) assessing the basic or nucleophilic strength of substances, and

(3) accurately describing the steps that take place and reactive intermediates that form during alkyl halide

reaction mechanisms. In addition, implications for teaching and future research are discussed.

Introduction

Organic chemistry is a course that is not only important andmandatory in chemistry and chemical engineering undergraduateprograms, but it is also an integral part in other undergraduatecurricula such as biology, biochemistry, medicine, pharmacy, andnanotechnology. There are many instructional strategies that havebeen developed to aid in the teaching of organic chemistry at theundergraduate level (Bradley, Ulrich, Jones, and Jones, 2002; Browneand Blackburn, 1999; Kingsbury and Schelble, 2001; Horowitz, 2007;Raker and Towns, 2010; 2012a, b). Nonetheless, there is a need forinstructional strategies that are based on rigorous research studiesthat have elucidated student understanding of organic chemistryconcepts. If the undergraduate organic chemistry curriculum wasdeveloped using research-based approaches and tools, it could beargued that instructors would be more successful in having studentstruly comprehend the topics of organic chemistry.

Student understanding of fundamental organic chemistry concepts

Organic chemistry has historically being considered difficult andpressure-packed (Bradley et al., 2002). It has been suggested that

its difficulty originates from the recognition that organic mole-cular reactivity is a function of multiple and interacting variables,such as steric and electronic variables (Kraft et al., 2010). Studentunderstanding of organic chemistry concepts has been eluci-dated to some extent for several topics. For the topic of Lewisstructures, it has been found that students are often confusedabout how to construct valid Lewis structures and are unable torecognize the implicit information that can be determined abouta molecule by not being able to integrate other knowledge, suchas polarity or boiling point, to the explicit information that isshown with Lewis structures (Cooper et al., 2010, 2012). Addi-tionally research has shown that organic chemistry instructorsshould not take for granted that students will necessarily abstracta salient feature from a pictorial representation of an organiccompound (Domin et al., 2008).

For the topic of acids and bases in an organic chemistrycontext, it has been found that students correctly define, giveexamples, and employ acids and bases according to theBrønsted–Lowry definition, but that they struggle with definingand employing acids and bases according to the Lewis defini-tion, which is the most prevalent one in organic chemistry(Cartrette and Dobberpuhl, 2009; Cartrette and Mayo, 2011). Inaddition, it has been found that students’ mental models ofacids and acid strength often rely on heuristic decision-makingand superficial characteristics of the molecules as opposed torelying on coherent cognitive constructions of molecular acidity

a University of South Florida, Department of Chemistry, 4202 East Fowler Ave.,

Tampa, FL 33620, USAb Purdue University, Department of Chemistry, 560 Oval Drive, West Lafayette,

IN 47906, USA. E-mail: [email protected]

Received 15th July 2013,Accepted 24th March 2014

DOI: 10.1039/c3rp00089c

www.rsc.org/cerp

Chemistry EducationResearch and Practice

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http://dx.doi.org/10.1039/C3RP00089C

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Chem. Educ. Res. Pract. This journal is©The Royal Society of Chemistry 2014

(Bhattacharyya, 2006; McClary and Talanquer, 2011). For thetopic of organic reaction mechanisms and the arrow-pushingformalism employed to show electron movement in them, it hasbeen found that the curved arrows used in the electron-pushingformalism hold no physical meaning for many students whenthey are performing exercises that require their employment(Bhattacharyya and Bodner, 2005; Ferguson and Bodner, 2008).

Some studies have focused on elucidating students’ alterna-tive conceptions regarding organic chemistry concepts. An alter-native conception, sometimes described as a misconception,means any concept that differs from the commonly acceptedscientific understanding of the term (Nakhleh, 1992). A list ofalternative conceptions related to organic chemistry that havebeen identified are: (1) the stability of the final products is moreimportant than the feasibility of the reaction mechanismrequired to arrive at said products, (2) hydrogen bonds can beinduced with hydrocarbons, (3) bond polarities depend onabsolute electronegativities of atoms only, whether they areconnected or not, (4) the functional group of a molecule deter-mines its acidic strength, (5) when alkyl halides are heated withstrong bases in the presence of alcohols, alcohols are generatedas the major product, and (6) the addition of water to an alkenein the presence of acid leads to the formations of ketones,aldehydes, and ethers (Taagepera and Noori, 2000; Henderleiteret al., 2001; Rushton et al., 2008; McClary and Bretz, 2012;S- endur, 2012; S- endur and Toprak, 2013).

Several reasons that explain why students struggle todevelop a thorough and coherent understanding of organicchemistry have been proposed. Grove and Bretz (2010)proposed that as students progress in an organic chemistrycurriculum, they perceive that organic chemistry becomesprogressively less straightforward. In particular, this notionwas applied to substitution and elimination reactions wheremore than one product could be produced, thus moving awayfrom an approach where there is one correct answer. As the yearprogressed, students also encountered synthesis problemswhere multiple pathways to a targeted product were possible.

Anderson and Bodner (2008) propose that the intense speedwith which material is covered in an organic chemistry course,along with the complexity of the material, forces students tobecome learners who focus on applying memorized rules, oftenincorrectly, without a coherent understanding of the reasonsfor the rules or when they should be applied.

Purpose of the study

The aim of this study is to elucidate student understanding oforganic chemistry concepts. In this case, the chosen organicchemistry topic is alkyl halide reactions because alkyl halidesare the first functional group that is traditionally covered inorganic chemistry courses. It is through studying the propertiesand reactivity of this functional group that students learn abouttwo of the main mechanisms that become a recurring themethroughout the course: substitution reactions and eliminationreactions. By assuring a sound understanding of this topic, itcould be argued that the foundation upon which the knowledgeof other functional groups is constructed has been strengthened.

Therefore, this strengthened foundation could assure betterperformance in the subsequent topics in an organic chemistrycourse, and possibly in the course as a whole.

To examine and elucidate student understanding of alkylhalide reactions in undergraduate organic chemistry, we askedthe following research question: Which mistakes or gaps inunderstanding emerge when students are asked to predictproducts or mechanisms of reactions involving alkyl halides?

Theoretical framework: personal constructivism

The theoretical framework that was selected for this study ispersonal constructivism (Geelan, 1997). This frameworkemphasizes the idea that individuals construct knowledge forthemselves through construing the repetition of events, andthat knowledge is individual and adaptive rather than objective.Bodner (1986) interpreted personal constructivism in thecontext of chemistry classrooms. He suggested that individualsuse what they already know to organize and make sense of newinformation, in other words, that ‘‘knowledge is constructed inthe mind of the learner’’ (p. 873). Therefore, the only way toelucidate what students know about a certain topic is bydirectly asking them to describe how they have constructedand connected the concepts relevant to that certain topic.

Methods

In order to discover how students had constructed their knowl-edge of alkyl halide reactions, a study was designed in whichparticipants were asked to solve a series of organic chemistryquestions which contained all the essential concepts that arerelevant to the topic of alkyl halide reactions. Semi-structuredinterviews were employed because it gave the researchers theopportunity to see the topic of alkyl halide reactions from theperspective of the participants whose meaning-making wasbeing studied. In addition to using semi-structured interviews,incorporating the use of a think-aloud protocol (Bowen, 1994)during these was essential to understand how the participantshad constructed the knowledge of alkyl halide reactions. In athink-aloud protocol, participants are asked to express vocally,or think aloud, what they are considering and how they areproceeding as they solve a given task through initial questionsor prompts and follow-up questions or prompts to probereasoning. This would permit the researchers to understandthe participants’ thought process in a deep and thoroughmanner, making it possible to pinpoint both the mistakesand gaps in understanding that lead them to incorrect answersand the correct warrants that lead them to correct answers.

Participants and setting

The setting for this study was a large, state-supported, researchintensive university in the Midwestern United States of America.The participants were enrolled in a three-credit undergraduate-level organic chemistry course that is offered every semester.The course was the first semester of a two-semester non-majorsorganic chemistry course. For recruitment, purposeful homogenous

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sampling was employed. Homogenous sampling (Patton, 2002)aims to understand and describe a particular group in depth. Inthe case of this study, the group to be described in depth isorganic chemistry students enrolled in the chosen course.Twenty-two participants were recruited through electronic mailand personal visitations to laboratory sections by one of theresearchers. All the participants were volunteers and were notoffered any compensation for participating in the study. Theparticipants were all undergraduate students, ranging fromtheir second to fourth year of study. The majors of the partici-pants were varied, and mostly included biological sciences,pre-professional programs (pre-medicine, pre-veterinary), healthsciences, and psychology. There were seventeen participants whoself-identified as female and five participants who self-identifiedas male; all were given pseudonyms to protect their identities. Theparticipants were interviewed immediately after the third exam ofthe course, which was the one with the relevant content knowl-edge, during the fall semester of 2011. Approval from the Institu-tional Review Board (IRB) of the university was received in 2010.

Interview questions

The participants were asked to perform nine organic chemistryquestions. These included predicting the major organic productof a given set of reagents, proposing a reasonable mechanism thatexplained a given organic reaction, and conceptual questionsasking about fundamental concepts that are covered prior toalkyl halide reactions. This report focuses on analyzing studentresponses to questions 1a, 1b, and 1c (the first three of the nine)that are shown in Fig. 1. These questions all require students topredict the product of an alkyl halide reaction.

Draw a structural formula for the major organic product ofeach reaction.

Data sources

All the interviews were recorded using a digital audio recorder anda Livescribe pen (Livescribe, 2010). The Livescribe pen is a ‘‘smartpen’’ with an embedded computer and digital audio recorder.When the pen is used with dot-pattern digital paper it recordswhat it writes for later uploading to a computer. When uploaded,

the student’s audio and writing are synchronized into one file.All of the interview questions were printed in the dot-patterndigital paper, one question per sheet of paper, and were presentedto the participants one question at a time. Each participantproposed an answer for the interview question and the interviewerasked follow-up questions regarding the reasoning behind theparticipant’s answer until the interviewer considered to have acomplete picture of the participant’s logic and reasoning behindtheir answer. After all the interviews were conducted, the mainresearcher listened to them and wrote memos for each of themmaking general observations. These memos served as a secondarydata source.

All interviews were transcribed verbatim using the InqScribercomputer software (InqScribe, 2005). The audio recordings andtranscriptions served as primary data sources, since they weretechnically created during the time of the study. In addition, therewas a third primary data source in the form of Livescriber penworksheets. This primary data source proved to be invaluablebecause it permitted the researcher to pinpoint exactly what theparticipants were drawing as they offered their explanations,therefore making sure that their understanding was elucidatedwith reliability. Linenberger and Bretz (2012) and Harle andTowns (2013) described this technology as a research tool andits usefulness when elucidating student understanding of topicsthat involve heavy drawing of diagrams or representations ofany kind.

Data analysis

Questions 1a, 1b, and 1c were analyzed using a modifiedversion of Toulmin’s model of argumentation as a framework(Toulmin, 1958). This methodology was adapted to chemistryeducation research from research on undergraduate mathe-matics education by Cole et al. (2012). Toulmin (1958) created amodel to describe the structure and function of argumentation.The model proposes that the core of an argument consists ofthree parts: the data, the claim, and the warrant. In an argu-ment, the participant makes a claim and presents evidence ordata to support that claim. To further improve the strength ofthe argument, participants often provide more clarification thatconnects the data to the claim, which serves as a warrant, or aconnector between the two. In addition, sometimes rebuttals orqualifiers arise to propel the argument forward. Nonetheless, forthe purpose of this analysis, Toulmin’s model of argumentationwas modified to only include claims, data, and warrants, whichwas deemed sufficient to describe the arguments presented by theparticipants. An illustration of the modified Toulmin’s model ofargumentation can be seen in Fig. 2.

Firstly, the interview transcripts for these three questionswere coded line by line for all the participants using the threeclassifications of the modified Toulmin’s model of argumenta-tion: claim, data, and warrants. The participants’ proposedmajor organic products were coded as their claim. The differentstatements they provided to provide evidence or explain theirreasoning for proposing the products were coded as either dataor warrants, depending on the nature of the statement. Anargumentation scheme was generated for each question, forFig. 1 Questions 1a, 1b, and 1c in the interview protocol.

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each participant. Secondly, all of the participants’ argumenta-tion schemes for each question were analyzed to identifyemergent trends and commonalities, in addition to beingcompared to an expert argumentation scheme generated bythe researchers and a faculty expert in organic chemistry. Thisstep in the analysis aided in identifying gaps in understandingand incorrect warrants exhibited by the participants. Thirdly,the trends and commonalities were used to categorize theparticipants into discrete groups described by the differenttypes of argumentation schemes that were identified. An over-view of this methodological approach is shown in Fig. 3.

Reliability and validity

The researchers met with three additional chemistry educationcolleagues at the beginning of the analysis to collaborativelycode a portion (questions 1a and 1b) of the whole transcriptsusing the modified Toulmin’s model of argumentation. Thiscollaborative coding was carried out across multiple participantsto achieve a consistent use of the coding scheme and involveddiscussing each transcript portion until agreement across allcoders was achieved. Afterwards, based on that exercise, themain researcher independently coded the remaining of thedata. Subsequently, the other researcher and one chemistryeducation colleague revised the coding done by the mainresearcher to confirm agreement with the original codingscheme and coherency. The interview protocol and the expertargumentation schemes were prepared by the main researcher

and corroborated by an organic chemistry faculty member toassure that they were coherent and correct.

Findings and discussion

In order to answer questions 1a, 1b, and 1c, it was expected forthe participants to primarily consider the four main reactions ofalkyl halides. Two of these reactions are substitution reactions:unimolecular nucleophilic substitution reactions (SN1) andbimolecular nucleophilic substitution reactions (SN2). An SN1reaction is a two-step interchange of chemical species, with bondbreaking preceding bond formation. The first step is ionizationto form a carbocation and the second step is the reaction of thecarbocation with a nucleophile. An SN2 reaction is a bimolecularconcerted displacement of one chemical species by another onan sp3 hybridized carbon atom. The two other main reactions ofalkyl halides are elimination reactions: first-order eliminationreactions (E1) and second-order elimination reactions (E2). AnE1 reaction is a multistep elimination where the leaving group islost in a slow ionization step and then a proton is lost in asecond step. The formation of the most substituted alkene isgenerally preferred. An E2 reaction is a concerted eliminationinvolving a transition state where the base is abstracting a protonat the same time that the leaving group is leaving. The anti-coplanar transition state is generally preferred (Wade, 2006). Thethree analyzed questions contained secondary alkyl halides whichwere purposefully chosen to force the participants to consider thepossibility of any of the four aforementioned mechanisms since

Fig. 2 Modified Toulmin’s model of argumentation.

Fig. 3 Summary of methodological approach for data analysis.


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primary and tertiary alkyl halides, unlike secondary alkyl halides,do not usually possess the possibility of undergoing all fourmechanisms. Most participants were able to identify the secondarynature of the alkyl halides; therefore the assessment of thatparticular characteristic of the substrate was not identified in thisstudy to be a significant gap in understanding.

Question 1a

Question 1a, seen in Fig. 1, has three molecules in the correctanswer. Cyclohexa-1,3-diene is the product of an E1 reaction while(S)-3-methoxycyclohexene and (R)-3-methoxycyclohexene are theproducts of an SN1 reaction. The expert argumentation schemefor question 1a is presented in Fig. 4. It is not expected that thestudent participants in this study will use every data and warrantin their arguments because an expert would have a greater depthand breadth of cognitive resources. In the discussion whichfollows students are able to construct arguments consisting of aclaim, data, and warrants that are reasonable and well supportedusing fewer pieces of data and warrants than are listed in Fig. 4.

The participants’ responses to question 1a were categorizedin three groups according to their claims. Group 1 had correctclaims, Group 2 had partially correct claims, and Group 3 hadincorrect claims.

Group 1: correct claim. Group 1 is composed of two parti-cipants, Barbara and Aurora. These participants responded that

there would be three main products, one product through E1and two products through SN1, as shown in the expert argu-mentation scheme. They correctly recognized the dual nature ofCH3OH which can act both as a base and as a nucleophile. Inaddition, these participants correctly assessed the basic andnucleophilic strength of CH3OH by classifying it as a weak baseand a weak nucleophile.

Barbara: . . .methanol is going to be a weak nucleophile, andalso kind of a crappy base, so looks to me like this is gonna be aracemic mixture, or it’s gonna go through SN1 and E1 reaction.

Barbara and Aurora also recognized the formation of astable carbocation during the mechanism, along with thestereochemical implications that this intermediate would havein the products. Knowing the mechanism proved to be animportant aspect of knowing why the SN1 reaction wouldproduce a racemic mixture of products.

Aurora: . . .I did SN1 and that has a carbocation which means it[the nucleophile] can come in from either side.

It is interesting that even though these two participants didnot mention all of the data and warrants in the expert argu-mentation scheme such as the nature of the substrate, theirreasoning generated a well-thought argument.

Group 2: partially correct claim. Group 2 is composed of tenparticipants. Their claims were classified as partially correctbecause they contain one or two of the three expected molecular

Fig. 4 Expert argumentation scheme for question 1a.


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products shown in the expert argumentation scheme. Group 2 hasbeen further divided into three sub-groups according to theirspecific partially correct claims.

Sub-group 2a. Sub-group 2a is composed of three participants,Anna, Dennis, and Isabel, who claimed that an SN1 reactionwould occur, yielding the two substitution products shown in theexpert argumentation scheme. These participants identified thenucleophilic nature of CH3OH and correctly classified it as aweak nucleophile. In addition, they were also aware of thecarbocation intermediate that exists in SN1 reactions, along withthe stereochemical implications for the products.

The participants in this sub-group were missing the productproduced by the E1 mechanism. The gap in understanding whichled to this was that they did not recognize the basic nature ofCH3OH. By not classifying CH3OH as a weak base, in addition to aweak nucleophile, they omitted an elimination product. Two incor-rect warrants were also identified. These incorrect warrants offerinsight as to why these participants did not consider an eliminationreaction as an option in their answer. The first incorrect warrant isthat the presence of a double bond in the starting material impliesthat a substitution is the reaction to be done.

Dennis: And I think that I’ll do a substitution reaction, uh, justbecause there’s a double bond.

The second incorrect warrant is that the presence of asolvent in the starting materials implies that a substitution isthe reaction to be done.

Interviewer: . . .is there any particular reason why you elimi-nated the elimination option?

Anna: I’m thinking because there’s a solvent, most likely.These two incorrect warrants demonstrate that participants

picked up cues from the question, such as which reagents areused and/or displayed, and attribute to them meanings thatwere unintended when the question was composed.

Sub-group 2b. Sub-group 2b is composed of six participants,Allison, Gabriel, Carl, Emily, Sonia, and Amy, who claimed thata substitution reaction would take place, yielding one product.The stereochemistry of the product was expressed either asabsent or with just one of the enantiomers shown in the expertargumentation scheme. The main piece of data used by theseparticipants was that CH3OH is a nucleophile.

These participants did not recognize the basic nature ofCH3OH, leading them to not take into account the formation ofthe elimination product through E1, exhibiting the same gap inunderstanding identified in sub-group 2a. In addition, they didnot assess the nucleophilic strength of CH3OH. Nonetheless,unlike sub-group 2a, they have gaps in understanding regard-ing the mechanism and the carbocation intermediate of thesubstitution reaction. This is why they only suggest the for-mation of one product, instead of two; they did not consider acarbocation intermediate which would produce two enantio-mers as substitution products.

Besides the gaps in understanding, there are several incor-rect warrants exhibited by the participants in sub-group 2b.Initially, there are two incorrect warrants which led participants

in this group to believe that only a substitution reaction wouldhappen. The first incorrect warrant is that the presence of aprotic solvent, as is the case with CH3OH, implies that asubstitution must be done.

Gabriel: I thought it was because if you have a protic solvent,it’s more likely to be an SN reaction than elimination, for a protic.

The second incorrect warrant is that the presence of theparticular CH3OH reagent always means that a substitutionreaction must be done.

Interviewer: Why did you choose a substitution over anelimination?

Sonia: I don’t think that’s what happens with the methanol, Ithink the methanol would come in and attach. . .I think it wouldcome in and replace it.

Finally, there are two incorrect warrants dealing with themechanism and stereochemistry of the SN1 reaction. The firstincorrect warrant is that SN1 reactions only cause inversion ofstereochemistry, as opposed to the racemic mixture that itreally produces. It seems that this participant confused theSN1 and SN2 mechanisms, as what he is describing is actuallyan SN2 mechanism.

Carl: Because. . .SN1, it attacks from the backside so it flips andit makes it go out.

The second incorrect warrant is that a racemic mixture willproduce one product through elimination and one productthrough substitution.

Interviewer: What does racemic mixture mean to you, ifanything?

Allison: That one of them is an SN1, and the other one is an E1.This warrant is incorrect because in a racemic mixture both

products will arise from the substitution mechanism.

Sub-group 2c. Sub-group 2c is composed of two participants,Allison and Robert, who claimed that an elimination reactionwould happen through E1 yielding the elimination productshown in the expert argumentation scheme as a result. Themain piece of data used by these participants was that CH3OHis a base. Even though these participants were able to make thepartially correct claim that an E1 reaction would occur, theyrelied on rote memorization without providing evidence ofdeep understanding about the E1 reaction mechanism.

Robert: I’m trying to remember what the chart says. Um, it’s aweak base and it would be elimination. . .it’s a weak base so Iwanna say that it’s E, it’s E1.

Robert expressed the correct evidence that CH3OH is a weakbase and the correct warrant that weak bases proceed by E1reactions, but he predicated his answer on a memorized chart,not on chemical and physical characteristics about the mecha-nism. A similar situation was observed with Allison.

Interviewer: . . .when you looked at the methanol you immedi-ately said SN1/E1, what is it about methanol that made youimmediately think that?

Allison: Well, in our notes in class she writes that it’s methanol,ethanol, and acetone [which do that].

It can be concluded that Allison and Robert exhibited a gapin understanding regarding the mechanism of the E1 reaction,


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including its carbocation intermediate and stereochemicalimplications. In addition, Robert exhibited a gap in under-standing about CH3OH not stating that it can act as a nucleo-phile in addition to it being able to act as a base.

Group 3: incorrect claim. Group 3 is composed of tenparticipants whose claims were considered incorrect becausethey contain products that are either mechanistically or structu-rally unfeasible given the reaction conditions in the question.Group 3 has been further divided into two sub-groups accordingto their specific incorrect claims.

Sub-group 3a. Sub-group 3a is composed of eight participants,Danisha, Melissa, Susan, Beth, Sabrina, Maggie, Amber, andMark, who claimed that a substitution reaction would occur.Nonetheless, they claimed that the functional group that sub-stitutes is a hydroxyl group (OH), and not the correct methoxygroup (OCH3), yielding cyclohexa-2-en-1-ol as a product. Themain piece of data used by these participants was that CH3OHis a nucleophile.

These participants did not recognize the basic nature ofCH3OH. This shows the same gap in understanding regardingthe dual (nucleophilic and basic) nature of CH3OH exhibited byall the participants in group 2. In addition, they did not speakabout the mechanism of the reaction or about the carbocationintermediate. This shows a significant gap in understandingregarding how substitution reactions take place. The partici-pants’ gap in understanding regarding the mechanistic aspectsof the reaction is further exemplified by the fact that theysubstituted with the OH group instead of the OCH3 group.This demonstrates gaps in understanding pertaining to thenature of covalent and ionic bonds, as well as interactionsbetween the reagents at the molecular level.

There are two specific incorrect warrants that have beenidentified which the participants used to justify their claim thatthe final product would have an OH group. The first incorrectwarrant is that CH3OH behaves as an Arrhenius base.

Interviewer: . . .it seems like, just by looking at it you know thatthe OH was the group to be added. How did you know that?

Beth: [the professor says] the ions attached to the front of it aren’timportant, she just puts [it] in there sometimes and sometimesdoesn’t, so we kind of only look at the, like, anion part, I guess.

From the way Beth explains her reasoning, it is evident that sheviews CH3OH as an ionic compound that is able to producehydroxide ions, instead of a covalent compound. The secondincorrect warrant which was identified is that if the nucleophilehas an OH group, that one is necessarily the group that substitutes.

Interviewer: . . .by looking at the methanol, how did you knowthat the group that was gonna be substituted was gonna be the OH?

Maggie: That’s just out of habit from lecture, it was just alwaysOH.

Apart from the OH group issue, two additional incorrectwarrants were identified in this group. The first of theseincorrect warrants was that if a starting material shows stereo-chemistry, the product must have an inverted stereochemistry.

Melissa: . . .since this is shown going one way, I would figurethat they’re doing SN2 ’cause they want you to do the inversion of

stereocenters, so I would just kind of assume that you would dothat.

The second of these incorrect warrants is that weak basesproceed by SN2 reactions and strong bases proceed by SN1reactions.

Danisha: . . .SN1 is the one that has the strong base, so I believeit would be SN2.

Interviewer: It would be SN2 because it’s not [a] super strongbase?

Danisha: Yes.Not only are the actual reaction pathways inverted in this

statement, but Danisha uses the word ‘base’ instead of the word‘nucleophile’ while speaking about nucleophilic substitutions.

Sub-group 3b. Sub-group 3b is composed of two participants,Sally and Brenda, who claimed that a substitution reactionwould take place yielding one where the CH3OH moleculebonded through the carbon replaces the chlorine (we note thatthey created a compound wherein the carbon has formed fivebonds). Despite further probing during the interview, partici-pants did not offer any data or warrants to support their claim.The claim offered demonstrate that the students do not under-stand how to construct proper Lewis structures, which is anessential skill in the undergraduate organic chemistry curri-culum. Sally exhibits a disagreement between her verbal expla-nation and her drawn structure, which exhibits this significantgap in understanding.

Interviewer: . . .would you tell me what specifically is the basicsite in that molecule [methanol]?

Sally: The O.Even though Sally seems to recognize that the oxygen atom

is the nucleophilic site in CH3OH, she draws her Lewis struc-ture as if the carbon is the nucleophilic site. The otherparticipant, Brenda, is more explicit about her confusion.

Brenda: I don’t know then how to go about it. . .the final productto me would be. . .that’s an ugly structure. . .but I know that’s,there’s no way that’s right.

Brenda seems to be aware that something in her claim isincorrect, but is unable to use her understanding of chemistryto amend it. All of the identified gaps in understanding andincorrect warrants for question 1a are listed in Table 1.

Question 1b

Question 1b, seen in Fig. 1, has one molecule as the correctanswer. (S)-3-Iodocyclopentene is the product of an SN2 reac-tion. The expert argumentation scheme for question 1b can beseen in Fig. 5.

Similar to question 1a, the participants’ responses to ques-tion 1b were categorized into three groups according to theirclaims. Group 1 had correct claims, Group 2 had partiallycorrect claims, and Group 3 had incorrect claims.

Group 1: correct claim. Group 1 is composed of nineparticipants: Sonia, Barbara, Isabel, Robert, Amy, Sabrina,Melissa, Anna, and Aurora, who correctly claimed that therewould be one main product through an SN2 reaction, as shownin the expert argumentation scheme. They correctly recognized


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that iodide is a strong nucleophile and that acetone is a polar,aprotic solvent. They also expressed the warrant that strongnucleophiles usually react through SN2 mechanisms. Further-more, they showed a thorough understanding of the concertednature of the SN2 mechanism and the stereochemical implica-tions this would have in the product.

Barbara: So the SN2 is going to have inversion of the stereo-chemistry, so we are going to do that, because it kicks, SN2 isgoing to come in at the um, to the alpha carbon, at the sametime the leaving group gets kicked out causing inversion ofstereochemistry.

As it was the case with question 1a, these participants didnot mention absolutely all of the data and warrants in the

expert argumentation scheme. Although they were still able toprovide a correct claim while exhibiting sufficient understand-ing, there is one significant gap in understanding that wasidentified. The participants did not mention that in addition toiodide being a strong nucleophile, it is also a weak base. Thispiece of information is important because it justifies why anelimination reaction does not take place.

Group 2: partially correct claim. Group 2 is composed ofeleven participants. Their claims were classified as partiallycorrect because of incorrect or absent stereochemistry in theirproduct. Group 2 has been further divided into two sub-groupsaccording to their understanding as evidenced by theirarguments.

Table 1 Gaps in understanding and incorrect warrants for question 1a

Question 1a

Gaps in understanding:1. Do not recognize that CH3OH is a base2. Do not classify CH3OH as a weak base3. Do not know the mechanism of the E1 reaction, including its carbocation intermediate and stereochemical implications4. Do not recognize that CH3OH is a nucleophile5. Did not classify CH3OH as a weak nucleophile6. Do not know the mechanism of the SN1 reaction, including its carbocation intermediate and stereochemical implications7. Do not understand the difference between covalent and ionic bonds8. Do not understand the acid–base step in the SN1 reaction through which the CH3OH loses its proton to another CH3OH molecule to become theOCH3 group in the final product9. Do not understand how to construct proper Lewis structures10. Do not recognize that the carbocation intermediate had an alternate resonance structure

Incorrect warrants:1. The presence of a double bond in the starting material implies that a substitution is the reaction to be performed.2. The presence of a solvent in the starting materials implies that a substitution is the reaction to be performed.3. The presence of a protic solvent implies that a substitution reaction must be performed.4. The presence of CH3OH as a reagent always implies that a substitution reaction must be performed.5. SN1 reactions only cause inversion of stereochemistry in the product.6. A racemic mixture will produce one product through an elimination reaction and one product through a substitution reaction.7. CH3OH behaves as an Arrhenius base8. If the nucleophile has an OH group, that one is necessarily the group that substitutes in a substitution reaction.9. If a starting material shows stereochemistry, the product must have an inverted stereochemistry.10. Weak bases proceed by SN2 reactions and strong bases proceed by SN1 reactions.

Fig. 5 Expert argumentation scheme for question 1b.


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Sub-group 2a. Sub-group 2a is composed of five participantsSally, Carl, Beth, Allison, and Mark who claimed was that an SN2reaction would yield the correct product. These participants classi-fied iodide as a strong nucleophile and expressed the warrant thatstrong nucleophiles perform SN2 reactions, leading them to maketheir claim. However, these participants had incorrect or absentstereochemistry in their product, which exposes a gap in under-standing regarding the concerted mechanism of SN2 reactions andit stereochemical implications. Two incorrect warrants were identi-fied which explain why the stereochemistry was incorrect in thissub-group’s claim. The first incorrect warrant is that SN2 reactionsconserve the stereochemistry of the starting material.

Mark: I think the stereochemistry would be the same, I think it’sjust the iodine switches with the bromine.

Mark believes the iodide and bromide would switch withoutany stereochemical effects demonstrating an erroneous under-standing of the SN2 mechanism. The second incorrect warrantis that elimination reactions are the only ones that affect thestereochemistry of the product.

Beth: . . .elimination is the one that affects stereochemistry, somaybe in this case I would keep it going back like the Br was, and Iwould just say that it’s SN2.

Thus, the claims of the students included incorrect orabsent stereochemistry because of their lack of understand ofthe bimolecular nature of the mechanism.

Sub-group 2b. Sub-group 2b is composed of six participantsDanisha, Susan, Maggie, Dennis, Amber, and Emily who claimedthat a substitution reaction would happen, yielding the correctproduct with unspecified stereochemistry. Even though they men-tion that a substitution reaction would take place, they do notspecify which kind of substitution reaction. The main aspect thatdistinguishes this sub-group from sub-group 2a is that theseparticipants did not express any deep understanding or knowledgeto justify their claim. Their only piece of shared data across the sixparticipants was that the iodide group reacts with the alkyl halide.

These participants exhibited gaps in understanding regardingmost aspects of the reaction. It followed that they did not specifystereochemistry in the product since they did not know the details ofthe reaction mechanism. These students relied on the superficialaspects or surface features of the representations of the molecules andobserved patterns that they recalled from lectures and homework.

Susan: . . .what’s acetone, like I don’t know what that is . . .I’m gettingrid of Br ’cause we always get rid of Br. . .Br’s gonna float off somewhere,it’s got a plus or a minus, I’m not sure which, I think minus maybe. . .NaI,it’s hard to memorize every single these things and these things which,which one does more likely, like E2 or SN1 and whatnot. . .acetone justhelps it out somehow by giving it electrons, or taking away electrons,I don’t think the whole NaI goes there, it’s probably just I.

Interviewer: Why did you just put the I?Susan: Because, from experience, the NaI generally isn’t all

there, it’s just the I because that’s gonna, the protons and electronsare gonna be given to something else somehow, they’re gonna be,you know, I don’t know what I’m doing.

Strangely, a partially correct claim can be offered even whenthe participant has very little understanding of the relevantchemical and physical concepts involved in the question.

Group 3: incorrect claim. Group 3 is composed of two partici-pants, Gabriel and Brenda. Gabriel’s claim was that iodide wouldact as a base, yielding cyclopenta-1,3-diene as a product. Thisclaim is incorrect because iodide is a weak base which would notbe able to perform an elimination reaction. Gabriel correctlyclassified the solvent, acetone, as an aprotic solvent. Nonetheless,he expressed the incorrect warrant that the presence of an aproticsolvent implies that an elimination reaction is going to occur.

Interviewer: So what does aprotic mean to you?Gabriel: To me it means elimination.Brenda’s claim was that sodium would be the species that

would substitute, therefore acting as a nucleophile, instead ofthe iodide.

Brenda: Sodium, let’s put sodium here, yup, I’m just gonna do that.It seemed that Brenda was guessing what to do in the question.

Nonetheless, her poor general understanding was evidenced bythe fact that she used the sodium cation, an electrophile, as anucleophile. All of the identified gaps in understanding andincorrect warrants for question 1b are listed in Table 2.

Question 1c

Question 1c, seen in Fig. 1, has one molecule as the correctanswer. (S)-3-(1-Methylethyl)cyclohexene is the product of an E2reaction. The expert argumentation scheme for question 1c canbe seen in Fig. 6. The chair confirmation of the alkyl halide isshown in Fig. 7 identifying the anti and coplanar hydrogenavailable for the E2 mechanism and the syn and coplanar

Table 2 Gaps in understanding and incorrect warrants for question 1b

Question 1b

Gaps in understanding:1. Do not recognize that iodide is a weak base2. Do not recognize that polar, aprotic solvents are useful for reactions with concerted mechanisms such as SN23. Do not understand the concerted mechanism of SN2 reactions and its stereochemical implications4. Do not recognize that iodide is a strong nucleophile

Incorrect warrants:1. The solvent plays no role in SN2 reactions.2. SN2 reactions conserve the stereochemistry of the starting material.3. Elimination reactions are the only ones that affect the stereochemistry of the product.4. The presence of an aprotic solvent implies that an elimination reaction is to be performed.5. A sodium cation can act as a nucleophile.


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hydrogen which is not. Even though the consulted expert facultymember agreed that the main organic product would be the oneshown in Fig. 6, he pointed out that there are other side productsthat could possibly be formed in the reaction. It was indicatedthat there is the minor possibility of the bromide group acting asa leaving group which would form a carbocation on the carbonatom which had the bromide group. The resulting carbocationwould be secondary; therefore it has the possibility of rearran-ging to a tertiary carbocation in the carbon atom with the1-methylethyl group through a 1,2 hydride shift. These twocarbocations could give rise to five side products through SN1and E1 mechanisms. These side products would have beencategorized as partially correct claims if they had been justifiedwith the appropriate evidence and warrants. Nonetheless, theparticipants who proposed any of these side products did notexhibit a thorough understanding of the relevant chemical con-cepts, as evidenced by incomplete sets of evidence and incorrectwarrants. Most importantly, none of these participants mentionedthe crucial carbocation intermediate to justify their claim. There-fore, it can be concluded that these participants’ claims were notproposed because of the argument offered by the expert facultymember, and their arguments can be analyzed by comparingthem to the agreed-upon expert argumentation scheme.

As with the other two questions, the participants’ responsesto question 1c can be categorized in two groups according totheir claims. Group 1 had correct claims while Group 2 hadincorrect claims.

Group 1: correct claim. Group 1 is composed of six partici-pants. This group has been further divided into two sub-groups

according to their exhibited level of understanding as shown byeach sub-group’s shared arguments.

Sub-group 1a. Sub-group 1a is composed of three participantsAurora, Barbara, and Emily who correctly claimed that therewould be one main product through an E2 reaction, as shown inthe expert argumentation scheme. They correctly classifiedmethoxide as a strong base and stated that strong bases performE2 reactions. Furthermore, they correctly expressed that theproduct would be the least substituted alkene because theproton abstraction happens in an anti-coplanar fashion. Onceagain, these participants did not mention absolutely all of thedata and warrants in the expert argumentation scheme, but wereable to express a correct well-reasoned argument. Barbara wasthe participant who exhibited the most thorough understandingof what happens in the reaction.

Barbara: I look at this and see a secondary carbon. . .here youhave methoxide and sodium, so when I see methoxide, that says

Fig. 6 Expert argumentation scheme for question 1c.

Fig. 7 Chair structure of alkyl halide in question 1c.


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strong base. . .so because it’s strong, bases tend to favor eliminationso this is gonna go through an E2 mechanism, so we’re going toeliminate. So they should be anti and coplanar. . .so then it shouldpull off this one because the bromine and the hydrogens are antiand coplanar, so that’s the one exception that we learned about.

The exception to which Barbara referred addresses the factthat this reaction produces the least substituted alkene as aproduct, as opposed to the most substituted alkene which isusually the case due to the increased stability of substitutedalkenes. The so-called exception is due to the anti-coplanarfashion of the mechanism, which Barbara and the other twoparticipants mention, but it is also due to the concerted fashionof the mechanism. The notion that the mechanism is concertedmay be implied when they express that the mechanismhappens in an anti-coplanar fashion, but none of them explicitlymention this concept which suggests a gap in understandingabout the concerted fashion of the mechanism. Another gap inunderstanding exhibited by this sub-group is that they do notmention why an SN2 reaction is not viable in this case, with theexception of Aurora who actually expresses the incorrect warrantthat strong bases perform both E2 and SN2 reactions simulta-neously, without taking into consideration the steric effects ofthe 1-methylethyl group. Moreover, she chooses the SN2 productas the main one in her proposed E2–SN2 mixture.

Aurora: I know that this is a strong base. . .I would do SN2 and E2.Interviewer: Ok. If you had to choose between, a main product

between the SN2 product and the E2 product, what would youchoose? Or you can say that they would both be equal, that’s alsoacceptable.

Aurora: I would choose SN2.Interviewer: Is there any particular reason why?Aurora: I don’t know, I always just think of E2 being bulky bases

to eliminate.From this interaction it can be seen that Aurora also exhibits

the incorrect warrant that E2 reactions always employ bulkybases. Even though Aurora chose the SN2 product as the mainone when forced to choose, she was classified in this sub-groupbecause she did propose the correct product as one of heroptions and, most importantly, because she expressed thecorrect warrant for proposing that product, which is the anti-coplanar fashion of E2 mechanisms. Another incorrect warrantexpressed by Emily demonstrated a lack of understandingabout the E2 reaction mechanism.

Emily: . . .the Br is going to be my leaving group, the two electronsin here are gonna come off, and then this Na plus is going to attractthe Br negative and they’ll form that bond. . .so I’m gonna have apositive, I guess, carbocation there, and the CH3O minus.

Emily proposed that the mechanism has a carbocationintermediate, even though she expressed that the reactionproceeded by an E2 pathway.

Sub-group 1b. Sub-group 1b is composed of three partici-pants Maggie, Carl, and Robert who correctly claimed that therewould be one main product through an elimination reaction, asshown in the expert argumentation scheme. The participantsmade the correct claim and justified it using only one piece of

data, that methoxide is a base. None of the students expressedany correct warrants connecting the claim and data. Theseparticipants exhibited gaps in understanding regarding mostaspects of the reaction. The first incorrect warrant is that duringthe elimination the hydrogen atom is abstracted from the carbonatom that has the most hydrogen atoms bonded to it.

Interviewer: Is there any reason why you put the double bondbetween these two carbons [forming the least substituted alkene]instead of these two carbons [forming the most substituted alkene]?

Maggie: I think it’s easier to take off that hydrogen, ’causethere’s more hydrogens on that carbon.

This warrant is incorrect because the decision of whichhydrogen atom to abstract should be based on the geometryof the transition state and reactive intermediates of the reac-tion, and on the stability of the resulting alkene. The secondincorrect warrant is that the presence of stereochemistry in thealkyl halide implies that the least substituted alkene must beformed as a product.

Robert: So then, I remember something in our notes, it saidnormally you would take the hydrogen off the most substituted,um, substituent, but if you have stereochemistry then I believe it’soff the opposite one.

The final incorrect warrant that was identified is that theelimination reaction must happen as far away as possible fromthe most substituted carbon atom.

Interviewer: Ok, so what made you decide to do the eliminationin the carbon far, in the carbon farthest away from the isopropylgroup, instead with the one who has the isopropyl group?

Carl: Um, because you want the alpha carbon to be further awayfrom the more substituted group, so instead of having it right here,with these two coming off, you put it down here, where it’s lesssubstituted.

This warrant is also incorrect because it does not take intoconsideration the reaction mechanism or the stability of theresulting product. Across this set of participants there is confu-sion about the way in which this concerted mechanism takesplace and they do not recognize that the H and the leaving groupmust be anti-coplanar position. The findings from sub-groups 1aand 1b for question 1c provide more evidence for the assertionthat it is possible for participants to offer acceptable claims whileexhibiting gaps in understanding and incorrect warrants aboutfundamental concepts of the reaction in the question.

Group 2: incorrect claim. Group 2 is composed of sixteenparticipants. Their claims are considered incorrect becausethey contain products that are not considered the main organicproduct given the reaction conditions in the question andbecause of the lack of relevant warrants to justify their claims.Group 2 has been further divided into two sub-groups accord-ing to their specific incorrect claims.

Sub-group 2a. Sub-group 2a is composed of six participantsAllison, Sabrina, Anna, Sonia, Mark, and Amy who claimed thatan elimination reaction would produce the most substitutedalkene, between the carbon atom that had the bromide groupand the carbon atom with the 1-methylethyl group. Theseparticipants recognized the basic nature of the methoxide


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group. The only warrant these participants had in common wasthat the most substituted alkene must always be formed whenperforming an elimination reaction. While it is correct that themost substituted alkene would be the more stable eliminationproduct, it is not always the product that will be formed giventhat there are mechanistic and steric issues that must be takeninto consideration. Given the reaction conditions in question1c, the most favored pathway was an E2 reaction. Through thispathway, the only mechanistically viable product is the leastsubstituted alkene. Nonetheless, all of the participants insub-group 2a expressed the incorrect warrant that the moststable alkene will always be formed.

Sonia: I put the double bond between these two because it’s, thisside has the most substituents, so it wants to form between themore stable.

This incorrect warrant brought to light several gaps inunderstanding these participants have about eliminationreactions. The main one is a general lack of knowledgeregarding the mechanism of E2 reactions, including thetransition state, the anti-coplanar proton abstraction, andthe stereochemical implications of these two aspects. Further-more, there is no assessment of the strength of the base andno association of base strength with type of eliminationreaction to be performed.

Additionally, there are two other incorrect warrants identi-fied in this sub-group. The first one is that the absence of adouble bond in the alkyl halide implies that an eliminationreaction is to be performed.

Amy: Ok, um, so in this case unlike the other cases, I guess Iwouldn’t immediately think substitution.

Interviewer: Why?Amy: Um, I guess because. . .just the fact that this doesn’t have a

double bond.The second incorrect warrant is that any structurally viable

alkene is possible when performing an elimination reaction.

Interviewer: Ok, let me ask you this, in your eliminationproduct, did you realize you had two options? You could’ve donethe double bond here or here.

Allison: Yes.Interviewer: Why did you choose that spot?Allison: I guess you could put it both ways; it doesn’t really

matter because you need four bonds to complete it, and [in] bothareas, that’s available.

Interviewer: So then you’re saying that both alkenes would’vebeen valid?

Allison: Mmhmm [yes].Allison did not consider what mechanistic pathway the

reaction could take; she is only considered the structuralviability of the end product.

Sub-group 2b. Sub-group 2b is composed of ten participantsIsabel, Melissa, Dennis, Brenda, Beth, Danisha, Amber, Gabriel,Susan, and Sally who claimed was that the methoxide group wouldact as a nucleophile in a substitution reaction. These participantsrecognized that methoxide can act as a nucleophile but they did notrecognize the more probable basic behavior of this reagent. There-fore, they opted to use methoxide as a nucleophile in a substitutionreaction instead of as a base in an elimination reaction. They hadno expressed warrants in common; even though all of themperformed a substitution reaction, there is no coherent, commonreasoning for this claim. Since substitution reactions are routinelytaught before elimination reactions, it might be the case that theparticipants defaulted to a substitution reaction just because theywere unsure of how to proceed; nonetheless, the participants didnot explicitly mention this as the reason for their claim.

Three incorrect warrants were identified that help explainwhy the participants in this group chose to do a substitutionreaction over an elimination reaction. The first incorrect warrantis that the explicit depiction of separate charges in the sodiummethoxide reagent implied that a substitution must occur.

Table 3 Gaps in understanding and incorrect warrants for question 1c

Question 1c

Gaps in understanding:1. Do not understand the concerted fashion of the E2 mechanism2. Do not understand the anti-coplanar proton abstraction in the E2 mechanism3. Do not classify the bromide as a good leaving group4. Do not recognize that an SN2 reaction is not viable because of steric hindrance5. Do not recognize that OCH3 is a base6. Do not classify OCH3 as a strong base7. Do not recognize that strong bases proceed through E2 reactions

Incorrect warrants:1. Strong bases perform both E2 and SN2 reactions simultaneously, regardless of steric hindrance.2. E2 reactions always employ bulky bases.3. E2 reactions have a carbocation intermediate.4. During an elimination reaction, the hydrogen atom is abstracted from the carbon atom that has the most hydrogen atoms bonded to it.5. The presence of stereochemistry in the alkyl halide implies that the least substituted alkene must be formed as a product.6. Elimination reactions must happen as far away as possible from the most substituted carbon atom.7. The most stable alkene will always be formed during an elimination reaction.8. The absence of a double bond in the alkyl halide implies that an elimination reaction is to be performed.9. Any structurally viable alkene is possible when performing an elimination reaction.10. The explicit depiction of separate charges in an ionic reagent implies that a substitution reaction must be performed.11. The presence of stereochemistry in the alkyl halide implies that an SN2 reaction is to be performed.12. Elimination reactions only happen with bulky bases.


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Dennis: . . .it has the charges separate, and I remember lookingin the book and when the charges were separate, we did thesubstitution.

This incorrect warrant seemed to be another example inwhich a participant makes an incorrect generalization fromother questions they have seen in lecture and homework. It alsodemonstrates the fragmented nature of the tapestry of knowl-edge that the students construct. The second incorrect warrantis that if stereochemistry is shown in the alkyl halide, then anSN2 reaction is to be performed.

Gabriel: In this case it would be SN2.Interviewer: Why would you choose to do an SN2?Gabriel: Uh, like the first time it was very familiar with the

solvent being protic, and then with the stereochemistry beingshown here I just assume that it’d be an SN2 again.

Gabriel was also a member of sub-group 2b for question 1a,in which it was shown that he expressed the incorrect warrantthat protic solvents imply a substitution reaction must be made.In this case, he used that warrant again and added the fact thatthe stereochemistry is shown in the alkyl halide as a secondwarrant to justify his claim that a substitution reaction is goingto be produced. Finally, the third incorrect warrant that wasidentified is that eliminations only happen with bulky bases.

Isabel: . . .it would’ve been elimination but it’s not stericallyhindered.

Interviewer: And bases used for elimination usually should besterically hindered?

Isabel: Yeah, the nucleophile should be sterically hindered.As Isabel correctly assessed, methoxide is not a sterically

hindered base. Nonetheless, her warrant is incorrect becauseelimination reactions can proceed with both sterically hinderedand unhindered bases. All of the identified gaps in under-standing and incorrect warrants for question 1c are listed inTable 3.

Concluding remarks

The objective of this study was to examine and elucidatestudents’ understanding of alkyl halide reactions in undergrad-uate organic chemistry. This study provides support for thisobjective by identifying gaps in understanding and incorrectwarrants exhibited by undergraduate students when solvingorganic chemistry questions about alkyl halides. There areseveral conclusions regarding how the students constructedtheir knowledge of alkyl halide reactions that can be reachedfrom the findings of the study.

One of the most important aspects of understanding alkylhalide reactions is being able to classify substances either asbases that are able to abstract a proton in elimination reactionsand/or as nucleophiles that are able to react with an electro-philic carbon atom in nucleophilic substitution reactions; thisincludes the ability to distinguish between basicity and nucleo-philicity. In addition, it is important to have the ability to assessthe strength of the bases and nucleophiles by making thedistinction between strong and weak species. The inclusion

of these pieces of data proved to be a distinguishing factorbetween groups with correct answers and groups with incorrectanswers. In addition, the arguments for the groups with correctanswers contained the correct warrants that connected the kindof reagent with the kind of reaction it performs (e.g. weaknucleophiles perform SN1 reactions), while the arguments forthe groups with incorrect answers contained either incorrector no warrants about this connection. Therefore, it can beconcluded that the inclusion of this specific type of correctwarrant that connects the kind of reagent with its inherentphysical and chemical characteristics with the type of reactionit performs is a distinguishing factor between groups withcorrect answers and groups with incorrect answers.

Another important aspect of understanding alkyl halidereactions is being able to describe the steps of the mechanisms,including the reactive intermediates or transition states. Thearguments for the groups with incorrect answers containedlittle or no information about the steps of the mechanisms ofthe reactions. Therefore, the inclusion of specific informationabout the mechanisms of the reactions, including the reactiveintermediates and transition states, is a distinguishing factorbetween groups with correct answers and groups with partiallycorrect or incorrect answers.

Another conclusion that can be reached when looking acrossthe incorrect warrants voiced by the students is that they oftenapply rules memorized from particular examples to new ques-tions which they deem to be similar based upon surfacefeatures of the molecules. By following these heuristics, thesestudents incorrectly apply rules that are not generalizable toother questions, which lead them to incorrect answers.

Implications for instruction

In conclusion, the findings from this study suggest that studentswho propose incorrect answers to organic chemistry questionsabout alkyl halide reactions exhibit significant gaps in under-standing and incorrect warrants mainly dealing with: (1) classi-fying substances as bases and/or nucleophiles, (2) assessing thebasic or nucleophilic strength of substances, and (3) accuratelydescribing the steps that take place and reactive intermediatesthat form during alkyl halide reaction mechanisms.

The findings from this work demonstrate that students havedifficulties assessing the acidic/basic or electrophilic/nucleophilicnature and strength of the reagent. To scaffold students’ devel-oping understanding of reaction mechanisms, it is importantthat instructors help students learn how to make these assess-ments and distinctions in detail, before teaching specific reactionmechanisms involving alkyl halides. Effective pedagogicalapproaches include small group discussions where studentsconcepts and understandings of acids/bases and electrophiles/nucleophiles are considered. As students describe their under-standings and compare them across group members they havethe opportunity to refine their knowledge to a more sophisticatedunderstanding.

The state of their evolving understanding could be assessedby using an tool that contains a list of ten to fifteen variedreagents, including acids, bases, electrophiles, and nucleophiles,


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and asks students not only to classify them using one of thosefour categories and assess their strength, but also to provide anexample of a chemical reaction that illustrates the classificationthat they assigned to the reagent. Again, this instrument couldbe used as a basis for classroom discussion and refinement ofconcepts. When instructors explain mechanisms, it would bebeneficial to require students to explicitly make these classifica-tions when answering a question that asks them to propose themajor organic product or mechanism of a reaction involvingalkyl halides. Thus, connecting the classifications in the assess-ments, instructors could better pinpoint and address the sourceof any mistakes by the students.

In addition, this study demonstrates that students havedifficulties when connecting which types of reagents performspecific types of reactions (e.g. weak nucleophiles perform SN1reactions). It would be beneficial to ask students to providereasons which support a proposed major product or mecha-nism particularly the chemical and physical characteristics ofmolecules and the steps and intermediates of reaction mechan-isms. That way, instructors can identify any incorrect warrantsexhibited by the students and respond to them in a timelyfashion that helps to refine student knowledge. It is alsoimportant to provide pedagogical support for the notion thatone starting compound reacting with one reagent can yieldmultiple products as Grove and Bretz (2010) have noted. Class-room discussions where students consider possible productsaccompanied by reasons why they would be produced wouldallow students to contextualize ad refine their knowledge.

This study also demonstrates that students can propose acorrect major organic product for a given set of reagents with-out understanding completely the physical and chemical char-acteristics of the molecules and the mechanism through whichthe product was formed. If the learning objectives involvestudents understanding the mechanism along with being ableto identify the major products, then requiring the students topropose a mechanism which details their reasoning would be amore valid assessment than simply suggesting major products.Setting the classroom expectation that reasoning is requiredand discussion is part of the normal activities should helpstudents refine and reorganize their knowledge of organicchemistry.

Implications for research

This study elucidated students’ understanding of alkyl halidereactions in undergraduate organic chemistry. After such eluci-dation, a possible next logical step in research could be to usethe findings described in this study to build a diagnosticinstrument for alkyl halide reactions. The instrument could beconstructed employing multiple-choice questions by using thegaps in understanding and incorrect warrants that have beenidentified as a guide, incorporating student quotes from thisstudy as distractors in the test items. This design would enableinstructors to identify the corresponding incorrect warrant thatprompted the students to make an incorrect choice, so that theycan correct it in a timely fashion. Once the instrument isdesigned and validated, subsequent research projects could

verify if the usage of such an instrument by an instructorimproves student performance in the first semester of under-graduate organic chemistry courses.

Beyond instrument design, this study points the waytowards possible pedagogical initiatives based upon scaffoldingstudent learning of alkyl halide reactions and their associatedmechanisms. Using this study to design more effective andevaluate pedagogies, whether they be learning progressions orportions of a spiral curriculum, and to evaluate these pedago-gical approaches is another area for further research.

In addition, this study showed that the methodologicalapproach described by Cole et al. (2012) can be applied tochemistry education research projects aimed at elucidatingstudents’ understanding of topics that require students tomake a claim and support it with evidence and warrants, suchas proposing the major product of an organic reaction. Byapplying a modified version of Toulmin’s model of argumenta-tion, gaps in understanding and incorrect warrants that leadstudents to make mistakes when answering organic chemistryquestions were identified. In the same manner, future researchprojects can apply this methodology to elucidate students’understanding of the reactions of any other functional groupin organic chemistry, as well as of any other topic in chemistrythat in some manner requires students to make claims andsupport them with evidence and warrants.

References

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Bhattacharyya G. and Bodner G. M., (2005), ‘‘It gets me to theproduct’’: how students propose organic mechanisms,J. Chem. Educ., 82, 1402–1407.

Bodner G. M., (1986), Constructivism: a theory of knowledge,J. Chem. Educ., 63, 873–878.

Bowen C. W., (1994), Think-aloud methods in chemistry educa-tion: understanding student thinking, J. Chem. Educ., 71,184–190.

Bradley A., Ulrich S. M., Jones M. and Jones S. M., (2002),Teaching the sophomore organic course without a lecture.Are you crazy? J. Chem. Educ., 79, 514–519.

Browne L. M. and Blackburn E. V., (1999), Teaching introduc-tory organic chemistry: a problem-solving and collaborative-learning approach, J. Chem. Educ., 76, 1104–1107.

Cartrette D. P. and Dobberpuhl M. R., (2009), The conceptinventory as a diagnostic of understanding and achievementin a year-long organic chemistry course, Chem. Educ., 14,1–10.

Cartrette D. P. and Mayo P. M., (2011), Students’ understandingof acids/bases in organic chemistry contexts, Chem. Educ.Res. Pract., 12, 29–39.


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Students' understanding of alkyl halide reactions in undergraduate organic chemistry