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In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/cerp Page 1 of 23 Chemistry Education Research and Practice 1 2 3 4 Organic Chemistry Students’ Ideas about Nucleophiles and 5 6 Electrophiles: The Role of Charges and Mechanisms 7 8 9 Mary E. Anzovino and Stacey Lowery Bretz* 10 11 Miami University, Department of Chemistry & Biochemistry, Oxford, OH USA. Email: [email protected] 12 13 Organic chemistry students struggle with reaction mechanisms and the electron-pushing formalism (EPF) used by Manuscript 14 15 practicing organic chemists. Faculty have identified an understanding of nucleophiles and electrophiles as one conceptual 16 prerequisite to mastery of the EPF, but little is known about organic chemistry students’ knowledge of nucleophiles and 17 electrophiles. This research explored the ideas held by second-semester organic chemistry students about nucleophiles and 18 19 electrophiles, finding that these students prioritize structure over function, relying primarily on charges to define and identify 20 such species, both in general and in the context of specific chemical reactions. Contrary to faculty who view knowledge of 21 nucleophiles and electrophiles as prerequisite to learning mechanisms and EPF, students demonstrated that they needed to know Accepted 22 23 the mechanism of a reaction before they were able to assess whether the reaction involved nucleophiles and electrophiles or not. 24 25 26 27 Introduction and background 28 Organic chemistry has a well documented history of being a challenging subject for students. Undergraduate students Practice 29 30 are prone to developing a wide variety of alternative conceptions, many of which persist to and beyond graduation. Additionally, 31 students often struggle to develop expert-like ways of thinking about the material, which leads to difficulty in applying prior and 32 knowledge and understanding new concepts. These difficulties have been chronicled in the organic chemistry education research 33 (OCER) literature. 34 35 36 Alternative conceptions 37 A wide variety of alternative conceptions have been documented among organic chemistry students. For example, 38 polarity is a poorly-understood concept: students believe that bond polarity is caused simply by the presence of an electronegative 39 Research 40 atom in a molecule, with no regard for the relative electronegativities of atoms involved in a particular bond (Taagepera and 41 Noori, 2000). A similar way of thinking has been exhibited by students who claim that functional group determines acid strength 42 43 without considering the emergent nature of acid strength as a function of a combination of factors (McClary and Bretz, 2012). 44 Students also hold incorrect ideas that arise from incorrect or inappropriate reliance on rote memorization. 45 Henderleiter, et al. (2001) reported that students in their study generated lists of atoms that could participate in hydrogen bonding 46 47 containing incorrect entries, such as chlorine, that likely arose from such a reliance on memorization. They also believed that Education 48 hydrogen bonding can be induced in hydrocarbons or that it is a type of covalent bond. This type of faulty recall has been 49 demonstrated in studies asking students to predict the products of reactions of alkenes; alternative conceptions in this realm 50 51 include that alcohol products are produced when an alkyl halide and strong base are reacted in an alcoholic solvent, and that 52 ether, ketone, and aldehyde products are produced by reacting water with an alkene in the presence of acid (Şendur, 2012). 53 54 55 Thought processes for concepts related to organic chemistry 56 Understanding students’ thought processes has been the focus of other OCER studies. Acid strength is one concept Chemistry 57 related to reactivity in organic chemistry, and Bhattacharyya (2006) has shown that even graduate students invoke 58 59 60 Chemistry Education Research and Practice Page 2 of 23 1 2 3 unsophisticated mental models of acidity, relying primarily on bond polarization to explain “weakening” of the bond between the 4 5 acidic hydrogen and the atom to which it is bonded. McClary and Talanquer (2011) identified a set of heuristics that 6 undergraduate students used in tasks asking them to rank various organic compounds based on acid strength. The majority of 7 students relied on lexicographic heuristic methods, one prominent example of which involved resonance structures and how they 8 9 contribute to strength of an acid via stabilization of a conjugate base. Although students often correctly attributed a trend in acid 10 strength to this factor, they were not able to articulate how or why a conjugate base having more resonance forms would be more 11 stable. 12 13 Domin and coworkers (2008) asked students to categorize organic compounds that differed on three significant 14 features: functional group (alcohol or ketone), stereochemical configuration at the carbon adjacent to the functional group, and Manuscript 15 whether the compounds were linear hexanols/hexanones or cyclic ones. Students could sort the compounds into any two groups 16 they wished. Most students used functional group as the critical attribute by which to classify the molecules, but struggled to 17 18 articulate why. It is possible that the students were simply grouping the compounds based on patterns of symbols rather than 19 considering their functional behaviors. In another research study, students were asked to select a set of reagents likely to effect a 20 given transformation in a multiple-choice item. The students typically resorted to one of three strategies: attribute substitution 21 Accepted 22 (looking for a familiar reagent set rather than considering the mechanism of reaction between given reactants and each set of 23 reagents), fluency processing (predominant focus on familiar features), and intuitive associations (“X does Y”) (Graulich, 2015). 24 25 26 Reaction mechanisms and related concepts 27 When chemistry majors were presented with multiple-choice items asking them to choose the most likely product given 28 Practice a set of reagents and reaction conditions, the students chose answers by considering product stability rather than evaluating the 29 30 feasibility of the mechanisms that would lead to the possible products (Rushton et al., 2008). Surprisingly, these undergraduate 31 chemistry majors, who were within one semester of graduation, did not consider reaction mechanisms to be important for product 32 and prediction. Grove and coworkers (2012) reported similar findings for students enrolled in introductory organic chemistry who 33 34 failed to use mechanistic thinking to predict the products of reactions - even when explicitly directed to do so. Graduate students 35 in chemistry who were asked to generate mechanisms for organic chemistry transformations simply drew arrows that would 36 result in electron pair and atom placement consistent with the products, without considering the chemical feasibility of the steps 37 38 they proposed (Bhattacharyya and Bodner, 2005). In a separate study, undergraduate chemistry majors enrolled in organic 39 chemistry encountered a variety of barriers to using the curved arrow formalism in a meaningful way when engaging in Research 40 mechanistic tasks (providing mechanisms for transformations, given starting materials and products), including an inability to 41 42 apply information (for example, mentioning the trivalent nature of boron but failing to apply it to evaluate the reactivity of 43 NaBH4) and gaps in content understanding (for example, using a very weak base in a deprotonation step) (Ferguson and Bodner, 44 2008). All of these reports on students’ understanding of mechanistic thinking suggest that the curved arrows that hold so much 45 46 meaning for organic chemists are essentially little more than symbolic decorations to the students, who persist in viewing the 47 mastery of organic chemistry as a Herculean task of memorization rather than one of process-oriented thinking and deep 48 Education conceptual understanding (Grove and Bretz, 2010, 2012). 49 50 To investigate the lack of alignment between faculty and student perspectives on the usefulness and meaning of the 51 arrows and associated electron-pushing formalism (EPF), Bhattacharyya (2013) surveyed faculty, asking them to identify 52 concepts which must be mastered before a student could develop fluency with the EPF. Among the conceptual prerequisites 53 54 identified were electronegativity,