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A deep understanding of organic chemistry requires a learner to understand many concepts and have fluency with multiple skills. This understanding is particularly necessary for constructing and using mechanisms to explain chemical reactions. Electrophilicity and nucleophilicity are two fundamental concepts to learning and understanding reaction mechanisms. Prior research suggests that learners focus heavily on explicit structural features ("e.g.," formal charge) rather than implicit features ("e.g.," an open p-orbital) when identifying and describing the role of electrophiles and nucleophiles in reaction mechanisms; however, these findings come from small-scale, interview-based investigations with a limited number of reaction mechanisms. The work reported herein seeks to further explore the meaning learners ascribe to electrophiles and nucleophiles by evaluating 19 936 written explanations from constructed-response items asking "what" is happening in reaction mechanisms and "why" it happens for 85 unique reaction mechanisms across a yearlong postsecondary organic chemistry course. To analyze these data, we developed an electrophile rubric to capture learners' level of explanation sophistication ("Absent," "Descriptive," "Foundational," and "Complex"); this electrophile rubric is complementary to a nucleophile rubric previously reported in the literature. Our data show proportional levels of explanation sophistication for electrophiles and nucleophiles ([tau][subscript b] = 0.402) across these written explanations of reaction mechanisms. We note that learners' explanations of nucleophiles tend to be at a higher level than their explanations of electrophiles. While this finding does support prior literature reports, we also found that explanations of mechanisms involving reductions of pi-bonds ("e.g.," carbonyls) tended to be more sophisticated for electrophiles than for nucleophiles. Overall, our results support the claim that learners are able to discuss both electrophilicity and nucleophilicity; however, learners discuss electrophilicity and nucleophilicity at different levels of sophistication where nucleophilicity predominates for most reaction types.

Acid-base chemistry is a key reaction motif taught in postsecondary organic chemistry courses. More specifically, concepts from the Lewis acid-base model are broadly applicable to understanding mechanistic ideas such as electron density, nucleophilicity, and electrophilicity; thus, the Lewis model is fundamental to explaining an array of reaction mechanisms taught in organic chemistry. Herein, we report the development of a generalized predictive model using machine learning techniques to assess students' written responses for the correct use of the Lewis acid-base model for a variety (N = 26) of open-ended formative assessment items. These items follow a general framework of prompts that ask: why a compound can act as (i) an acid, (ii) a base, or (iii) both an acid and a base (i.e., amphoteric)? Or, what is happening and why for aqueous proton-transfer reactions and reactions that can only be explained using the Lewis model. Our predictive scoring model was constructed from a large collection of responses (N = 8520) using a machine learning technique, i.e., support vector machine, and subsequently evaluated using a variety of validation procedures resulting in overall 84.5-88.9% accuracies. The predictive model underwent further scrutiny with a set of responses (N = 2162) from different prompts not used in model construction along with a new prompt type: non-aqueous proton-transfer reactions. Model validation with these data achieved 92.7% accuracy. Our results suggest that machine learning techniques can be used to construct generalized predictive models for the evaluation of acid-base reaction mechanisms and their properties. Links to open-access files are provided that allow instructors to conduct their own analyses on written, open-ended formative assessment items to evaluate correct Lewis model use.

Assessments that aim to evaluate student understanding of chemical reactions and reaction mechanisms should ask students to construct written or oral explanations of mechanistic representations; students can reproduce pictorial mechanism representations with minimal understanding of the meaning of the representations. Grading such assessments is time-consuming, which is a limitation for use in large-enrollment courses and for timely feedback for students. Lexical analysis and logistic regression techniques can be used to evaluate student written responses in STEM courses. In this study, we use lexical analysis and logistic regression techniques to score a constructed-response item which aims to evaluate student explanations about what is happening in a unimolecular nucleophilic substitution ("i.e.," S[subscript N]1) reaction and why. We identify three levels of student explanation sophistication (i.e., descriptive only, surface level "why," and deeper "why"), and qualitatively describe student reasoning about four main aspects of the reaction: leaving group, carbocation, nucleophile and electrophile, and acid--base proton transfer. Responses scored as Level 1 (N = 113, 11%) include only a description of what is happening in the reaction and do not address the why for any of the four aspects. Level 2 responses (N = 549, 53%) describe why the reaction is occurring at a surface level (i.e., using solely explicit features or mentioning implicit features without deeper explanation) for at least one aspect of the reaction. Level 3 responses (N = 379, 36%) explain the why at a deeper level by inferring implicit features from explicit features explained using electronic effects for at least one reaction aspect. We evaluate the predictive accuracy of two binomial logistic regression models for scoring the responses with these levels, achieving 86.9% accuracy (with the testing data set) when compared to human coding. The lexical analysis methodology and emergent scoring framework could be used as a foundation from which to develop scoring models for a broader array of reaction mechanisms.

The Lewis acid-base model is key to identifying and explaining the formation and breaking of bonds in a large number of reaction mechanisms taught in the sophomore-level year-long organic chemistry course. Understanding the model is, thus, essential to success in organic chemistry coursework. Concept-inventories exist to identify misunderstandings and misconceptions of acid-base theories; open-ended problems, though, have been shown to provide a more nuanced and holistic understanding of how students use acid-base models to explain reactions. The time necessary to score such problems, however, limits their use, especially in large student enrollment courses. Given the efficacy of open-ended problems, there is occasion for the development of methods to efficiently and effectively analyze open-ended assessment responses. In this study, we establish the importance of assessing "use of the Lewis acid-base model to explain a chemical reaction" by determining the association of model use with summative examination performance. In addition, we generate and evaluate a binomial logistic regression model based on lexical analysis techniques for predicting Lewis acid-base model use in explanations of an acid-base proton-transfer reaction. Our work results in a predictive model that can be used to score the open-ended problem used in our study.

A well-developed understanding of the Lewis acid-base model is highly important for the understanding of organic chemistry. As such, students should receive instruction and be assessed on use of the model. Online tutorials and constructedresponse items provide a means for confirming that students have a well-developed conceptualization of the Lewis acid-base model. In a prior study, a predictive logistic regression model was presented that can be used with constructed-response assessment items to determine use of a Lewis acid-base model in written responses. In this study, we use that predictive model to evaluate the effectiveness of a tutorial designed to promote meaningful understanding of the Lewis acid-base model in three different instructional contexts: first-semester organic chemistry students before summative assessment, first-semester organic chemistry students after summative assessment, and second-semester organic chemistry students. Additionally, we evaluated the learning gains of one set of first-semester students after a 3-week time delay. McNemar's test results suggest that the tutorial had a net positive impact in all three instructional contexts, with the most significant impact observed with the second-semester students. This work has implications for further development of literature-based tutorials to promote meaningful understanding of organic chemistry reaction mechanisms assessed by constructed-response items. Key words: chemical education research, organic chemistry, computer-based learning, testing, assessment, Lewis acids and bases. Pour bien comprendre la chimie organique, il est tres important d'acquerir une bonne maîtrise du modele acidobasique de Lewis. Dans cette perspective, on doit enseigner aux etudiants comment utiliser ce modele et evaluer l'acquisition de la competence. A ce titre, les tutoriels en ligne et les questions a developpement offrent un moyen de s'assurer que les etudiants ont acquis une bonne maîtrise du concept acido-basique de Lewis. Dans une etude anterieure, nous avons presente un modele predictif de regression logistique pouvant être utilise dans le cadre d'une evaluation par questions a developpement pour determiner si le modele acido-basique de Lewis a ete evoque dans les reponses ecrites. Dans la presente etude, nous avons employe ce modele predictif pour evaluer l'efficacite d'un tutoriel conçu pour favoriser une reelle comprehension du modele acido-basique de Lewis dans trois differents contextes pedagogiques : etudiants au premier trimestre en chimie organique avant la premiere evaluation recapitulative, etudiants au premier trimestre en chimie organique apres la premiere evaluation recapitulative et etudiants au deuxieme trimestre en chimie organique. En outre, nous avons evalue l'acquisition des connaissances chez un groupe d'etudiants au premier trimestre apres un delai de trois semaines. Les resultats du test de McNemar semblent indiquer que le tutoriel a eu une incidence positive dans les trois contextes pedagogiques, l'effet le plus important ayant ete observe chez les etudiants au deuxieme trimestre. Ces travaux presentent un interêt pour le raffinement de tutoriels fondes sur la litterature qui visent a favoriser un reel apprentissage des mecanismes reactionnels en chimie organique evalues par des questions a developpement. [Traduit par la Redaction] Mots-cles : recherche pedagogique en chimie, chimie organique, enseignement assiste par ordinateur, examens, evaluations, acides et bases de Lewis., Introduction The Lewis acid-base model is the most useful acid-base model for explaining reactivity in organic chemistry. (1-6) It has been shown that the understanding and application of the Lewis [...]

The palladium-catalyzed C-O bond formation was applied for efficient synthesis of diporphyrins from bromoporphyrin precursors. The method could also be applied to obtain a series of homo-diporphyrins containing different types of spacers in high to excellent yields.

A study to demonstrate that the readily accessible 5,10-bis(2',6'-dibromophenyl)porphyrins are versatile synthons for modular construction of chiral porphyrins via palladium-catalyzed multiple amidation reactions with chiral amides is presented. Findings show that Cobalt(II) complexes of the D2-symmetric chiral prophyrins are active catalysts for highly enantioselective and diastereoselective cyclo-propanation under a practical one-pot protocol.

Cobalt complexes of N-confused porphyrins and benziphthalocyanine, which both feature organometallic bonds at the macrocycle cores, catalyze the cyclopropanation of styrene with a higher trans-selectivity than the corresponding porphyrin and phthalocyanine complexes. [ABSTRACT FROM AUTHOR]

This article focuses on bromoporphyrins. Metalloporphyrins have been found to catalyze a range of fundamentally and practically important chemical transformations, some of which represent the first demonstrations of such catalytic processes. Due to the unique ligand environment and metal coordination mode, unusual reaction selectivities and excellent catalyst turnovers have been observed for metalloporphyrin-based catalysts. Although significant progress has been made, catalytic reactions based on metalloporphyrins have not been developed into practical methodologies that can be used in asymmetric synthesis.

Diporphyrins can be efficiently synthesized from bromoporphyrin precursors via palladium-catalyzed C-O bond formation. The synthetic methodology is general and can be applied to various diols, forming a series of homo-diporphyrins containing different types of spacers in high to excellent yields. Chiral diporphyrins can be readily constructed through the use of optically active diols. A similar strategy allows access to hetero-diporphyrins and triporphyrins, including free-base and metalloporphyrin hetero dimers.