Communication pubs.acs.org/jchemeduc
An Improved Decision Tree for Predicting a Major Product in Competing Reactions Kate J. Graham* Department of Chemistry, College of Saint Benedict and Saint John’s University, Saint Joseph, Minnesota 56374, United States ABSTRACT: When organic chemistry students encounter competing reactions, they are often overwhelmed by the task of evaluating multiple factors that affect the outcome of a reaction. The use of a decision tree is a useful tool to teach students to evaluate a complex situation and propose a likely outcome. Specifically, a decision tree can help students predict a major product in substitution and elimination reactions.
KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Mnemonics/Rote Learning, Problem Solving/Decision Making, Reactions
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n organic chemistry courses, students typically struggle when presented with competing reactions that could potentially result in multiple products. Many students first encounter this problem when introduced to substitution and elimination reactions. Concrete thinkers who have been trained to look for “the correct answer” are often frustrated when attempting to make sense of a multitude of factors that can influence the major product formed. At this point, some students will throw logic to the wind and attempt to memorize reactions: a futile approach. In an attempt to consolidate these factors into a coherent package, many different authors have attempted to provide a framework for the material; McMurray’s Organic Chemistry textbook 1 provides a table, Karty2 asks students to create a checklist table, and Buonora and Lim3 have published a Disc method. While these resources are comprehensive, students do not seem to find these approaches to be particularly useful as they are too cumbersome. For the first introduction to a complex situation, undergraduate students need a streamlined decision making process. Hagen4 reported the use of a simple flowchart, and McClelland5 published a decision chart. These charts provide a preliminary overview that is easy to follow, yet students are left with the idea that base strength and substrate structure are the only two factors. McClelland simply avoids the role of solvent or beta hydrogen. Hagen does not address sterically hindered bases or the geometry required of E2 eliminations. A new decision tree (Figure 1) introduces students to a more complete set of factors affecting SN1, SN2, E1, and E2 reactions while still maintaining a straightforward decision making process. The roles of solvent, temperature, and stereochemistry are included, as well as nucleophilicity/basicity and reactant structure. Note that the E1cb mechanism is not specifically addressed in this activity. © XXXX American Chemical Society and Division of Chemical Education, Inc.
SUMMARY While instructors may choose to present this decision tree in a variety of ways, typically 1−2 class periods are used to introduce the key ideas of leaving groups, steric hindrance, charge stabilization, nucleophiles, and base strength that are needed to utilize this decision tree. Students then spend a class period implementing this decision tree to predict the major product of a reaction under specific conditions. Students have found that this decision tree helps them to navigate the murky territory of competing reactions with less confusion and in a shorter time period. At this point, it is possible to progress to more nuanced discussions of competing reactions, kinetics, and controlling reactions for synthetic processes.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) McMurray, J. E. Organic Chemistry, 8th ed.; Cengage Learning: Belmont, CA, USA, 2011; pp 372−407. (2) Karty, J. Predicting the Products of an SN1/SN2/E1/E2 Competition, Teach the Mechanism. http://teachthemechanism.com/ 2012/10/30/predicting-the-products-of-an-sn1sn2e1e2-competition/ #more-160 (accessed Jun 2014). (3) Buonora, P. T.; Lim, Y. J. The Substitution−Elimination Mechanistic Disc Method. J. Chem. Educ. 2004, 81 (3), 368−372.
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dx.doi.org/10.1021/ed400908g | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Figure 1. Decision tree to determine major product in competing nucleophilic substitution and elimination reactions. (4) Hagen, J. P. Flow charting leaving group reactions. J. Chem. Educ. 1988, 65 (7), 620. (5) McClelland, B. W. Chart for Deciding Mechanism for Reaction of Alkyl Halide with Nucleophile/Base. J. Chem. Educ. 1994, 71 (12), 1047−1048.
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dx.doi.org/10.1021/ed400908g | J. Chem. Educ. XXXX, XXX, XXX−XXX
