My favorite chapter to teach in first-semester organic chemistry has to be Chapter 9 (“Nucleophilic Substitution and Elimination Reactions 1”) from Karty’s text. The introductions of SN2/E2 and SN1/E1 reactions begin in Chapters 7 and 8, respectively, but Chapter 9 puts these reactions to the test and suggests to the students that reactions do not always work as planned. Competition reactions occur in nearly every real-world synthesis and the competitions between simple substitutions and eliminations are the students’ first exposure to the challenges of being a synthetic chemist.
Chapter 9 may be my favorite chapter of first-semester organic chemistry, but I know the topics can be difficult for my students. Springing on them the real possibility of synthetic traps and undesired products, after providing fully detailed mechanisms of these seemingly flawless reactions, can be tough on moral. Some students, however, embrace the challenges of synthetic chemistry, finding reaction competition and synthetic traps a nice change from structural theory and stereochemical concepts. The students that learn to approach synthetic problems one step at a time, evaluating every aspect of the reaction, are not only the most successful, but find the systematic approach to synthesis refreshingly approachable. Karty’s text assists the students in effectively breaking down a synthesis, giving them the tools and tricks to evaluate and solve complicated syntheses. By including a variety of simplistic and difficult retrosynthetic examples, as well as the multiple stepwise functional-group transformation problems, students are given a diverse set of problems to better prepare them for examinations.
Chapter 9 (in conjunction with Chapter 10) is also the students’ first introduction to total syntheses, rather than just the observation and analysis of individual reactions. This requires some finesse in the chapter’s delivery; not only are there four different mechanisms and outcomes to discuss, but now multiple factors determining the major product of a reaction. The factors include the strength and concentration of the nucleophile (substitution) or base (elimination), the strength of the leaving group, the hybridization of the leaving group’s carbon, the protic v. aprotic solvent effects, and the temperature of the reaction. With so many factors to analyze, Karty allows instructors to take the time to teach the importance of accessing each factor individually, rather than allowing the problem to overwhelm the student. The analysis of individual traits is a skill better learned before synthesis chapters, like Chapters 13 and 19, become the focus of the second-semester course.
Having spent most of my professional career as a carbohydrate chemist, I finish Chapter 9 with SN1 reactions involving glucose pyranoses and SN2 reactions with competing intramolecular reactions in order to train the students to identify the hybridization of the electrophilic carbon, the leaving group, and the nucleophilic species hidden within a complex reactant. The easier it becomes for a student to identity the functional group of interest, the easier it will be to analyze and solve the more complicated syntheses of second-semester organic chemistry.
Chapter 9 is fun to teach and exciting for its introductions into the meat-and-potatoes aspects of organic synthesis. The topics can be difficult for students because of the many synthetic elements deriving from substitutions and eliminations. However, the organization of the four-chapter series on substitutions and eliminations (Chapters 7-10) creates a story that is easily followed by the student, providing them with the fundamental concepts and techniques necessary to be successful in organic chemistry.
-Cliff Coss, Northern Arizona University