The Benefits of a Mechanistically Organized Book When Teaching a 2-cycle Approach

Two-cycle organic chemistry is a pedagogical approach that has gained in popularity over the last couple decades. It’s a rather simple idea: The first semester course is treated as something of a survey, dealing primarily with the fundamentals, whereas the second semester revisits many of the same topics from the first semester, but treating them in greater depth. This two-cycle approach seems to be particularly advantageous for institutions whose biology majors (and other nonmajors) are required to take just one semester of organic chemistry. With less depth in each first-semester topic, nonmajors are exposed to more topics, and the material, moreover, can seem less intimidating. For chemistry majors and pre-health students, a significant benefit might come from the way that second-semester material is treated. Revisiting the first semester topics in greater depth represents an inherent review of the earlier material, allowing students to stay fresh on that material throughout the entire year. And because the second semester maintains a focus on the more challenging material, students should be better prepared for the final exam.

Despite these potential benefits, instructors who teach (or want to teach) a two-cycle organic course face a significant problem: Which book to use. The first semester course is typically not an issue. For many instructors, a standard 1-semester textbook does the trick. But what about the second semester course? The organization of most textbooks—by functional group—makes it difficult to rearrange topics to focus on just the higher-level material. It can certainly be done, but the textbooks don’t naturally lend themselves to this kind of reorganization.

The way that I have organized material mechanistically, however, may make it easier to use in a two-cycle approach. In my textbook, each reaction type is discussed in back-to-back chapters. The first of each pair of chapter covers the fundamentals of the reaction type, and the second goes into greater depth—especially in terms of mechanistic complexity.

For example, in my first of two nucleophilic substitution and elimination chapters, factors are discussed that enable students to determine the winner of an S_N1/S_N2/E1/E2 competition. The second nucleophilic substitution and elimination chapter introduces other substitution and elimination reactions that are useful for synthesis, such as the bromination of alcohols using PBr₃, alpha halogenation and alkylation, nucleophilic attack on epoxides, and Hofmann elimination. My first of two electrophilic addition chapters discusses just the addition of strong Brønsted acids to alkenes and alkynes—relatively simple reactions mechanistically. The second electrophilic addition chapter discusses electrophilic addition reactions that proceed through a cyclic transition state, such as the addition of carbenes, the addition of molecular halogens, oxymercuration-reduction, and hydroboration-oxidation.

The basis of designing a two-cycle organic course would then involve teaching the first of these paired chapters in the first semester, and the second paired chapter in the second semester. The sequence of topics throughout the entire year might then look something like this:

First Semester

• Chapter 1: Atomic and Molecular Structure
• Chapter 2: Three-Dimensional Geometry, Intermolecular Interactions, and Physical Properties
• Chapter 3: Orbital Interactions 1: Hybridization and Two-Center Molecular Orbitals
• Chapter 4: Isomerism 1: Conformational and Constitutional Isomers
• Chapter 5: Isomerism 2: Chirality, Enantiomers, and Diastereomers
• Chapter 6: The Proton Transfer Reaction: An Introduction to Mechanisms, Thermodynamics, and Charge Stability
• Chapter 7: An Overview of the Most Common Elementary Steps
• Chapter 8: An Introduction to Multistep Mechanisms: S_N1 and E1 Reactions
• Chapter 9: Nucleophilic Substitution and Elimination Reactions 1: Competition Among S_N2, S_N1, E2, and E1 Reactions
• Chapter 11. Electrophilic Addition to Nonpolar Pi Bonds 1. Addition of a Brønsted acid.
• Chapter 17. Nucleophilic Addition to Polar Pi Bonds 1. Addition of strong nucleophiles.
• Chapter 20. Nucleophilic Addition–Elimination Reactions 1. The general mechanism involving strong nucleophiles.
• Chapter 22. Electrophilic Aromatic Substitution 1. Substitution on benzene and symmetric benzene rings.

Second Semester

• Chapter 10: Nucleophilic Substitution and Elimination Reactions 2. Reactions that are useful for synthesis.
• Chapter 12. Electrophilic Addition to Nonpolar Pi Bonds 2. Reactions involving cyclic transition states.
• Chapter 13. Organic Synthesis 1: Beginning Concepts
• Chapter 14. Orbital Interactions 2: Extended Pi Systems, Conjugation, and Aromaticity.
• Chapter 15. Structure Determination 1: UV-Vis and IR Spectroscopies
• Chapter 16. Structure Determination 2: NMR Spectroscopy and Mass Spec
• Chapter 18. Nucleophilic Addition to Polar Pi Bonds 2. Addition of weak nucleophiles; acid and base catalysis.
• Chapter 19. Organic Synthesis 2. Intermediate topics.
• Chapter 21. Nucleophilic Addition–Elimination Reactions. 2. Weak nucleophiles.
• Chapter 23. Electrophilic Aromatic Substitution 2. Substitution involving mono and disubstituted benzene and other aromatic rings.
• Chapter 24. The Diels–Alder Reaction and Other Pericyclic Reactions.
• Chapter 25. Reactions Involving Free Radicals.
• Chapter 26. Polymers

— Joel Karty