Chapter 39: Determining Reaction Mechanisms

The foundational approach is retrosynthetic analysis, a problem-solving method that works backwards from a desired product to identify key bond disconnection points and determine the ideal precursor fragments needed to construct it. Students learn to recognize synthons, which are idealized molecular fragments derived from strategic disconnections, and then map these synthons onto actual chemical reagents available in the laboratory, accounting for their real reactivity profiles and selectivity patterns. The chapter emphasizes target structure analysis as the critical first step, requiring students to systematically evaluate the functional groups present, identify the carbon skeleton framework, and recognize any ring systems that will influence synthesis planning. A core concept is understanding reactivity hierarchies and the sequence in which transformations must occur to avoid side reactions and incompatibilities between functional groups. The authors introduce the practical considerations of chemoselectivity, regioselectivity, and stereoselectivity early in the planning stage, ensuring that synthetic routes are not merely theoretically sound but also experimentally feasible and efficient. The chapter explores functional group interconversions as tools for preparing intermediates and discusses how reversible reactions can guide backward planning. Students learn to evaluate multiple competing synthetic routes by comparing their step economy, the accessibility of starting materials, the robustness of individual transformations, and the overall complexity of execution. Throughout, the emphasis remains on integrating prior mechanistic knowledge into a coherent strategy for molecular construction, preparing students for more complex total synthesis problems and enabling them to approach novel synthetic challenges with systematic thinking.