Chapter 11: General Principles of Mechanisms

The authors classify mechanisms into three primary categories—polar, radical, and pericyclic—each governed by distinct electronic principles and occurring through different intermediate species. Curved arrow notation is presented as the universal language for depicting electron movement and bond reorganization, enabling chemists to visualize and communicate mechanistic proposals. A central focus examines reactive intermediates including carbocations, carbanions, radicals, and carbenes, with detailed analysis of their formation conditions, relative stabilities, and typical lifetimes. The chapter emphasizes that intermediate stability directly correlates with reaction feasibility and selectivity. Kinetic and thermodynamic considerations are explored through rate-determining steps, reaction coordinate diagrams, and the principle of microscopic reversibility, which states that forward and reverse reaction pathways must be identical at the molecular level. The authors explain how experimental evidence—particularly kinetic isotope effects that reveal which bonds are broken during rate-limiting steps and linear free-energy relationships that quantify how molecular structure affects reactivity—can be employed to deduce or validate proposed mechanisms. Rate-determining steps are shown to control overall reaction rates and explain observed reaction order and kinetic dependencies. By synthesizing these conceptual elements, the chapter provides students with both theoretical understanding and practical tools for analyzing, predicting, and mechanistically explaining organic transformations across diverse reaction types.