This second semester of Freshman Organic Chemistry builds on the first semester’s treatment of molecular structure and energy* to discuss how reaction mechanisms have been discovered and understood. It also treats the spectroscopy and synthesis of organic molecules. Reactions and their rates can be understood in terms of reaction-coordinate diagrams involving the passage of a set of atoms through the “transition state” on the potential-energy surface. Analysis of bond-dissociation energies suggests a chain mechanism for free-radical halogenation of alkanes. Experimental determination of kinetic order provides insight into complex reaction schemes, especially when one step is rate-limiting.