Michael McBride opens Yale's CHEM 125B, the second semester of Freshman Organic Chemistry, by shifting from structure to mechanism: how chemists work out the pathway a reaction actually takes. He builds reaction-coordinate diagrams that plot potential energy as atoms pass through a transition state, then uses bond-dissociation energies to argue for a chain mechanism in free-radical halogenation of alkanes. The lecture's third chapter turns to kinetics, showing how measuring the experimental order of a reaction reveals which step is rate-limiting in a multi-step scheme. McBride works through the reasoning at the board with running commentary rather than slides, tying each mechanistic claim back to the energy concepts from the first semester. Recorded in spring 2011 as lecture one of the course, it sets up the mechanistic toolkit the rest of the semester will apply to organic transformations.