Michael McBride lectures on electrophilic addition mechanisms in Yale's Freshman Organic Chemistry II (CHEM 125B). He opens with alkene thermochemistry and the thermodynamics of acid addition, then walks through free-radical, metal-catalyzed, and stepwise electrophilic pathways, the last proceeding through a cation intermediate. McBride explains how electrostatics position an attacking proton but orbital mixing determines the more stable cation and thus Markovnikov selectivity, drawing on hyperconjugation, hybridization, and solvation to rank carbocation stability. He covers methide shifts as a competing pathway to solvent trapping and closes by examining the puzzling relative rates of HCl and HBr addition to alkynes, where halogen substituents act as both electron donors and electron withdrawers. Recorded in spring 2011 as part of the Open Yale Courses series, the lecture runs just under an hour and includes chapter markers for each topic.