Michael McBride continues Yale's Freshman Organic Chemistry II with a lecture on how electrophilic addition to alkenes can avoid skeletal rearrangement when a nucleophile participates simultaneously with the electrophile. He covers Markovnikov hydration via hydroxymercuration, the three-membered-ring halonium ion mechanism for halogen addition, cyclopropane formation from carbenes and what its stereochemistry reveals about simultaneous bond formation, and anti-Markovnikov hydration through hydroboration followed by oxidation with hydroperoxide. McBride draws an orbital analogy between the frontside C-O bond formation in the borane hydroperoxide rearrangement and backside attack in SN2 substitution, using syn-addition as evidence that nucleophilic and electrophilic attack occur together rather than through a free carbocation. The lecture is organized into four chapters moving from hydroxymercuration through halonium ions, carbenes, and hydroboration-oxidation, recorded as part of the Open Yale Courses series.