Yale's Freshman Organic Chemistry II (CHEM 125B), lecture 10 with Michael McBride. McBride examines how bridged pentavalent carbon structures act as intermediates or transition states during cation rearrangement in SN1 reactions, and how short-lived ion pairs account for net inversion of stereochemistry. He contrasts how preparative and mechanistic organic chemists interpret reaction yields differently, using a study of t-butylhydrazine designed to show that molecular rotation can become rate-limiting in viscous solvents. The lecture then turns to electrophilic addition to alkenes, framed as the reverse of E2 or E1 elimination and studied with analogous kinetic and stereochemical techniques. McBride closes by using NIST Webbook thermochemical data to compare the relative stability of isomeric alkenes. Recorded in Spring 2011, the lecture moves from mechanism to data with the blackboard derivations and reaction diagrams typical of the course.