Michael McBride continues Yale's Freshman Organic Chemistry II with a lecture testing whether carbon can ever hold five bonds. He walks through X-ray analysis of molecules designed to favor a pentavalent carbon intermediate in SN2 reactions, showing why the data actually rule it out, while boron can achieve pentavalency in a similar environment. The lecture then turns to the reactions that compete with SN2: E2 elimination, examined through kinetic isotope effects and its stereochemistry and regiochemistry, and SN1 and E1 reactions, which proceed through carbocation intermediates prone to skeletal rearrangement. McBride connects these mechanistic details to practical synthesis strategy, discussing how the competition between E2 and SN2 pathways shapes the design of synthetic schemes involving carbon nucleophiles. Recorded in Spring 2011 as part of the Open Yale Courses series, the talk builds directly on kinetic and structural evidence rather than abstract rules, tying each mechanism to the experiments that revealed it.