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Lecture 11 - Carbocations and the Mechanism of Electrophilic Addition to Alkenes and Alkynes

Lecture 11 - Carbocations and the Mechanism of Electrophilic Addition to Alkenes and Alkynes

This video was recorded at CHEM 125b: Freshman Organic Chemistry II. Substitution stabilizes alkenes, and addition of acids is thermodynamically favorable in acidic media. Addition to alkenes can involve free-radical, metal-catalyzed, and stepwise electrophilic mechanisms, the last via a cation intermediate. Electrostatics can help position an attacking electrophile like H+, but bonding en route to Markovnikov addition requires orbital mixing to form the more stable cation. Relative cation stability can be understood in terms of hyperconjugation, hybridization, and solvation or polarizability. Stabilization of a carbocation via methide shift can compete with its trapping by solvent. The curious relative rates in stepwise addition of HCl or HBr to alkynes show that halogen substituents are... Show More
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