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CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 11 - Professor J. Michael McBride

Carbocations and the Mechanism of Electrophilic Addition to Alkenes and Alkynes

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 both electron withdrawing and electron donating.

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