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org rxn mech overview - 7 Hyperconjugation • Hyper what...

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1 Overview of Organic Reaction Mechanisms Thermodynamics vs. Kinetics
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2 Thermodynamics What are those bond dissociation energies?
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3 What can we say about the reaction mechanism in terms of the thermodynamics and kinetics?
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4
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5 Reactive Carbon Intermediates Carbocations (carbon cations) —six electrons Free Radicals—seven electrons Carbanions (carbon anions) —eight electrons You guessed it: electrophiles, electron pairing-abstraction mechanisms, and nucleophiles, respectively
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6 More about Carbocations • Hybridization? How are they stabilized? Where do we see them? [an example] Carbocations are sp 2 hybridized Why is this better than sp 3 ? Because the substituents are farther apart Better overlap with adjacent bonding orbitals
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Unformatted text preview: 7 Hyperconjugation • Hyper what? • An awkward term that refers to an orbital overlap mechanism (delocalization of a positive charge or electron density, if you like) Fig. 4.15 In VB, the C-H bond (or other C-H/C-C bonds) overlap with the empty p-orbital. In MO, the p-orbital is part of the entire molecular calculation. 8 Fig. 4.13 The more opportunities for overlap, the more stable the cation The S N 1 Reaction (substitution, nucleophilic, unimolecular 9 The Bimolecular version: S N 2 • Inversion of configuration: The electrophilic addition mechanism: CH 2 CH 2 + HBr CH 3 CH 2 Br 10 Details: CH 2 CH 2 CH 3 CH 2 Br H Br CH 3 CH 2 + Br carbocation intermediate...
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