ZLM-chapter 7-delocalization and resonance

ZLM-chapter 7-delocalization and resonance - LMZ/CHEM109A/...

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LMZ/CHEM109A/ Winter 2010 Chapter 7: Electron Delocalization, Resonance structures and More Molecular Orbital Theory I. Benzene (sec. 7.1-7.2): not cyclohexatriene (literally) All six hydrogens are equivalent, and carbon-carbon bonds are of the same length, 1.39Å A . Benzene is more stable than a triene. Heat of hydrogenation Hydrogenation Hydrogenation H = - 28.6 kcal/mol H = - 49.8 kcal/mol (< 3x28.6) " " H 2 3H 2 B . Unlike alkenes, benzene does not undergo electrophilic addition reactions with bromine C . Bonding in Benzene: electron delocalization II. Resonance structures: electron delocalization (mainly π or n electrons) (sec. 7.4) a simple but powerful method to explain bonding and stability A. examples B. Rules for drawing resonance structures (contributors)-hyperthetical structures 1. only electrons move, atoms never move; 2. electron numbers stay the same 3. the octet rule (nobel gas electron configuration) can not be exceeded (but could be unfulfilled, e.g. carbocation) 4. only π or n electrons move (exceptions with hyperconjugation, where σ electrons involved) e.g. 5. The weighted average of all the resonance structures (resonance hybrid) represents the real picture of the molecule. C. Predict relative stability of resonance structures
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ZLM-chapter 7-delocalization and resonance - LMZ/CHEM109A/...

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