Electron Delocalization, Resonance structures Orbital theory.pdf

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  LMZ/CHEM109A/ Winter 2010 Chapter 7: Electron Delocalization, Resonance structures and More MolecularOrbital TheoryI. 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 HydrogenationHydrogenation  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. examplesB. Rules for drawing resonance structures (contributors)-hyperthetical structures 1. only electrons move, atoms never move;2. electron numbers stay the same3. the octet rule (nobel gas electron configuration) can not be exceeded (but could beunfulfilled, e.g. carbocation)4. only π or n electrons move (exceptions with hyperconjugation, where σ electronsinvolved)e.g.5. The weighted average of all the resonance structures (resonance hybrid) representsthe real picture of the molecule. C. Predict relative stability of resonance structures factors make resonance structures with higher energy (with less stability)1. an atom with an incomplete octet2. a negative charge not on the most electronegative atom or a positive charge not onthe least electronegative atom3. charge separation D. Electron delocalization (resonance) offers stability (sec. 7.6) - resonance structure contribution to stability- the more stable a resonance structure is, the more contribution it has to the realmolecule, the more the structure is stable, the more similar it is to the real moleculestructurally/reactivity-wise.- the greater the number of relatively stable resonance structures, the more stable themolecule is (more resonance energy/delocalization energy) ( not just the sheer numberof resonance structures) . E. Examples:- conjugated dienes vs nonconjugated dienes vs cumulative dienes (allene)  LMZ/CHEM109A/ Winter 2010 - allylic and benzylic cations III. Molecular Orbital Description of Stability - 1,3-butadiene and 1,4-pentadiene and 1,2-propadienefor delocalization, the p orbitals need to be parallel (double bonds coplanar)- HOMO (highest occupied molecular orbital) -- electrons in these orbitals are the mostenergetic and reactive except lone electron pairs (n electrons)- LUMO (lowest unoccupied molecular orbital) -- the most stable orbital for fillingadditional electrons.- 1,3,5-hexatrienes vs benzeneall p orbitals are parallel in order toachieve side-by-side overlap, thusdelocalization- benzenearomaticunusually stabledue to largedelocalizationenergy IV. Delocalization on p  K  a  - resonance structures and inductive effect V. Delocalization on reactivity: regioselectivity (sec. 7.10) - benzylic cation vs secondary carbocation - allyl cation vs secondary carbocation
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