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What is the structure of benzene? (C 6 H 6 ) first ideas:
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or, it could be this...
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actual structure (refer to models)
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orbitals and electron densities:
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8-membered version (cyclo-octa-tetra-ene): not in same plane – less delocalization, single/double bonds
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Resonance contributors
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four electrons in three p orbitals
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Rules for resonance: 1: Only electrons move – never atoms 2: Only electrons and lone pair electrons can move – never electrons 3: Number of electrons never changes. Net charge is conserved
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carbocations two electrons in three p orbitals
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Relative stability (importance) of resonance contributors A is more stable – no sep of charge. A is closer to reality
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equal stability
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neg on carbon, pos on oxygen
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rules for stability: contributor is less stable if it has: 1: atom(s) with incomplete octet 2: neg charge NOT on most electronegative atom 3: charge separation
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which is more stable?
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resonance delocalization gives stability! which is more stable carbocation?
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conjugated dienes more stable:
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heats of hydrogenation: Why? conjugated p electrons are delocalized (refer to model) also – sp2-sp2 overlap is closer than sp2-sp3 – meaning closer overlap too
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cumulated double bonds - allene
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stability of allylic / benzylic carbocations
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rate of reaction with HBr?
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MO theory
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1,3-butadiene LUMO HOMO
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1,3,5-hexatriene HOMO-LUMO gap getting smaller!
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Benzene don’t worry about pictures of higher energy MO’s (p. 311 fig)
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Resonance delocalization and acid/base strength
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what about ortho? meta?
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explain regioselectivity:
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1,2 vs 1,4 addition
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Thermodynamic vs kinetic control
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kinetic control: low temps
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thermodynamic control: higher temps
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more sub alkene – more stable
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kinetic control: why is 1,2 product formed faster? consider the reaction:
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proximity effect:
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kinetic control, but no thermodynamic control:
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Diels-Alder
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increases reactivity of dienophile
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MO picture:
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prediciting regiochem:
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prefered:
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conformation of diene:
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in general, endo favored
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stereochem of Diels-ALder
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