lec 7 - Resonance Resonance Resonance Resonance Resonance...

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Resonance Resonance Resonance Resonance Resonance Resonance Resonance Resonance Structures Reading: Gray: (2-14) OGC: (3.8)
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VII - 2 CC H H H H H C H H H H H C C C H H HH H C Resonance Structures [C 3 H 5 ] : two structural possibilities for - - however, the actual molecule is neither of the two structures: - δ - δ
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VII - 3 H C C H H H H H C Resonance Structures: Another Example Two structural possibilities for benzene: CC C H C C H H H H H C C H C C H H H H H C C Each C - C bond can be thought of as a 1.5-electron bond in the actual structure
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VII - 4 CC H H H H H C - Formal Charge Formal Charge : the difference between the number of valence shell electrons for an element and the number of electrons surrounding it in a molecule (including half of the bond electrons and all the free electrons) Example: The leftmost carbon has 3 bonds with 2 electrons each, plus 2 free electrons; so it has 5 electrons surrounding it. The formal charge equals the valence electrons minus surrounding electrons: 4 - 5 = –1 = Formal Charge
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VII - 5 “Reasonable” Resonance Structures 1. Form octets if possible 2. Maximize bonding 3. Distribute formal charges in a reasonable way -minimize if possible -more negative on more electronegative atoms -avoid same sign on neighboring atoms
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VII - 6 Drawing Resonance Structures Count the number of electrons available, and devise a reasonable structure having that many electrons example: [CO 3 ] 2 - 8e - in bonds +16e - as free electrons 24e - total - O C OO O C O C - C 4e - O 3(6e - ) + 2 extra electrons 24 electrons total - - - -
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VII - 7 O NN O N NO N N 2 O 2(5e - ) + 6e - = 16e - this structure won’t work: no octets on N’s nor this one: high formal charges +2 O nor this one: such configurations are destabilized by “ring strain” + best structures possible Resonance Structures of N 2 O - - - - +
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VII - 8 C C CC C C H H
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lec 7 - Resonance Resonance Resonance Resonance Resonance...

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