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Unformatted text preview: Resonance Resonance Resonance Resonance Resonance Resonance Structures Reading: Gray: (214) OGC: (3.8) VI1 C C H H H H H C C C H H H H H C C C H H H H H C Resonance Structures [C 3 H 5 ] : – two structural possibilities for   however, the actual molecule is neither of the two structures:  δ δ VI2 H C C H H H H H C Resonance Structures: Another Example Two structural possibilities for benzene: C C C H C C H H H H H C C C C H C C H H H H H C C C C Each C  C bond can be thought of as a 1.5electron bond in the actual structure VI3 C C 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 VI4 “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 VI5 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 O O O C O O O C O O  C 4e O 3(6e ) + 2 extra electrons 24 electrons total     VI6 O N N O N N N N O N N O 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 N N nor this one: such configurations are destabilized by “ring strain” + best structures possible Resonance Structures of N 2 O     + VI7 C...
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This note was uploaded on 10/27/2010 for the course CH 1a taught by Professor Lewis during the Fall '08 term at Caltech.
 Fall '08
 Lewis
 Chemistry, Mole

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