Series06ResonanceStructures

- Resonance Resonance Resonance Resonance Resonance Resonance Structures Reading Gray(2-14 OGC(3.8 VI-1 C C H H H H H C C C H H H H H C C C H H H

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Unformatted text preview: Resonance Resonance Resonance Resonance Resonance Resonance Structures Reading: Gray: (2-14) OGC: (3.8) VI-1 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 VI-2 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 1.5-electron bond in the actual structure VI-3 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 VI-4 “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 VI-5 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 - - - - VI-6 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 - - - - + VI-7 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.

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- Resonance Resonance Resonance Resonance Resonance Resonance Structures Reading Gray(2-14 OGC(3.8 VI-1 C C H H H H H C C C H H H H H C C C H H H

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