resonance structures handout -...
TRANSCRIPT
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Resonance StructuresUCI Chem 51A
Dr. Link
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Goals1. Explain the need for resonance theory.
2. Explain what is a resonance structure and what is not.
3. Draw valid resonance structures.
4. Properly use curved arrow notation.
5. Identify major and minor resonance contributors.
6. Draw the resonance hybrid for a structure.
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Resonance Theory
Resonance structures are NOT REAL.
Resonance structures are NOT IN EQUILIBRIUM. (Note the arrow type.)
Resonance structure are NOT ISOMERS.
So what are they?!
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Resonance Hybrids
Resonance structures we draw are the extremes of a spectrum.
The “real” structure of the molecule is called a resonance hybrid and is somewhere between the extremes.
How do we know that? -1 -1 -1
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Evidence that Lewis Structures Fail
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Bond LengthC-O 143 pmC=O 122 pm
carbonate 129 pm
All C-O bonds equal!
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More on Resonance Hybrids
Fictitious Fictitious Real!
+ =
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Not All Resonance Structures are Created Equally!
Maximize bonds and octets.
Minimize formal charge.
When formal charge is necessary, (-) on more electronegative atoms, (+) on less electronegative atoms.
Major contributors vs. minor contributors.
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Major and Minor Contributors
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Wrapping Up
Practice drawing resonance structures, including formal charge.
Practice using the curved arrow notation.
Practice determining whether a resonance structure would be a major or minor contributor to the resonance hybrid.
Practice drawing the resonance hybrid.