2 Resonance

Organic Chemistry: Structure and Function

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1 Resonance Lecture supplement: Thinkbook page 13
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2 What Is Resonance? Example Carbon monoxide Which accurately represents the structure? Open octet on carbon vs. formal charges - either decreases stability Double bond versus triple bond versus C O C O
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3 What Is Resonance? The Lewis structure of CO C-O or C=O? Laboratory analysis of C-O bond lengths Conclusion The Lewis structure that accurately represents CO is... C O C O C-O length 1.13Å shorter than C=O C-O length 1.20Å C-O length 1.21Å carbon monoxide O=C=O H 2 C=O Empirical observations of dipole moment rule out triple bond. Empirical observation: actual structure is somewhere in between. X X
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4 What Is Resonance? The Lewis structure of CO Perhaps an equilibrium between two structures that differ in electron distribution? C O C O
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5 Curved Arrows C O C O Indicates electron redistribution Starts at electron source; points to electron destination Electron source Electron destination When arrow starts at bond : bond is broken When arrow points to atom or space between atoms : bond is formed A curved arrow... C Cl H H H C H H H Cl C H H H Cl C Cl H H H
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6 C O C O What Is Resonance? The Lewis structure of CO Repeating the question: Perhaps CO structure is an equilibrium between two structures that differ in electron distribution? Equilibrium ruled out by empirical evidence. ? C O C O X
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7 What Is Resonance? The Lewis structure of CO Empirical CO structure is intermediate between the Lewis structures: C O Inadequate Inadequate C O Adequate structure What is this “between” structure? Each inadequate structure has some, but not all, actual features of CO CO cannot be adequately represented by a single Lewis structure Therefore “blend” or “average” inadequate structures composite structure
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8 What Is Resonance? A definition Resonance Molecule cannot be adequately represented by just one Lewis structure Two or more Lewis structures must be used.
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