Chapter 7-8 (29-10-2019).pdf - Electron Delocalization Dienes and Benzene Bruice’s Organic Chemistry Chapter 8 Learning Outcomes 1.  Understand

Chapter 7-8 (29-10-2019).pdf - Electron Delocalization...

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Electron Delocalization: Dienes and Benzene Bruice’s Organic Chemistry : Chapter 8 1. Understand resonance theory and how it describes the properties of molecules such as dienes, allyl cations, benzene 2. Understand molecular orbital theory and how it describes properties of molecules such as dienes, allyl cations, benzene 3. Understand the reactivity of conjugated dienes 4. Understand the mechanism of the Diels Alder reaction Learning Outcomes
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Stability due to delocalization The two π systems are isolated. The two π systems are overlapped Why delocalization results in extra stability? § Two descriptions of bonding to understand delocalization: § 1. Resonance Theory § 2. Molecular Orbital Theory
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Resonance Theory 1. A molecule is stabilized by resonance à when several valid Lewis structures can be written for it which differ only in the position of the electrons . 2. Each valid structure is called a “ resonance contributor § Note the arrows used for showing resonance 3. They can be interconverted between each other by pushing electrons resonance contributor resonance contributor resonance contributor resonance contributor resonance contributor All are valid Lewis structures (e.g. C can only have 4 bonds) Only in the position of the electrons is changed
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Resonance Theory 4. No single resonance contributor is a valid representation of the real species: the true species is a hybrid of the resonance contributors § For a conjugated diene, the hybrid or true species has double bond character in the central bond, as implied from its resonance contributors Resonance contributor #1 Resonance contributor #2 resonance hybrid resonance contributor resonance contributor resonance contributor real conjugated diene not real, but a good representative
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Resonance Theory § From the resonance contributors of acetate, we deduce that the hybrid or true species has double bond character in both C - O bonds § There is negative charge on both oxygens, but the charge is not associated with the carbon resonance contributor resonance contributor resonance hybrid real carboxylate not real, but a good representative
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Resonance Theory § The hybrid or true species is always more stable than any of its contributors § Explains why the conjugated diene is more stable than isolated diene isolated diene conjugated diene
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Resonance Theory isolated diene real conjugated diene § The hybrid or true species is always more stable than any of its contributors § Explains why the conjugated diene is more stable than isolated diene
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Resonance Theory 6. The resonance contributors may or may not be equally stable , and will differ in significance § For acetate, both resonance contributors are equivalent and therefore equally stable § Each contributor are equally significant, and each contributes 50% character to the hybrid § Each oxygen has negative charge of ½ real carboxylate 1/2 1/2 50% 50% >> 50% << 50% and positive charge on electronegative atom more stable more contribution to the real hybrid
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