Questions and Answers
Define localized electrons.
Electrons that are restricted to a particular region or belong to a single atom or are shared between two atoms.
Define delocalized electrons.
Electrons that are shared between three or more atoms.
Describe benzene's structure.
A planar molecule with all six carbon-carbon bonds the same length, all six carbons are sp2 hybridized (120º bond angle), and six π electrons are delocalized.
Define resonance contributor.
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Define resonance hybrid.
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When is electron delocalization most effective?
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What do delocalized electrons result from?
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What are the rules for drawing resonance contributors?
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What causes resonance contributors to contribute more to the structure of the resonance hybrid?
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What causes resonance contributors to be more similar to the real molecule?
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What is a molecule with separated charges?
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When electrons can be moved in more than one direction, how is the most stable resonance contributor obtained?
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When is it acceptable to move electrons away from the most electronegative atom?
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List features that decrease the predicted stability of a resonance contributor.
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Define delocalization energy.
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What factors cause a greater delocalization energy?
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What is important in determining the delocalization energy?
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Define dienes.
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Define isolated dienes.
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Define isolated double bonds.
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Define conjugated dienes.
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Define conjugated double bonds.
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Which is more stable, a conjugated or isolated diene?
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Why is a conjugated diene more stable than an isolated diene?
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Define allenes.
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Define cumulated double bonds.
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Study Notes
Localized and Delocalized Electrons
- Localized electrons are confined to a specific atom or shared between two atoms.
- Delocalized electrons are shared among three or more atoms, contributing to molecular stability.
Benzene Structure
- Benzene is a planar molecule with uniform carbon-carbon bond lengths.
- Each of the six carbon atoms is sp² hybridized, yielding 120º bond angles.
- Six π electrons in benzene are delocalized, enhancing stability.
Resonance Concepts
- A resonance contributor (or structure) depicts an arrangement with localized electrons but is not an actual representation.
- The resonance hybrid is the true structural representation that reflects delocalized electrons.
Electron Delocalization
- Most effective in planar arrangements to maximize p orbital overlap among atoms.
- Caused by the overlap of p orbitals from adjacent atoms.
Drawing Resonance Contributors
- Only electrons can be moved; atoms remain static.
- Movement is restricted to π electrons and lone pairs.
- Total electron count and net charge must remain unchanged across contributors.
Stability of Resonance Contributors
- Greater stability in a resonance contributor correlates with its contribution to the hybrid structure.
- Contributors resembling the real molecule provide better representations when they contribute more.
Charge Separation in Molecules
- Molecules with separated charges are inherently unstable.
- They consist of distinct positive and negative charges that can be neutralized via electron movement.
Stability and Electron Movement
- To achieve maximum stability, electron movement should preferentially direct toward the most electronegative atom.
- If necessary, it is acceptable to move electrons away from the electronegative atom when other alternatives are nonexistent.
Features Decreasing Stability
- Incomplete octets reduce predicted stability.
- Negative charges should ideally reside on electronegative atoms, while positive charges should not.
Delocalization Energy
- The extra stability afforded by delocalized electrons is termed delocalization energy or resonance energy.
- It increases with the number of stable resonance contributors and their structural similarity.
Dienes
- Dienes are hydrocarbons containing two double bonds.
- Isolated dienes possess double bonds separated by more than one single bond.
- Conjugated dienes have double bonds separated by a single bond, leading to greater stability.
Stability in Dienes
- Conjugated dienes are more stable than isolated dienes due to enhanced electron delocalization and stronger orbital overlaps (sp²-sp² versus sp²-sp³).
Allenes
- Allenes are compounds characterized by cumulated double bonds, which have a unique geometric arrangement leading to specific properties.
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Explore the fundamentals of delocalized and localized electrons through flashcards. This quiz covers key concepts such as the structure of benzene and electron sharing among atoms. Perfect for students studying organic chemistry.