Chemistry Chapter 8: Delocalized Electrons

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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.

The approximate structure with localized electrons (imaginary). Signup and view all the answers

Define resonance hybrid.

The actual structure with delocalized electrons (real). Signup and view all the answers

When is electron delocalization most effective?

Electron delocalization is most effective if all the atoms sharing the delocalized electrons lie in the same plane so that their p orbitals can maximally overlap. Signup and view all the answers

What do delocalized electrons result from?

Delocalized electrons result from a p orbital overlapping the p orbital of two adjacent atoms. Signup and view all the answers

What are the rules for drawing resonance contributors?

Only electrons move, atoms never move; only π electrons and lone-pair electrons can move; the total number of electrons in the molecule does not change; each resonance contributor for a particular compound must have the same net charge. Signup and view all the answers

What causes resonance contributors to contribute more to the structure of the resonance hybrid?

The greater the predicted stability of a resonance contributor, the more it contributes to the structure of the resonance hybrid. Signup and view all the answers

What causes resonance contributors to be more similar to the real molecule?

The more the resonance contributor contributes to the structure of the resonance hybrid, the more similar the contributor is to the real molecule. Signup and view all the answers

What is a molecule with separated charges?

A molecule with separated charges has a positive charge and a negative charge that can be neutralized by the movement of electrons and are relatively unstable. Signup and view all the answers

When electrons can be moved in more than one direction, how is the most stable resonance contributor obtained?

The most stable resonance contributor is obtained by moving the electrons toward the most electronegative atom. Signup and view all the answers

When is it acceptable to move electrons away from the most electronegative atom?

When the movement of electrons away from the most electronegative atom is the only way the electrons can move. Signup and view all the answers

List features that decrease the predicted stability of a resonance contributor.

An atom with an incomplete octet, a negative charge that is not on the most electronegative atom, a positive charge that is on an electronegative atom, and charge separation. Signup and view all the answers

Define delocalization energy.

The extra stability a compound gains from having delocalized electrons. Signup and view all the answers

What factors cause a greater delocalization energy?

The greater the number of relatively stable resonance contributors and the more nearly equivalent the structures of the resonance. Signup and view all the answers

What is important in determining the delocalization energy?

The number of relatively stable resonance contributors, not the total number of resonance contributors. Signup and view all the answers

Define dienes.

Hydrocarbons with two double bonds. Signup and view all the answers

Define isolated dienes.

Dienes that have isolated double bonds. Signup and view all the answers

Define isolated double bonds.

Double bonds that are separated by more than one single bond. Signup and view all the answers

Define conjugated dienes.

Dienes that have conjugated double bonds. Signup and view all the answers

Define conjugated double bonds.

Double bonds that are separated by only one single bond. Signup and view all the answers

Which is more stable, a conjugated or isolated diene?

A conjugated diene. Signup and view all the answers

Why is a conjugated diene more stable than an isolated diene?

Greater electron delocalization and hybridization of the orbitals (an sp2-sp2 overlap is shorter and stronger than sp2-sp3 overlap). Signup and view all the answers

Define allenes.

Compounds that have cumulated double bonds. Signup and view all the answers

Define cumulated double bonds.

Double bonds that are directly adjacent to each other. Signup and view all the answers

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.