Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Problem 68(7)

Chapter 9, Problem 68(7)

a. Draw resonance contributors for the following species. Do not include structures that are so unstable that their contributions to the resonance hybrid would be negligible. Indicate which are major contributors and which are minor contributors to the resonance hybrid.
b. Do any of the species have resonance contributors that all contribute equally to the resonance hybrid?
7.

Verified step by step guidance

Step 1: Analyze the given structure. The molecule is a cycloheptatrienyl cation, also known as the tropylium ion. It consists of a seven-membered ring with alternating double bonds and a positive charge on one of the carbons.

Step 2: Recognize that the tropylium ion is aromatic. It follows the Huckel rule (4n + 2 π electrons, where n = 1), as it has 6 π electrons. This aromaticity makes the molecule highly stable and allows for resonance.

Step 3: Draw resonance contributors by moving the π electrons around the ring. Start by shifting one double bond to the adjacent position, while moving the positive charge to the next carbon. Repeat this process systematically around the ring to generate all possible resonance structures.

Step 4: Indicate that all resonance contributors are equivalent. In the tropylium ion, each resonance structure contributes equally to the resonance hybrid because the positive charge and π electrons are delocalized symmetrically across the ring.

Step 5: Conclude that the resonance hybrid is a fully delocalized structure, where the positive charge and π electrons are evenly distributed over all seven carbons. This delocalization is responsible for the stability of the tropylium ion.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Resonance Structures

Resonance structures are different ways of drawing a molecule that show the delocalization of electrons. They represent the same molecule but differ in the arrangement of electrons, particularly pi electrons and lone pairs. The actual structure of the molecule is a resonance hybrid, which is a weighted average of all valid resonance contributors, with more stable structures contributing more to the hybrid.

Major and Minor Contributors

In resonance theory, major contributors are those resonance structures that are more stable and thus have a greater influence on the resonance hybrid. Factors that enhance stability include full octets, minimized formal charges, and the presence of electronegative atoms bearing negative charges. Minor contributors, on the other hand, are less stable and contribute less to the overall resonance hybrid due to higher energy configurations or unfavorable charge distributions.

Equally Contributing Resonance Structures

Some molecules may have resonance contributors that contribute equally to the resonance hybrid. This occurs when all structures have similar stability and energy levels, often seen in symmetrical molecules. Identifying such structures is crucial for understanding the overall electronic distribution in the molecule, as it indicates that no single structure dominates, leading to a more uniform distribution of electron density.

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Drawing Contributing Structures

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Textbook Question

a. Draw resonance contributors for the following species. Do not include structures that are so unstable that their contributions to the resonance hybrid would be negligible. Indicate which are major contributors and which are minor contributors to the resonance hybrid.