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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
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of BenzeneProblem 111a
Chapter 9, Problem 111a

Draw the resonance contributors of the cyclooctatrienyl dianion.
a. Which of the resonance contributors is the least stable?

Verified step by step guidance
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Step 1: Begin by understanding the structure of the cyclooctatrienyl dianion. Cyclooctatrienyl refers to an eight-membered ring with three alternating double bonds. The dianion indicates that the molecule has two extra electrons, which contribute to its resonance.
Step 2: Identify the locations of the two extra electrons. These electrons are typically placed in positions where they can delocalize across the conjugated π-system of the ring. Use curved arrows to show the movement of electrons to generate resonance structures.
Step 3: Draw all possible resonance contributors by systematically moving the π-electrons and the extra electrons around the ring. Ensure that each resonance structure adheres to the rules of resonance, such as maintaining the octet rule for atoms and avoiding overloading any atom with more than eight electrons.
Step 4: Evaluate the stability of each resonance contributor. Resonance structures with minimal formal charges, evenly distributed charges, and charges on atoms with appropriate electronegativity (e.g., negative charges on oxygen or nitrogen) are generally more stable. Structures with localized charges or disrupted conjugation are less stable.
Step 5: Identify the least stable resonance contributor. This is typically the structure where the negative charges are localized on less electronegative atoms, or where conjugation is disrupted, leading to a less delocalized and less stable electronic configuration.

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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 Lewis structures for the same molecule that depict the delocalization of electrons. They help illustrate how electrons are distributed across a molecule, particularly in systems with conjugated double bonds. The actual structure of the molecule is a hybrid of these resonance forms, which contributes to its stability and reactivity.
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Cyclooctatriene and its Dianion

Cyclooctatriene is a cyclic compound with alternating double bonds, specifically containing three double bonds in an eight-membered ring. When it forms a dianion, it gains two additional electrons, leading to increased electron density. Understanding the structure and electron distribution in the dianion is crucial for analyzing its resonance contributors and stability.
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Stability of Resonance Contributors

The stability of resonance contributors is determined by factors such as the octet rule, charge distribution, and the presence of electronegative atoms. Contributors that minimize charge separation and maintain full octets are generally more stable. In the case of the cyclooctatrienyl dianion, identifying the least stable resonance contributor involves analyzing these factors to see which structure has the most unfavorable charge distribution.
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Related Practice
Textbook Question

The experiment shown next and discussed in Section 8.13 shows that the proximity of the chloride ion to C-2 in the transition state causes the 1,2-addition product to form more rapidly than the 1,4-addition product.

a. Why was it important for the investigators to know that the preceding reaction was being carried out under kinetic control?

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Textbook Question

Identify the product of the following reaction.

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Textbook Question

In 1935, J. Bredt, a German chemist, proposed that a bicycloalkene could not have a double bond at a bridgehead carbon unless one of the rings contains at least eight carbons. This is known as Bredt's rule. Explain why there cannot be a double bond at this position.

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Textbook Question

Draw the resonance contributors of the cyclooctatrienyl dianion.

b. Which of the resonance contributors makes the smallest contribution to the hybrid?

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