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 11(8)

Chapter 9, Problem 11(8)

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?
11.

Verified step by step guidance

Analyze the given structure: The molecule contains a benzene ring with a negative charge and a cyano group (-CN) attached to the ring. The negative charge indicates the presence of delocalized electrons, which can participate in resonance.

Step 1: Identify possible resonance contributors. The negative charge on the benzene ring can delocalize through the π-system of the ring. Additionally, the cyano group (-CN) is an electron-withdrawing group, which can influence the resonance contributors.

Step 2: Draw the resonance contributors. Move the negative charge around the benzene ring by shifting the π-electrons. Ensure that the cyano group remains attached to the ring and does not participate directly in the resonance.

Step 3: Evaluate the stability of each resonance contributor. Contributors where the negative charge is on more electronegative atoms (e.g., oxygen or nitrogen) or in positions that stabilize the charge (e.g., through conjugation) are major contributors. Contributors where the negative charge is on less electronegative atoms or in positions that destabilize the charge are minor contributors.

Step 4: Determine if all resonance contributors contribute equally. If the resonance contributors have similar stability (e.g., the negative charge is equally delocalized across the ring), they may contribute equally to the resonance hybrid. Otherwise, identify the major and minor contributors based on their stability.

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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 same arrangement of atoms but differ in the distribution of electrons. These structures help illustrate the delocalization of electrons within a molecule, which contributes to its stability. The actual structure of the molecule is a resonance hybrid, a weighted average of all valid resonance forms, where more stable forms contribute more to the hybrid.

Major and Minor Contributors

In resonance theory, major contributors are 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 in appropriate positions. Minor contributors, on the other hand, are less stable and contribute less to the overall resonance hybrid, often due to unfavorable charge distributions or incomplete octets.

Equal Contribution of Resonance Contributors

Some resonance contributors can contribute equally to the resonance hybrid if they are all equally stable and have similar energy levels. This typically occurs when the structures have similar arrangements of electrons and atoms, leading to no significant differences in stability. Identifying such contributors is crucial for understanding the overall electronic structure and reactivity of the molecule.