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 6b

Chapter 9, Problem 6b

Draw resonance contributors for each of the following species and rank them in order of decreasing contribution to the resonance hybrid. Then draw the resonance hybrid.
b.

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Step 1: Identify the resonance contributors by analyzing the structure. Look for atoms with lone pairs, π bonds, and the ability to delocalize electrons. In this molecule, the nitrogen atom has a lone pair, and there is a conjugated π system involving the C=C double bond.

Step 2: Draw the first resonance structure. Move the lone pair of electrons on the nitrogen atom to form a double bond between nitrogen and the adjacent carbon. Simultaneously, shift the π electrons from the C=C double bond to the terminal carbon atom.

Step 3: Draw the second resonance structure. Move the π electrons from the newly formed N=C double bond back to the nitrogen atom, restoring the lone pair on nitrogen. Shift the π electrons from the terminal carbon back to the C=C double bond.

Step 4: Rank the resonance contributors based on their stability. Resonance structures with full octets on all atoms and minimal formal charges contribute more to the resonance hybrid. Structures with charges or incomplete octets contribute less.

Step 5: Draw the resonance hybrid by combining the features of all resonance contributors. Represent delocalized electrons as dashed lines and partial charges where applicable, showing the averaged electron distribution across the molecule.

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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.

Resonance Hybrid

The resonance hybrid is the actual structure of a molecule that is represented as a combination of its resonance contributors. It reflects the delocalization of electrons across the molecule, resulting in bond lengths and angles that are intermediate between those predicted by individual resonance structures. The resonance hybrid is often more stable than any single resonance contributor due to this electron delocalization.

Stability of Resonance Contributors

The stability of resonance contributors is determined by factors such as the octet rule, charge distribution, and the presence of formal charges. Contributors that have full octets, minimal formal charges, and charge separation are generally more stable and thus make a greater contribution to the resonance hybrid. Ranking these contributors helps predict the most significant structures that influence the overall properties of the molecule.

Draw resonance contributors for each of the following species and rank them in order of decreasing contribution to the resonance hybrid. Then draw the resonance hybrid. b.