Textbook Question
For each of these ions, draw the important resonance forms and predict which resonance form is likely to be the major contributor.
(a)
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1. A Review of General Chemistry
Resonance Structures
3:44 minutesProblem 44c
Textbook Question
For each of these ions, draw the important resonance forms and predict which resonance form is likely to be the major contributor.
(c) 
Verified step by step guidance1
Identify the structure of the ion: The given ion is a pyridinium ion with a positive charge on the CH2 group attached to the nitrogen-containing aromatic ring.
Determine the possible resonance forms: The positive charge on the CH2 group can be delocalized into the aromatic ring. This involves moving electrons from the double bonds in the ring to form new double bonds and shifting the positive charge around the ring.
Draw the first resonance form: Move the electrons from the double bond adjacent to the CH2 group towards the CH2 group, forming a new double bond between the CH2 and the ring, and shifting the positive charge to the nitrogen atom.
Draw additional resonance forms: Continue moving the positive charge around the ring by shifting electrons from adjacent double bonds, ensuring that each atom maintains a valid valence structure.
Predict the major resonance contributor: The major resonance form is typically the one with the least charge separation and the most stable arrangement of charges. In this case, the form where the positive charge is on the nitrogen atom is likely to be the major contributor, as nitrogen can better accommodate a positive charge due to its electronegativity.
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3mKey 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 a molecule that depict the same arrangement of atoms but differ in the distribution of electrons. They are used to represent delocalized electrons within certain molecules where the bonding cannot be expressed by a single Lewis structure. The true structure is a hybrid of all possible resonance forms, with some contributing more to the hybrid than others.
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Major and Minor Resonance Contributors
In resonance, not all structures contribute equally to the resonance hybrid. The major contributor is the resonance form with the lowest energy, typically having the most covalent bonds, the least charge separation, and negative charges on the most electronegative atoms. Minor contributors have higher energy due to less favorable charge distributions or incomplete octets.
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Aromaticity and Stability
Aromaticity refers to the increased stability of certain cyclic, planar molecules with delocalized pi electrons, following Huckel's rule (4n+2 pi electrons). In the context of resonance, aromatic systems often have resonance forms that maintain aromaticity, contributing significantly to the stability of the molecule. The presence of an aromatic ring can influence the major resonance contributor by stabilizing positive or negative charges through delocalization.
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